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A 2H and 14N NMR study of molecular motion in polycrystalline choline salts
JOURNAL
OF MAGNETIC
RESONANCE
81,350-370
(1989)
A 2H and 14NNMR Study of Molecular Motion in Polycrystalline Choline Salts T.
M.P.
K.PRATUM*-~AND
KLEIN+
*Department of Chemistry, University of Washington, Seattle, Washington 98195, and $Chemical Biodynamics Division, Lawrence Berkeley Laboratory, Berkeley California 94720 Received March 14, 1988; revised May 26,1988 ‘H and 14N solid-state NMR spectra of polycrystalline choline chloride, bromide, and iodide indicate that 180” cation flipping motion occurs in all three salts. From the temperature dependence of these spectra, the activation energy for this motion is determined to be 5.8 + 1 kcal/mol in the iodide salt and 11 + 1.5 kcal/mol in the chloride salt. In the bromide salt the reorientation rate is too rapid to be determined from the NMR lineshape, but the temperature dependence of the *H quadrupole coupling parameters is indicative ofa second-order phase transition at approximately 273 K. The spectral distortions in the 14N NMR spectra of the chloride and iodide salts are adequately explained using the motional model derived from the ‘H NMR results, while the ‘&N spectra of the bromide salt show no motional effects. The axis of reorientation which is inferred from these data appears to be consistent with that indicated in a previous X-ray crystallographic study. 0 1989 Academic PISS, Inc.
Two crystalline choline salts exhibit an amazing sensitivity to ionizing radiation. Both choline chloride, [ ( CH3)3NCH&H20H] + Cl-, and choline bromide, [ ( CH3)3NCH2CH20H ] + Br-, decompose via a chain mechanism which yields acetaldehyde and trimethylamine hydrochloride as its final products ( 1). Other choline salts and a large number of analogous compounds fail to show extreme radiation sensitivity, as do solutions of the two radiation labile salts (2). These observations have inspired a considerable research effort in an attempt to elucidate the mechanism of this unusual decomposition. Previous studies have included X-ray structural (36)) chemical (2, 7)) kinetic ( I, 7)) infrared (8-Z I ), calorimetric ( 22)) ESR ( 7, 9)) and NMR (6, 13-15) experiments. Studies of the temperature dependence of the radiation sensitivity (I) indicate that a thermally activated molecular reorientation process may be important. The utility of NMR in the study of molecular dynamics makes this an attractive technique for investigation of this process. Most of the previous NMR experiments have focused on the behavior of the proton second moment ( 13)) or the spin-lattice relaxation times T, and TIP ( 14, IS). These studies, which have for the most part been confined to the chloride, bromide, and iodide salts of choline, indicate that these compounds undergo, with increasing temperature, methyl group rotation (about 140”)) trimethylammonium group rotation (about 250”)) and isotropic reorientation of the entire choline ion (which occurs t To whom correspondence should be addressed. 0022-2364/89
$3.00
Copyright 0 1989 by Academic F’ress, Inc. All tigbu of reproduction in any form reserved.
350
MOTION
IN CHOLINE
SALTS
351
above the high-temperature phase transition in all three salts). The high-temperature cutoff of the radiation sensitivity correlates well with the onset of isotropic reorientation (6), but the low-temperature initiation seems not to correlate with any of the afore-mentioned motions. Recently Wemmer et al. (6) studied the NMR of these salts labeled with 13C at C4 and ‘H at the hydroxyl position (see Fig. 1). Their results indicate that significant motion sets in well before the high-temperature phase transitions, but the exact nature of this motion could not be elucidated. Additionally, they
a
b
C(3) C(5) C(4)/
------
-------0
---------N
C(I) C(4)
0: C(5)’
C(2) FIG. 1. (a) Anisotropic thermal ellipsoids from the X-ray crystallographic data of Hjortas and Sorum (4). The plotted axis is that corresponding to the major eigenvalue of&ration asdetermined by parkming a rigid body motion analysis (17) on these data. (b) The axis of apparent 180” reorientation in &oIine iodide as presented in Ref. (6) [reproduced with permission from J. Phys, Chem. 87,999, copyright 1983 American Chemical Society].
352
PRATUM
AND
KLEIN
obtained room temperature X-ray structural data which indicate a dynamic disorder for the cation in choline iodide between two positions related by a mirror plane (see Fig. lb). In an earlier study of 14N NMR in tetraalkylammonium salts ( 16), 14N NMR powder spectra were observed for the chloride and iodide salts which very much departed from a rigid lattice appearance. At that time, it was hypothesized that the distortion in the iodide spectra was related to the 180” flip-flop process revealed by the X-ray study, but there was no known anisotropic motion to account for the distortion observed in the chloride spectra. The crystallographic symmetry of the choline chloride unit cell allows for a twofold rotation coupled with a translation (a 2r screw axis), but an actual molecular motion corresponding to this seems quite unlikely. Fortunately, Hjortas and Sorum determined the anisotropic thermal parameters for all nonhydrogen atoms in their detailed crystallographic analysis of choline chloride ( 4)) and we have performed a rigid body analysis on these data (using the computer program THMV- I( 17)) in an attempt to reveal hidden librational freedom. The result showed one major axis of rotational freedom with a rms amplitude of lo” passing close to the nitrogen and oxygen atoms (see Fig. 1a), and another smaller rotational disorder of rms amplitude 3” approximately along the C-N ( Me)s axis. This latter motion is most likely associated with threefold rotation of the trimethylammonium group, while the former may have some relation to the flip-flop of the iodide salt. Because of the problems one encounters in assigning electric field gradient (efg) parameters to 14N in very symmetric environments, where crystalline field effects may become very large, it was not possible to analyze these spectra without further information concerning the nature of the motion affecting them. Since the deuteron quadrupolar interaction is not likely to show large crystalline field effects, a 2H NMR study of selectively labeled cholines was undertaken to correlate the 14N lineshapes with motion revealed in the *H spectra. EXPERIMENTAL
The samples used for the 14N study were prepared from commercial choline halides (Sigma Chemical Co., St. Louis, Missouri) which had been recrystallized from isopropanol and dried in wcuo at 85°C for at least 12 hr prior to being sealed in glass tubes under a dry argon atmosphere. The deuterated compounds were prepared as follows: Choline-C4-d2, [ ( CH3)3NCD2CH20H]+ X -: Betaine free base (Sigma Chemical Co.) was heated to a temperature above 3 10°C in vucuo to give a crude quantity of N,N-dimethylglycine methyl ester. This crude product was then distilled three times, and only the fraction boiling near 135°C was retained. The purified ester was then exchanged in MeOD/ MeONa twice to yield N, N-dimethylglycine-C4 - d2 methyl ester. Subsequent reduction with lithium aluminum hydride and methylation with methyl iodide gave choline iodide-c, - d2, which was recrystallized from isopropanol and ion exchanged on Amberlite IRA-402 to yield the chloride and bromide salts. The level of deuteration, as determined by mass spectroscopy, was 8 1%. Choline-C5 - d2, [ ( CH3 )3 NCH2CD20H ] + X -: N, N-Dimethylglycine methyl ester, prepared as above, was reduced with lithium aluminum deuteride and methylated
MOTION
IN
CHOLINE
SALTS
353
with methyl iodide to produce choline-C5 - d2 iodide. Recrystallization from 2-propano1 was followed by ion exchange to give the chloride and bromide salts. The level of deuteration was determined by mass spectroscopy to be 89%. Choline-C,, C2, CJ-d3, [ ( CD3)3NCH2CH20H]+ X -: To a mixture of ethanol and &CO3 was added one part ethanolamine (reagent, BDH Chemicals, Vancouver, B.C., Canada) and slightly over three parts methyl iodide-d3. The entire mixture was refluxed overnight and recrystallized to yield choline iodide- d3. Ion exchange, as before, generated the chloride and bromide salts. All samples were dried in vucuo at approximately 1WC over P205 for at least 12 hr prior to being sealed in glass tubes under vacuum. To check for sample-preparationdependent effects, the C4 - d2 bromide salt was treated in four different ways: ( 1) it was recrystallized from 2-propanol, (2) it was recrystallized from ethanol-ether, (3) it was taken straight off the ion exchange column without recrystallization, and ( 4) the iodide was first exchanged for the hydroxide, then the hydroxide was titrated to neutrality with HBr. The 2H NMR spectra of all four were essentially superimposable at room temperature (the only temperature checked), and the T, ‘s were identical to within experimental error. Therefore, it is concluded that there are no large samplepreparation-dependent effects. In some cases our room temperature 2H NMR spectra contained a sharp peak at the Larmor frequency. Initially, this was thought to be due to the entry of water into the sample, but it was later found to be a result of pyrolysis of a small amount of sample adhered to the walls of the sample tube during sealing. Because this peak could obscure the onset of isotropic reorientation, we took special care to ensure its absence in those samples which were taken through their high-temperature phase transition. Spectroscopy. The 14N spectra and the 2H spectra acquired at temperatures below ambient were taken on a homebuilt spectrometer operating at frequencies of 19.507 MHz ( “N), 4 1.442 MHz ( 2H), and 270 MHz ( ‘H) at the University of California Lawrence Berkeley Laboratory. These spectra were acquired in the presence of a 50 kHz proton-decoupling field using a phase-alternated quadrupolar echo sequence ( f8), with 90” pulses of 3.3-3.7 ps for 14N and 2.5 ps for 2H. On this spectrometer, the sample temperature is measured with a thermocouple placed directly beneath the sample and is regulated to within 1°C. Previous work on calorimetricaIly known phase transitions has indicated that the sample temperature and the thermocouple reading differ by at most 2°C ( 19). Results which will be presented later in this paper indicate that an additional 3°C temperature gradient may be present across the sample. Deuterium spectra acquired above ambient temperature were taken at the University of British Columbia on a homebuilt spectrometer operating at a deuterium resonance frequency of 3 5.5 5. MHz. A phase-alternated quadrupolar echo sequence was employed with 90“ pulses of 2.5 ps. The sample temperature on this spectrometer is measured with a thermocouple placed inside a copper oven which surrounds the sampIe. The temperature gradient across the sample using this arrangement is estimated to be less than 0.5”C. The echo pulse spacing (T) used to acquire the 2H NMR spectra was always 40 /IS, while “N spectra were acquired using several T values. Because of the possibility of spectral distortion from anisotropic TI effects (20, 21) ” data were acquired using recycle delays of at least several times T,
354
PRATUM
AND KLEIN
The 14N quadrupolar echoes were left shifted to the echo maximum, and the resulting half echoes were exponentially multiplied prior to Fourier transformation. The *H data were left shifted to the echo maximum and Fourier transformed without exponential multiplication. The data in the out of phase channel were almost always retained, except as indicated. The lineshapes were analyzed by successively fitting the experimental data to a rigid lattice lineshape convolved with a broadening function and corrected for the response roll-off due to the finite duration of the excitation and echo pulses (22). In cases of apparently overlapping powder patterns, the procedure of dePaking (23) was used to help unravel the resulting spectrum. Relaxation time data which are presented here are not intended to be quantitative, but only to give an indication of general trends. Tr data on 2H were obtained using a quadrupole echo inversion-recovery sequence with compensated 180” pulses (24)) while those for 14N employed progressive saturation. The transverse relaxation time of the quadrupolar echo ( T2e) was measured by varying the pulse spacing (7) of the echo sequence in the presence of proton decoupling. Proton decoupling has a large effect on the values obtained, especially for the 2H T2e (e.g., choline-C5-d2 bromide at 25°C: undecoupled T2, = 370 ps, decoupled T2, = 1.1 ms). In all cases the data were fitted to a single exponential and no attempts at multiple exponential fits were made. RESULTS
AND DISCUSSION
Relaxation time data. We assume that a large-amplitude anisotropic motion is responsible for the temperature dependence of T, and T2e. This assumption will be further justified as we proceed. An idea of the rate of this motion may be gleaned from the 14N T2, data presented in Fig. 2 (we have observed 2H T2e data for the deuterated salts which show the same general trends). The Tze of choline iodide decreases as the temperature is lowered, but the T2, minimum is not attained. A similar
0 - choline x : choline
ctdonde iodide
+ = choline
bromide
FIG. 2. 14N Tze data plotted against reciprocal temperature for the chloride (0)) bromide ( * ) , and iodide ( X) salts of choline.
MOTION
IN CHOLINE
SALTS
355
observation is made for choline bromide, although the T,, decrease is much less pronounced. For choline chloride a clear Tze minimum is observed at approximately -20°C. These data indicate that, within the temperature range we have investigated, the motional rates appear to follow choline bromide > choline iodide @ choline chloride. The 2H Tl data (Fig. 3) further show this trend. No T, minimum is observed for choline chloride (the apparent minimum is the result of the phase transition at 3 5 1 K) , while the bromide and iodide salts show T, minima at approximately room temperature. These data tell us roughly that, at room temperature, the reorientation rate must be in the tens of megahertz range for the bromide and iodide salts, and in the hundreds of kilohertz range for the chloride salt. NMR lineshapes. For the two I = 1 nuclei we are considering, the NMR spectrum will be dominated by the first-order quadrupolar splitting, uQ
= 0.75e2qQ/h(3
cos*8 - 1 + rl sin20 cos 2+),
Ill
where the asymmetry parameter q = (V, - VYY)/ VZZand 0 and 4 relate the efg to the static magnetic field direction (HO). In our polycrystalline samples, this leads to a powder spectrum characterized by three doubly degenerate inflection points ( j vZZ1 3 I &fI ’ I KXI): V, = +(l - v)0.375e2qQ/h, I4 V,, = f(1 + v)0.375e2qQ/h,
[-iI
V,, = k0.750e2qQ/h.
VI
and In the presence of rapid motion, where the rate of the motion overwhelms the quadrupolar interaction, the powder spectrum reflects the nature of the motion. Motion of cubic or higher symmetry will completely average the quadrupolar splitting to a single line, while motions of lower symmetry yield the spectrum of an averaged, but nonzero, efg. From this averaged efg we can learn of the nature of the motion by envisioning the process which would be required to lead from the static to the observed spectrum, keeping in mind that the efg behaves under rotation as a tensor of rank two. Here we investigate the occurrence of fixed angle planar rotational jumps. if the jumps are of C, or higher symmetry, then the spectrum of an axial efg will result (irrespective of the symmetry of the static efg) whose magnitude will be given by a geometrical factor (similar to Eq. [ 11) relating the reorientation axis to the principalaxis system of the efg. If the jumps are of lower symmetry, then an axially asymmetric spectrum may result whose magnitude and asymmetry depend upon similar geometrical factors. The motion which is most commonly observed to yield spectra of an axially asymmetric nature is the two-site flip-flop (25-2 7). As the rate of the motion (K) slows to the order of the quadrupolar interaction, the NMR spectrum will collapse due to a short T2 (because the quadrupolar frequency is no longer well defined). In the situation of anisotropic motion, this spectral collapse will be anisotropic, producing characteristic spectra from which we can learn more about both the type of motion and the rate at which it is occurring. As discussed by Spiess and Sillescu ( 25 ) , the quadrupolar echo lineshape of a powder in this coalescence region will change as the pulse spacing T is varied due to anisotropic T2 effects.
'd: (b) 0
6
x 0 0 00
0
x
0
x
‘< 2 ‘$1
x
0
0
x
x
x
3
4
(c)
c
IO’-: t=%
G
0
-
L
0 0
lO-0 0
0 Oo
I
I 2
I
fl
0
1
3
I
I
I
'009qo!,0
)
1
1
4
FIG. 3. *H Tr data for selectively deuterated salts of choline. In each case the arrow indicates the hightemperature phase transition point of the nondeuterated material ( 12). For the data taken above 293 K (1000/T less than 3.4) the Larmor frequency was 35.55 MHz, while below this temperature (1000/T greater than 3.4) the Larmor frequency was 41.442 MHz. (a) Choline-C.& chloride TI data; the transition temperature is reported to be 35 1 K. (b) Choline bromide T, data for the Cd-d, (0) and C,-dr (X) salts. The transition temperature is reported to be 363 K. (c) Choline-C.+-dr iodide T, data; the transition temperature is reported to be 367 K. 356
MOTION
IN CHOLINE
SALTS
357
In order to take advantage of these facts, lineshapes must be calculated for various motional models at various rates, and compared with experimental data. The calculations of the theoretical spectra presented here were performed on an Amdahl470-V8 computer at the University of British Columbia using an exchange matrix formulation (25 ) . The effects of finite pulse length were taken into account by considering evolution under a static first-order quadrupolar interaction (22). This has been shown to lead to small errors in iineshape calculations in the intermediate motion limit due to stochastic evolution during the RF pulses (28)) and we must consider our neglect of this effect to be a possible source of error. The nature of the motion. The deuterium NMR spectra of all three C,-d2 salts at room temperature are presented in Fig. 4a. Assuming the static *H efg in these compounds to be approximately axial, we observe in the chloride and iodide salts an apparent effect of 180” flip-flop motion (25, 26) on the intermediate (K - e*qQ/ h) and fast (K 9 e*qQ/ h) time scales. While the choline bromide spectrum appears to be rigid, the 2H T, is much shorter, and the observed e*qQ/ h of 140 kHz is somewhat lower than the 170 kHz we would expect for a rigid C-D bond (29). As the temperature is raised (Fig. 5), all three salts show the growth of an isotropic (chloride. bromide) or nearly isotropic (iodide, in which a small residual coupling remains) peak in the spectrum. Above the respective phase transitions, the *H spectra of these salts are dominated by this feature. These results are completely in agreement with those of Wemmer et al. (6). In Fig. 4b the room temperature spectra of the three C,-d2 salts are presented. The primary point of interest here is that the bromide salt shows a fairly large asymmetry parameter (9 = 0.84), while the chloride and iodide spectra are very similar to those of the G-d2 salts. Assuming that there are no large differences in the cation conftguration between the isotopically labeled compounds, this observation can only be taken to mean that the orientation of the C-D bond vectors on carbons 4 and 5 with respect to the axis of motion must be approximately the same (there are different but
FIG.4. Quadrupole echo 2H NMR spectra of the three C4-dz (a) and C,-d, (b) choline salts taken at 293 K. The quadrupole coupling parameters derived from these spectra are (a) Br - &Q/h = 140 kHz, VJ= 0.05, I - e2qQ/h = 76 kHz, g = 0.93; and (b) Br - e’qQ/h = 73 kHz, v = 0.84,1e’gQ/h = 76 kHz, 7 = 0.96. In each case, the chloride spectrum is distorted by slow motions, and no quadrapole coupling parameters may be assigned to it.
358
PRATUM
do
:
Frequency
AND KLEIN
-lb0
IkHzl
364O I
II
III IO0
1 0 c _^-.._-^..
III ,L”-I
1 -100
II
FIG. 5. Choline-C4-dz chloride (a), bromide (b), and iodide (c) 2H quadrupole echo NMR spectra at temperatures 293 K and above.
indistinguishable orientations, as will be described later) in the chloride and iodide salts, while the bromide salt shows a significant difference. The room temperature spectra of the three choline- d9 salts are observed to be much narrower, and completely consistent with static efg’s which have been twice averaged by threefold jump motion, once by rapid methyl group rotation, and then by trimethylammonium group rotation. The spectra are all of intermediate motion character, and the rates which we may crudely estimate from these spectra are consistent with what may be assessed from the proton second moment study (i.e., approximately 350 kHz for Br, 100 kHz for I, and 70 kHz for Cl). As the temperature dependence of these spectra was found to be dominated by rather uninteresting threefold rotational motions, no further data are presented on these compounds. Interpretation in terms of 180” reorientations. Assuming that a simple 180” tlip-flop motion is dominant, we may interpret our observed average of efg’s in terms of a static efg, and the Euler angles (Yand ,6 which describe the orientation of the molecular flipping axis with respect to the principal-axis system of the efg (see Fig. 6). An equation which expresses a second-rank tensor interaction in terms of fixed angle planar rotational jumps about the axis z” in Figure 6 has previously been presented by Mehring (30). We may use this expression to determine the average quadrupolar fre-
MOTION
IN CHOLINE
359
SALTS
FIG. 6. The definition of the angles a and j3 which relate the axis of planar jumps (z”) to the principle axis of the electric field gradient (efg) tensor.
quency observed in the rapid motion limit for arbitrary orientations of the molecular llipping axis with respect to H,, (described by the angles 8 and 4), ~Q=0.75e2q~/h[~2(cQse)~2(COs~)-0.75((~~-~2)sin28sin28cos~ - sin*P sin28 cos 24) + 0.5q(P2(cos @sin28 cos 2a + (pl - p2)sin B sin 26( cos p cos 2cu cos r#~- sin 2a sin $) +sin28(0.5(1
+cos2p)cos2ff
cos2$ -cospsin2asin2&))],
[5]
where P2(x) = (3x2 - 1)/2, and pI and p2 are the normalized a priori occupation probabilities of the two sites which are in exchange. When pI and p2 are equal, this expression reduces to vQ = o.75e2qQ/h[P2(cos + 0.75
Sill’fl
Sin28
,d)&(cos 6) COS
24 + 0.5~{
pz(COS
8)SiI12/?
COS
2a
+ sin28(0.5( 1 + cos2/3)cos 2c~cos 24 - cos p sin ICYsin 24) }I.
[6]
For this situation, one of the principal values of the averaged efg will appear when the axis of reorientation is aligned with Ho (i.e., when fl= 0” and 4 = 0”) and is given by V,, = 0.75e2qQ/h{P2(cos@)+0.5~
sin2/3cos2a}.
VI The other two principal values will occur when 8 = 90” and 4 = $‘, I$‘, where 14 - 4” 1 = 90”. These other values may be obtained by finding the maximum and minimum values of Eq. [ 61 subject to 6 = 90“; one of these will occur at 4’ = 0.5 atan{ -(2 q cos /3 sin 2a)/( 3 sin26 + v( 1 + cos*~)cos 2cy)1,
VI
and the other will be related to this by 90”. Note that if 17is small, or cyis near 0” or 90”,$’ = 0” and 4” = 90”. In this case v,, = -0.75eZqQ/h(l
+ ?j cos 2a)/2,
]91
and V,, = -VI,-
V,, = -0.75e2qQ/h{P2(cosp)-
1 -0.5~cos2acos2~}.
[lo]
360
PRATUM
AND
KLEIN
Equations [ 71, [ 91, and [lo], for the situation in which cy= O”, have previously been derived by Soda and Chiba (26). When q is large, and LYis not close to 0” or 90”, one has no choice but to substitute Eq. [ 81 into Eq. [ 61. If 7 is small enough to be neglected, which should be an adequate approximation for aliphatic deuterons, then (Y has no meaning, and the observed e’qQ/ h and 17take on a particularly simple form: 0"
v,, = v,
(e’qQlh)obs = PZ(COSP)(e2qQ/h)static t&s
=
(1
-
P,(cos
P))I~2(cos
[Ill 1121
P),
35.3” < ,LI< 45”; I’,, = V,,, Vi, = V,,, V,, = V, and 45” < ,f3< 54.7”; V2, = V,, V33 = v,, v,, = v,
(e2qQlhb = (e2qQlh)sd2 %bs=
54.7"
1131
16cos*P-31,
[I41
v,, = v,
(e2&?lh>obs = (10 - p2(cosB))(e2sQlh),tic %bs
=
(l/2
+
p2(cos
8))/t1
/2
-
p2(c0s
P)).
[I61
Equations [ 1 I]- [ 161 imply that the observed e*qQ/h and 71values for 45” < /? < 90” are a reflection of those for 0” < B < 45” (e.g., p = lo” values are equivalent to those with 0 = 80”); therefore only a 45” range of @ will result in distinguishable powder spectra, and there will be several values of /I which could correspond to our observed 2H NMR spectra. Cholineiodide.Both C, and C5 compounds show approximately q= 1 .Olineshapes at room temperature (Cd rl = 0.93, Cs 7 = 0.95), so ,f3must be near 35.3” or 54.7”. The exact values would be Cd: 34.5” or 55.4, Cg: 34.8” or 55.2” for a static v = 0.0; or Cs: 33.8” or 56.2”, Cs: 34.0” or 56.0“ for a static q= 0.05 and LY= 0”. The only way to choose between the degenerate values of P would be through geometrical arguments. To determine the activation energy, we have obtained spectra at several lower temperatures (see Fig. 7) and compared these with calculated spectra (see Fig. 8; note that the exact choice of p is not an important factor in determining rates). Comparison of Figs. 7 and 8 yields the following rates (Key; ~~~ = K~,): 253 K w 5 X lo6 Hz, 233 K x 2.5 X lo6 Hz, 213 K a 650 X lo3 Hz, 193 K = 150 X lo3 Hz. The subscripts of K refer to the site labels; thus ~~2 defines the rate of flipping from site 1 to site 2. To these rates we have fitted an Arrhenius equation with an activation energy of5.8 + 1 kcal/mol. Cholinechloride.Here we do not obtain a rapid motion limit lineshape due to the onset of isotropic reorientation below the phase transition (see Fig. 5a). Nonetheless, the fact that we see no spectral intensity beyond l/2 V,, and we observe significant intensity in the center of these motionally distorted spectra, indicate that reorientation must occur through at least an angle of 70.5” (this comment applies if @is 90”; if /3 is other than 90”, the angle must be even larger). Therefore, the amplitude determined from the anisotropic thermal analysis is far too small to explain our spectra,
MOTION
-
100
IN CHOLINE
0 Frequency
SALTS
361
-100 IkHzI
FIG. 7. Quadrupole echo 2H NMR spectra of choline-C5-d2 iodide at temperatures (A) 293 K, (B) 253 K, (C) 233 K, (D) 2 13 K, (E) 193 K. The symmetric structure which permeates these lineshapes is due to a sample which was not finely divided enough to obtain a completely isotropic distribution of orientations. Very similar spectra are obtained for the Cd-d2 salt.
and we will henceforth assume that 180” jumps are occurring. Taking this assumption into account, the presence of the peak in the center indicates that p must be nearly 35.3” or 54.7”. This type of motion is expected to produce a very sharp feature in the center of the spectrum (see, for example, Ref. (25)). From a two-site exchange
FIG. 8. Simulated quadrupole echo spectra for a spin- I nucleus whose electric field gradient undergoing 180” tlips using the following parameters: a = o”, j3 = 56.2”, e*qQ/h = 152 kHz, 7 = 0.05, r = 40 es, and K,~ = K~, = (A) 10 X 103kHz, (B) 7 X 103kHz, (C) 5 X lO’kHz, (D) 2.5 X lo3 kHz, (E) 1 x 103kHz, (F) 650 kHz, (G) 150 kHz. All of these spectra have had 500 Hz of isotropic Lorentzian line broadening applied and have been multiplied by a function to account for the effect of finite duration pulses (22). The small amplitude wiggles are the result of truncation of the calculated half echoes.
362
PRATUM
AND KLEIN
model, the broad feature in the center of our spectrum could have two origins: (a) there is a distribution of jump angles 6, centered about some particular value, or (b) the angle /3 defining the molecular reorientation axis is distributed, or possibly diffuses over a range of values. These two possibilities were tested by adding an anisotropic exponential decay ( Tza) to each component of the powder lineshape, where (a) T;i cc (dvQ/dS)AS,
[I71
(b) T;i’ cc (+/4-3A/A
[I81
with A6 and A/? representing the assumed breadth of the distribution in 6 and & respectively. Although this treatment is quantitatively incorrect for either a static distribution or the effect of small-amplitude Brownian motion (21, .?I), it will give us a qualitatively correct feeling for the possible effects of (a) and (b). We have found that (a) will not reproduce the observed anisotropic line broadening unless 6 is substantially different from 180”. This is easily understood when one realizes that the averaged efg principle values are fairly insensitive to jump angle when this is nearly 180”. On the other hand, (b) will reproduce the observed anisotropic broadening in a qualitative sense (see Fig. 10). Unfortunately, we cannot mimic the exact shape of the central feature in Fig. 9 using this method. It is possible that the broad central feature would be better simulated using a very anisotropic rotational diffusional model (32)) but the sharpness of the edges of these spectra indicate that the character of the motion is strongly jump-like. Therefore, we have only a very rough idea as to the reorientation rates (K,~; K12 = Key): 293 K = 800 X lo3 Hz, 273 K N 150 X lo3 Hz, 253 K = 20 X lo3 Hz, 233 K x 5 X lo3 Hz. The value we estimate at 233 K is primarily based upon our measured Tze ( = 200 PSfor the C4 and Cs deuterated salts), which should approximately equal K ;i in the very slow motional region (33, 34). The fit to an Arrhenius equation yields an activation energy of 11 + 1.5 kcal/ mol.
0 Frequency
-100 IkHzI
RG. 9. Quadrupole echo ‘H NMR spectra of choline-C4-dz chloride at temperatures (A) 293 K, (B) 273 K, (C) 253 K, (D) 233 K. The spectrum at 233 K appears to have two barely resolved quadrupolar splittings, and will approximately fit to a sum of e’qQ/h = 160.0 kHz, q = 0.07, and e*qQ/h = 155.5 kHz, 7 = 0.0. Very similar spectra are obtained for the C,-d2 salt.
MOTION
i,
’
I
I
I
IN CHOLINE
II
160
I 6
Frequency
363
SALTS
I
I-
R I
,--ri -160
IkHzI
FIG. 10. Simulated quadrupole echo spectra for 180” jump motion using the following parameters: (Y = 90”, fl = 53.6”, A@ = 1” (A, B), Afl= o’(C, D, E), e’qQ/h = 156 kHz, n = 0.05, r = 40 us, and K,~ = K?, =(A)800kHz,(B)150WIz,(C)20kHz,(D)lOkHz,(E)5kHz.Thesespectrahavebeenmultipiiedby a function to account for finite duration pulses (22). The isotropic Lorentzian linewidth was assumed to be 500 Hz. The inset shows a simulation using the parameters of (A), but with A/3 = 0”.
Cholinebromide.For both of the isotopically labeled compounds a T, minimum is observed at nearly the same temperature as the iodide salt, but a slow or intermediate motion spectrum is not observed above 193 K (see Figs. 11 and 12 ) . Interestingly, the apparent asymmetry parameter of the C5 compound drops rapidly below 273 K, approaching zero in a more or less exponential manner as the temperature is lowered
FIG. 11. Quadrupole echo ‘H NMR spectra ofcholine-G-d2 bromide. The temperatures and associated quadrnpole coupling parameters are (A) 273 K, e’qQ/h = 141 kHz, 11z=0.03; (B) 253 K, e*qQ/h = 147 kHz, n = 0.0; (C) 233 K, e*qQ/h = 152 kHz, n = 0.036, and e*qQ/h = 152 kHz, 1) = 0.0; (D) 213 K, e*qQ/ h = 156 kHz, n = 0.058, and e2qQ/ h = 158 kHz, n = 0.028. The out of phase channel was zeroed prior to Fourier transformation of(B) , (C) , and (D) .
364
PRATUM
I ,
AND KLEIN
R I
I 100
I
I Frequency
I IkHzl
I
I
I
I
-100
FIG. 12. Quadrupole echo ‘H NMR spectra of choline-Cs-dz bromide. The temperatures and best fit coupling parameters (for which there are considerable errors, as explained in the text) are (A) 293 K, e’qQ/h = 73 kHz, 7 = 0.84; (B) 273 K, e*qQ/h = 75 kHz, 7 = 0.86; (C) 263 K, e*qQ/h = 103 kHz, 9 = 0.47; (D) 253 K, e*qQ/h = 112 kHz, 7 = 0.35; (E) 233 K, e*qQ/h = 133 kHz, q = 0.16; (F) 213 K, e*qQ/h = 146 kHz, v = 0.09; (G) 193 K, e*qQ/h = 154 kHz, T = 0.06.
(Fig. 13), while the C, salt shows nearly axial spectra of slightly reduced quadrupole coupling throughout this temperature range (Fig. 1 I). Assuming that twofold jumps are occurring in the C, salt, then the reorientation axis must lie nearly along one of the twofold axes of the efg (i.e., p is near 0” or 90”). Examination of the C5 spectra taken below 273 K shows that the turning points corresponding to V, and V,, take on a distinctly washed out appearance, while V, remains fairly sharp. A plot of V,, V,,, and V,, as a function of temperature (Fig. 13) indicates that V,, and V,, switch places just below 273 K, and VW then shows a small approximately linear temperature dependence (70 Hz/K), while V, and V,, increase exponentially as the temperature is lowered. Because the rate of change of V, and V,, with temperature is so large, even a small temperature gradient, 3°C in our case, will broaden the spectra to a significant extent. The interpretation of these results follows from the observation by Lemmon et al. of a very small peak at about 270 K in their thermogram of choline bromide ( 12). This is approximately the same temperature at which our spectra begin to change, so we may conclude that a low enthalpy phase transition is responsible for our observations. The fact that V, and V,, behave approximately as would an order parameter shows this phase transition to have considerable second-order character. There are two possible phase-transition mechanisms which are consistent with our observa-
MOTION
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Temp (OK)
FIG. 13.FQinciplevaluesofthe2Hefg(~V,j~)derivedfromFig. 12; V,(O), P’,(x),andV,(*).The solid line through V,, below 273 K follows Vz, = 56.4 1 + I 15.4. R”.53, while that through V, follows V,, = 4.34 + 102.4. R”.56, where R represents the reduced temperature ((273 - T)/273).
tions: (a) the angle 6 suddenly begins to change below the phase transition temperature ( T,), or (b) the occupation probabilities of the two sites involved in the flip-flop (P, and p2) suddenly begin to change below T, . For a discussion of (a), assume that 0 is initially 36.87” (although it could just as easily be 53.13”, as both of these values lead to the observed 17of 0.84 when the static Q = 0.0). We then calculate /3 at each temperature from our observed T)using Eqs. [ 121 and [ 141; the result is plotted in Fig. 14. We find that A/3 = 36.87 - fi fits quite well to an exponential equation in the reduced temperature (R = ( T, - T)/ T,) with a critical exponent of 0.5 1 -+ 0.05. Because the spectra of the C4 deuterons change very slightly through the phase transition, we must conclude that this mechanism would only involve a change in j3 at Cs , a highly improbable event. As the probability of occupation of the two sites varies, the time-averaged efg tensor tilts toward the site of highest probability, so this becomes equivalent to changing the angle fi. Assuming /I?is given, the new efg may be calculated numerically for given values of p1 using Eq. [ 5 ] and related to our observed r]. The value of p1 is equated to a value of residual disorder M = pI - p2 and plotted against temperature in Fig. 14. These data fit an exponential equation in R with a critical exponent of 0.29 St 0.05. This type of phase transition, where the cations are disordered between the two sites above T, and become ordered below, is completely consistent with what is observed in the C4 salt. The spectra at the C4 position show only a regular increase in Vz,, and a growing inequivalence of the two deuterons as the temperature is lowered, as evidenced by the splitting of the perpendicular edge. As the likely mechanism here is order-disorder, we will never expect to see a truly
366
PRATUM
AND KLEIN I
60
0
II
III 200
1
III
210
( 280
III
1
I
320
Temp PK) FIG. 14. Interpretation of the temperature dependence of the choline bromide-C5-d2 spectra in terms of a change in the angle @(X ), and a change in the relative occupation probabilities of the two sites (p, - pz = aP; (0)). The error bars represent a liberal estimate of the error which results from deriving these parameters from the spectra presented in Fig. 12. The solid line which originates on the right-hand side follows j3 = 36.87 - 49.11. R".5', while that which originates on the left follows AP = 1.35. R'.", where R represents the reduced temperature (( 273-T)/273).
slow or intermediate motion spectrum since the reorientation will be frozen out by the variation ofp, , and the rates will be determined by the condition of equilibrium. It is rather surprising that a discontinuity is not observed in the 2H T, data near T,; it may exist, but be too small for our crude measurements to reveal. The unambiguous characterization of the reorientation axis (z” in Fig. 6) in relation to the molecular geometry presented in Fig. 1 is not straightforwardly possible from our 2H data. This is because we observe only powder spectra and therefore cannot discriminate between orientations which yield equivalent principal axis values. Consider the following example, which follows very closely our observations at C4 and C5 in the chloride and iodide salts: assume that our observed 7 = 1.0, our static 3 = 0.0, and our observed e’qQ/h is half its static value for both deuterons on a CD2 moiety. Here, /3’s of 3X3”, 54.7”, 125.3”, and 144.7” all lead to equivalent powder spectra. If we assume that the principal axis of the 2H efg lies along the C-D bond, and that the geometry of the CD2 group is tetrahedral, then the simplest conclusion would be that z” bisects the two C-D bond vectors (yielding B = 54.7“ for both), or it lies in the plane of the bond vectors but is perpendicular to the CD2 bisector (yielding p, = 35.3” and a2 = 144.7”). Neither of these possibilities is physically reasonable because they are each nearly perpendicular to the long axis of the choline ion as presented in Fig. 1. It is more reasonable to assume that z” must be nearly perpendicular to the plane of the CD2 group, and thus be more nearly parallel to the long axis of the choline ion. This results in 0 = 90” for both deuterons and is not in agreement with our spectra. However, a slight tilt of z” relative to the CD2 plane yields PI = 35.3” and & = 125.3”, which is in general agreement with our chloride and
MOTION
IN CHOLINE
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367
iodide spectra. This assignment is not unique, and there are other orientations which give similar values of fi and maintain consistency with the axes plotted in Fig. 1. In the bromide salt our *H NMR data indicate that the orientation of z” relative to the choline cation differs very much from that present in the other two salts, but the lack of X-ray crystallographic data allows us to say no more than this. ‘4 N data. Deviations from rigid lattice lineshapes are observed in the chloride and iodide salts, but not for the bromide salt (16). For the chloride salt we observe a collapse of the central portion of the line (between -t I’,,,,) with increasing 7 (Fig. 16 ) , while for the iodide salt we observe a collapse of the entire lineshape, save the turning points (i.e., spikes form at V,, VW, and I’,,; see Fig. 15 ) , Because the 14N efg in these tetraalkylammonium salts is likely to be axially asymmetric ( 16)) two angles (Yand /3 (see Fig. 6) will generally need to be specified in order to describe the relationship between the molecular flipping axis and the static efg. For the iodide salt, the following parameters fit many of the observed spectral features: LY= 90”, p = 25”, 7 = 0.5, and e*qQ/h = 67 kI-Iz. The sharp features of the experimental spectra are reproduced in the simulations, but the apparent filling in of the lineshape between +-V, is not (see Fig. 15 ). This may be because 17is not equal
A
(bl
1
T=-40°C
'
FIG. 15. (a) The effect of the pulse spacing 7 on the appearance of choline iodide quad-pole echo 14N NMR spectra at 233 K. (b) The calculated effect of 7 using the parameters a = 90”, @= 25”, e*qQfh = 67 kHz, VJ= 0.50, and setting ~~2 = ~2, = 3 X lo3 kHz. Note that this is approximately the reorientation rate determined from the *H NMR data at this temperature.
368
PRATUM
AND KLEIN
T =48”C
T (psec)
III
1
III
(
III
1
III
c= 3r106Hz
FIG. 16. (a) The effect of the pulse spacing 7 on the appearance of choline chloride quadrupole echo 14N NMR spectra at 321 K. (b) Calculated effect of 7 using the parameters e*qQ/h = 57.3 kHz, 9 = 0.20, (Y = 50”, /3 = 25”, A@ = I”, and setting K,~ = K*, = 3 X lo3 kHz. Note that this is about the rate we would expect at 32 1 K based upon an extrapolation of the Arrhenius equation fit to our ‘H NMR data for this salt.
to 0.5, and CYis different from 90”, or it could represent the freezing out of an additional motion at low temperature. The agreement between the rates derived from i4N and ‘H data shows that our assignment of i4N parameters must be approximately correct. The choline chloride 14N NMR results are difficult to analyze because of the very washed out appearance of the spectra and a very long Ti . In order to simulate our results we assumed ,8 equal to that of the iodide salt, but found that LYhad to be closer to 0“ in order to reproduce the experimentally observed collapse between k V,. The value of 50” for CYwas chosen to account for the apparent change in 9 between the low- and high-temperature spectra. This may not be necessary, as the asymmetry parameter could have a temperature dependence owing to some other process (16, 19). Here we merely indicate that, while fi may be the same for both the chloride and the iodide salts, czis clearly different. The poorly defined nature of the edges of these spectra is consistent with the conclusion from the deuterium results that a range of ,fYsexists, or the molecular flipping axis is diffusing over a small range of angles. We see, in Fig. 16, that our model parameters give reasonable agreement between calcu-
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lated and experimental spectra as a function of 7 at fixed temperature using a nipping rate extrapolated from ‘H data. The correspondence between the 14N and 2H results can be considered good, but this may only be because the 14N S/N is so poor. In surveying these data, one realizes that the 14N efg is sensitive to 180” reorientations of the choline cation, yet insensitive to threefold trimethylammonium group rotation. The lack of observed effect of threefold ammonium group rotation on the 14N efg in glycine has been rationalized as being due to relaxation of the local electronic environment to that determined by the lattice in a time short compared to the correlation time for the molecular motion (35). It seems reasonable to invoke the same explanation for the absence of threefold rotational effects in the 14N NMR of the choline salts. The reason that 180” reorientations do have such a large effect is undoubtedly because the entire choline ion flips, not just the substituents on the nitrogen. In this situation it is unreasonable to assume that the electronic structure would relax to its previous value between jumps, and so the orientation of the efg changes with each jump. The relationship between the 14N efg and the axis of reorientation differs slightly between the chloride and the iodide salts, while for the bromide salt it remains unknown. Since the orientation of the 14N efg tensor is not known for any tetraalkylammonium salt, we can make no statements as to whether our values of (Yand p make any sense in relation to Fig. 1. The absence of motional effects in the 14N spectra of the bromide salt is consistent with the 2H NMR results which show a rapid flip-flop motion at all temperatures investigated. Finally we note that the temperature dependence of the bromide efg parameters, from previous work ( 16, 19)) shows no effect of the phase transition at 270 K. CONCLUSIONS
We have confirmed the existence of 180” cation flip-flop motions in the chloride, bromide, and iodide salts of choline. This motion has previously been proposed on the basis of X-ray data in the iodide, but has not been proposed for the chloride or bromide. The activation energies are about 6 kcal for the iodide and 11 kcal for the chloride. These activation energies are similar to those observed for reorienting water molecules in crystalline hydrates (26). As the reorientation in the chloride salt is not obvious in the X-ray results, it is possible that it is associated with the 21 screw axis. The reorientation in the bromide salt seems to be rapid at all temperatures investigated and is associated with an order-disorder phase transition close to 0°C. The flipflop motion in this salt is probably connected with the C2” unit cell symmetry inferred from infrared measurements ( I1 ). The presence of the phase transition, and the rather interesting 14N efg parameters, makes this salt a particularly attractive candidate for a future X-ray structure determination. The fact that very rapid motion is occurring in the iodide salt may be the reason that it lacks radiation-sensitive behavior. Unfortunately, this conclusion cannot be extended to the bromide salt where rapid reorientation is occurring, yet radiation sensitivity is observed. It is possible that the flip-flop motion is necessary for chain propagation to occur, and the lack of radiation sensitivity in choline iodide is related to some other crystallographic parameter, such as a larger distance between choline ions. If this is the case, the low-temperature onset of the radiation-sensitive behavior
370
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indicates that other thermal activity is also important since this onset does not correlate well to the initiation of 180” reorientations, particularly in the bromide salt. ACKNOWLEDGMENTS We thank Professor G. S. Bates, Professor E. E. Bumell, Professor Myer Bloom, Dr. A. L. MacKay, Dr. D. E. Wemmer, Dr. R. M. Lemmon, Dr. S. R. Holbrook, and Dr. T. S. Young for fruitful discussions. We also thank Dr. Lemmon for his permission to reproduce Fig. 1 b from Ref. (6). This work has heen supported by the Office of Energy Research, Office of Health and Environmental Research, Health Effects Research Division ofthe U.S. Department of Energy under Contract DE-AC03-76SF00098, and the Natural Sciences and Engineering Research Council of Canada. T.P. thanks the University of Washington Chemistry Department for their continuing support during the preparation of this manuscript. REFERENCES 1. V. PETROULEAS, A. NATH, AND R. M. LEMMON, Radiut. Phys. Chem. 16,113 (1980). 2. R. M. LEMMON, P. K. GORDON, M. A. PARSONS, AND F. MAZZETTI, J. Am. Chem. Sot. 80,273O (1958). 3. M. E. SENKO AND D. H. TEMPLETON, Acta Crystallogr. 13,28 1 (1960). 4. J. HJORTAS AND H. SORUM, Actu CrystuZ/ogr. B 27, 1320 ( 197 1). 5. V. PETROULEAS AND R. M. LEMMON, J. Chern. Phys. 68,2243 (1978). 6. D. WEMMER, V. PETROULEAS, N. PANAGLOTOFQULOS, S. E. FILLIPPAKIS, AND R. M, LEMMON, J.
Phys. Chem. 87,999 ( 1983). 7. 8. 9. IO. II. 12. 13. 14. 15. 16. 17. 18.
R. A. A. K. K. V. J. C. L. T. V. J.
19. 20. 21. 22. 23. 24. 25. 26. 27.
T. D. R. M. E. R. H. G. D.
28. 29. 30. 31. 32. 33. 34.
T. D. M. A. E. M. K.
35. R.
0. LINDBLOM, Ph.D. thesis, University of California, Berkeley, 1959. THEORET AND C. SANDORFY, Spectrochim. Ada 22,1527 ( 1966). NATH, R. AGARWAL, AND R. M. LEMMON, J. Chem. Phys. 61,1542 (1974). M. HARMONANDG. F. Aver, J. Mol. Strut. 140,261(1986). M. HARMON, G. F. AVCI, AND A. C. THIEL, J. Mol. Struct. 145,83 (1986). PETROULEAS AND R. M. LEMMON, J. Chem. Phys. 69, I3 15 (I 978 ). D. GRAHAM AND R. H. HANNON, J. Gem. Phys. 64,1204 (1976). A. MCDOWELL, P. RAGHUNATHAN, AND D. S. WILLIAMS, J. Chem. Phys. 64,1204 (1977). J. BURNETT, R. M. KNOWLES, AND J. D. GRAHAM, J. Chem. Phys. 68,25 14 (1978). K. PRATUM AND M. P. KLEIN, J. Magn. Reson. 53,473 (1983). SCHOMAKER AND K. N. TRUEBLOOD, Acta Crystallogr. B 42,63 ( 1968). H. DAVIS, K. R. JEFFREY, M. BLOOM, M. I. VALIC, AND T. P. HIGGS, Chem. Phys. Lett. 42,390 (1976). K. PRATUM, Ph.D. thesis, University of California, Berkeley, 1983. A. TORCHIA AND A. SZABO, J. Magn. Resort. 49,107 (1982). J. WITTEBORT, E. T. OLEJNICZAK, AND R. G. GRIFFIN, J. Chem. Phys. 86,5411(1986). BLOOM, J. H. DAVIS, AND M. 1. VALIC, Can. .I. Phys. 58,151O (1980). STERNIN, M. BLOOM, AND A. L. MACKAY, J. Magn. Reson. 55,274 (1983). TYCKO, E. SCHNEIDER, AND A. PINES, J. Chem. Phys. 81,680 (1984). W. SPIESS AND H. SILLESCU, J. Magn. Reson. 42,38 1 (198 1). SODA AND T. CHIBA, J. Chem. Phys. 50,439 (1969). M. RICE, R. J. WITTEBORT, R. G. GRIFFIN, E. MEIRO~TCH, E. R. STIMSON, Y. C. MEINWALD, J. H. FREED, ANDH. A. SCHERAGA, J. Am. Chem. Sot. 103,7707 (1981). M. BARBARA, M. S. GREENFIELD, R. L. VOLD, AND R. R. VOLD, J. Magn. Reson. 69,311(1986). T. EDMONDSAND C. P. SUMMERS, Chem. Phys. Lett. 41,482 (1976). MEHRING, “Principles of High Resolution NMR in Solids,” Springer-Verlag, New York, 1983. ABRAGAM, “Principles of Nuclear Magnetism,” Oxford Univ. Press, London/New York, 196 1. MEIROVITCH AND J. H. FREED, Chem. Phys Lett. 64,3 ll(1979). BLOOM, L. W. REEVES, AND E. J. WELLS, .I Chem. Phys. 49,16 15 (1965). P. PAUL& A. L. MACKAY, 0. SODERMAN, M. BLOOM, A. K. TANJEA, AND R. S. HODGES, Eur. J. Biophys. 12, l(1985). A. HABERKORN, R. E. STARK, H. VAN WILLIGEN, AND R. G. GRIFF’IN, J. Am. Chem. SOC.103, 2534(1981).
OF MAGNETIC
RESONANCE
81,350-370
(1989)
A 2H and 14NNMR Study of Molecular Motion in Polycrystalline Choline Salts T.
M.P.
K.PRATUM*-~AND
KLEIN+
*Department of Chemistry, University of Washington, Seattle, Washington 98195, and $Chemical Biodynamics Division, Lawrence Berkeley Laboratory, Berkeley California 94720 Received March 14, 1988; revised May 26,1988 ‘H and 14N solid-state NMR spectra of polycrystalline choline chloride, bromide, and iodide indicate that 180” cation flipping motion occurs in all three salts. From the temperature dependence of these spectra, the activation energy for this motion is determined to be 5.8 + 1 kcal/mol in the iodide salt and 11 + 1.5 kcal/mol in the chloride salt. In the bromide salt the reorientation rate is too rapid to be determined from the NMR lineshape, but the temperature dependence of the *H quadrupole coupling parameters is indicative ofa second-order phase transition at approximately 273 K. The spectral distortions in the 14N NMR spectra of the chloride and iodide salts are adequately explained using the motional model derived from the ‘H NMR results, while the ‘&N spectra of the bromide salt show no motional effects. The axis of reorientation which is inferred from these data appears to be consistent with that indicated in a previous X-ray crystallographic study. 0 1989 Academic PISS, Inc.
Two crystalline choline salts exhibit an amazing sensitivity to ionizing radiation. Both choline chloride, [ ( CH3)3NCH&H20H] + Cl-, and choline bromide, [ ( CH3)3NCH2CH20H ] + Br-, decompose via a chain mechanism which yields acetaldehyde and trimethylamine hydrochloride as its final products ( 1). Other choline salts and a large number of analogous compounds fail to show extreme radiation sensitivity, as do solutions of the two radiation labile salts (2). These observations have inspired a considerable research effort in an attempt to elucidate the mechanism of this unusual decomposition. Previous studies have included X-ray structural (36)) chemical (2, 7)) kinetic ( I, 7)) infrared (8-Z I ), calorimetric ( 22)) ESR ( 7, 9)) and NMR (6, 13-15) experiments. Studies of the temperature dependence of the radiation sensitivity (I) indicate that a thermally activated molecular reorientation process may be important. The utility of NMR in the study of molecular dynamics makes this an attractive technique for investigation of this process. Most of the previous NMR experiments have focused on the behavior of the proton second moment ( 13)) or the spin-lattice relaxation times T, and TIP ( 14, IS). These studies, which have for the most part been confined to the chloride, bromide, and iodide salts of choline, indicate that these compounds undergo, with increasing temperature, methyl group rotation (about 140”)) trimethylammonium group rotation (about 250”)) and isotropic reorientation of the entire choline ion (which occurs t To whom correspondence should be addressed. 0022-2364/89
$3.00
Copyright 0 1989 by Academic F’ress, Inc. All tigbu of reproduction in any form reserved.
350
MOTION
IN CHOLINE
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above the high-temperature phase transition in all three salts). The high-temperature cutoff of the radiation sensitivity correlates well with the onset of isotropic reorientation (6), but the low-temperature initiation seems not to correlate with any of the afore-mentioned motions. Recently Wemmer et al. (6) studied the NMR of these salts labeled with 13C at C4 and ‘H at the hydroxyl position (see Fig. 1). Their results indicate that significant motion sets in well before the high-temperature phase transitions, but the exact nature of this motion could not be elucidated. Additionally, they
a
b
C(3) C(5) C(4)/
------
-------0
---------N
C(I) C(4)
0: C(5)’
C(2) FIG. 1. (a) Anisotropic thermal ellipsoids from the X-ray crystallographic data of Hjortas and Sorum (4). The plotted axis is that corresponding to the major eigenvalue of&ration asdetermined by parkming a rigid body motion analysis (17) on these data. (b) The axis of apparent 180” reorientation in &oIine iodide as presented in Ref. (6) [reproduced with permission from J. Phys, Chem. 87,999, copyright 1983 American Chemical Society].
352
PRATUM
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KLEIN
obtained room temperature X-ray structural data which indicate a dynamic disorder for the cation in choline iodide between two positions related by a mirror plane (see Fig. lb). In an earlier study of 14N NMR in tetraalkylammonium salts ( 16), 14N NMR powder spectra were observed for the chloride and iodide salts which very much departed from a rigid lattice appearance. At that time, it was hypothesized that the distortion in the iodide spectra was related to the 180” flip-flop process revealed by the X-ray study, but there was no known anisotropic motion to account for the distortion observed in the chloride spectra. The crystallographic symmetry of the choline chloride unit cell allows for a twofold rotation coupled with a translation (a 2r screw axis), but an actual molecular motion corresponding to this seems quite unlikely. Fortunately, Hjortas and Sorum determined the anisotropic thermal parameters for all nonhydrogen atoms in their detailed crystallographic analysis of choline chloride ( 4)) and we have performed a rigid body analysis on these data (using the computer program THMV- I( 17)) in an attempt to reveal hidden librational freedom. The result showed one major axis of rotational freedom with a rms amplitude of lo” passing close to the nitrogen and oxygen atoms (see Fig. 1a), and another smaller rotational disorder of rms amplitude 3” approximately along the C-N ( Me)s axis. This latter motion is most likely associated with threefold rotation of the trimethylammonium group, while the former may have some relation to the flip-flop of the iodide salt. Because of the problems one encounters in assigning electric field gradient (efg) parameters to 14N in very symmetric environments, where crystalline field effects may become very large, it was not possible to analyze these spectra without further information concerning the nature of the motion affecting them. Since the deuteron quadrupolar interaction is not likely to show large crystalline field effects, a 2H NMR study of selectively labeled cholines was undertaken to correlate the 14N lineshapes with motion revealed in the *H spectra. EXPERIMENTAL
The samples used for the 14N study were prepared from commercial choline halides (Sigma Chemical Co., St. Louis, Missouri) which had been recrystallized from isopropanol and dried in wcuo at 85°C for at least 12 hr prior to being sealed in glass tubes under a dry argon atmosphere. The deuterated compounds were prepared as follows: Choline-C4-d2, [ ( CH3)3NCD2CH20H]+ X -: Betaine free base (Sigma Chemical Co.) was heated to a temperature above 3 10°C in vucuo to give a crude quantity of N,N-dimethylglycine methyl ester. This crude product was then distilled three times, and only the fraction boiling near 135°C was retained. The purified ester was then exchanged in MeOD/ MeONa twice to yield N, N-dimethylglycine-C4 - d2 methyl ester. Subsequent reduction with lithium aluminum hydride and methylation with methyl iodide gave choline iodide-c, - d2, which was recrystallized from isopropanol and ion exchanged on Amberlite IRA-402 to yield the chloride and bromide salts. The level of deuteration, as determined by mass spectroscopy, was 8 1%. Choline-C5 - d2, [ ( CH3 )3 NCH2CD20H ] + X -: N, N-Dimethylglycine methyl ester, prepared as above, was reduced with lithium aluminum deuteride and methylated
MOTION
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with methyl iodide to produce choline-C5 - d2 iodide. Recrystallization from 2-propano1 was followed by ion exchange to give the chloride and bromide salts. The level of deuteration was determined by mass spectroscopy to be 89%. Choline-C,, C2, CJ-d3, [ ( CD3)3NCH2CH20H]+ X -: To a mixture of ethanol and &CO3 was added one part ethanolamine (reagent, BDH Chemicals, Vancouver, B.C., Canada) and slightly over three parts methyl iodide-d3. The entire mixture was refluxed overnight and recrystallized to yield choline iodide- d3. Ion exchange, as before, generated the chloride and bromide salts. All samples were dried in vucuo at approximately 1WC over P205 for at least 12 hr prior to being sealed in glass tubes under vacuum. To check for sample-preparationdependent effects, the C4 - d2 bromide salt was treated in four different ways: ( 1) it was recrystallized from 2-propanol, (2) it was recrystallized from ethanol-ether, (3) it was taken straight off the ion exchange column without recrystallization, and ( 4) the iodide was first exchanged for the hydroxide, then the hydroxide was titrated to neutrality with HBr. The 2H NMR spectra of all four were essentially superimposable at room temperature (the only temperature checked), and the T, ‘s were identical to within experimental error. Therefore, it is concluded that there are no large samplepreparation-dependent effects. In some cases our room temperature 2H NMR spectra contained a sharp peak at the Larmor frequency. Initially, this was thought to be due to the entry of water into the sample, but it was later found to be a result of pyrolysis of a small amount of sample adhered to the walls of the sample tube during sealing. Because this peak could obscure the onset of isotropic reorientation, we took special care to ensure its absence in those samples which were taken through their high-temperature phase transition. Spectroscopy. The 14N spectra and the 2H spectra acquired at temperatures below ambient were taken on a homebuilt spectrometer operating at frequencies of 19.507 MHz ( “N), 4 1.442 MHz ( 2H), and 270 MHz ( ‘H) at the University of California Lawrence Berkeley Laboratory. These spectra were acquired in the presence of a 50 kHz proton-decoupling field using a phase-alternated quadrupolar echo sequence ( f8), with 90” pulses of 3.3-3.7 ps for 14N and 2.5 ps for 2H. On this spectrometer, the sample temperature is measured with a thermocouple placed directly beneath the sample and is regulated to within 1°C. Previous work on calorimetricaIly known phase transitions has indicated that the sample temperature and the thermocouple reading differ by at most 2°C ( 19). Results which will be presented later in this paper indicate that an additional 3°C temperature gradient may be present across the sample. Deuterium spectra acquired above ambient temperature were taken at the University of British Columbia on a homebuilt spectrometer operating at a deuterium resonance frequency of 3 5.5 5. MHz. A phase-alternated quadrupolar echo sequence was employed with 90“ pulses of 2.5 ps. The sample temperature on this spectrometer is measured with a thermocouple placed inside a copper oven which surrounds the sampIe. The temperature gradient across the sample using this arrangement is estimated to be less than 0.5”C. The echo pulse spacing (T) used to acquire the 2H NMR spectra was always 40 /IS, while “N spectra were acquired using several T values. Because of the possibility of spectral distortion from anisotropic TI effects (20, 21) ” data were acquired using recycle delays of at least several times T,
354
PRATUM
AND KLEIN
The 14N quadrupolar echoes were left shifted to the echo maximum, and the resulting half echoes were exponentially multiplied prior to Fourier transformation. The *H data were left shifted to the echo maximum and Fourier transformed without exponential multiplication. The data in the out of phase channel were almost always retained, except as indicated. The lineshapes were analyzed by successively fitting the experimental data to a rigid lattice lineshape convolved with a broadening function and corrected for the response roll-off due to the finite duration of the excitation and echo pulses (22). In cases of apparently overlapping powder patterns, the procedure of dePaking (23) was used to help unravel the resulting spectrum. Relaxation time data which are presented here are not intended to be quantitative, but only to give an indication of general trends. Tr data on 2H were obtained using a quadrupole echo inversion-recovery sequence with compensated 180” pulses (24)) while those for 14N employed progressive saturation. The transverse relaxation time of the quadrupolar echo ( T2e) was measured by varying the pulse spacing (7) of the echo sequence in the presence of proton decoupling. Proton decoupling has a large effect on the values obtained, especially for the 2H T2e (e.g., choline-C5-d2 bromide at 25°C: undecoupled T2, = 370 ps, decoupled T2, = 1.1 ms). In all cases the data were fitted to a single exponential and no attempts at multiple exponential fits were made. RESULTS
AND DISCUSSION
Relaxation time data. We assume that a large-amplitude anisotropic motion is responsible for the temperature dependence of T, and T2e. This assumption will be further justified as we proceed. An idea of the rate of this motion may be gleaned from the 14N T2, data presented in Fig. 2 (we have observed 2H T2e data for the deuterated salts which show the same general trends). The Tze of choline iodide decreases as the temperature is lowered, but the T2, minimum is not attained. A similar
0 - choline x : choline
ctdonde iodide
+ = choline
bromide
FIG. 2. 14N Tze data plotted against reciprocal temperature for the chloride (0)) bromide ( * ) , and iodide ( X) salts of choline.
MOTION
IN CHOLINE
SALTS
355
observation is made for choline bromide, although the T,, decrease is much less pronounced. For choline chloride a clear Tze minimum is observed at approximately -20°C. These data indicate that, within the temperature range we have investigated, the motional rates appear to follow choline bromide > choline iodide @ choline chloride. The 2H Tl data (Fig. 3) further show this trend. No T, minimum is observed for choline chloride (the apparent minimum is the result of the phase transition at 3 5 1 K) , while the bromide and iodide salts show T, minima at approximately room temperature. These data tell us roughly that, at room temperature, the reorientation rate must be in the tens of megahertz range for the bromide and iodide salts, and in the hundreds of kilohertz range for the chloride salt. NMR lineshapes. For the two I = 1 nuclei we are considering, the NMR spectrum will be dominated by the first-order quadrupolar splitting, uQ
= 0.75e2qQ/h(3
cos*8 - 1 + rl sin20 cos 2+),
Ill
where the asymmetry parameter q = (V, - VYY)/ VZZand 0 and 4 relate the efg to the static magnetic field direction (HO). In our polycrystalline samples, this leads to a powder spectrum characterized by three doubly degenerate inflection points ( j vZZ1 3 I &fI ’ I KXI): V, = +(l - v)0.375e2qQ/h, I4 V,, = f(1 + v)0.375e2qQ/h,
[-iI
V,, = k0.750e2qQ/h.
VI
and In the presence of rapid motion, where the rate of the motion overwhelms the quadrupolar interaction, the powder spectrum reflects the nature of the motion. Motion of cubic or higher symmetry will completely average the quadrupolar splitting to a single line, while motions of lower symmetry yield the spectrum of an averaged, but nonzero, efg. From this averaged efg we can learn of the nature of the motion by envisioning the process which would be required to lead from the static to the observed spectrum, keeping in mind that the efg behaves under rotation as a tensor of rank two. Here we investigate the occurrence of fixed angle planar rotational jumps. if the jumps are of C, or higher symmetry, then the spectrum of an axial efg will result (irrespective of the symmetry of the static efg) whose magnitude will be given by a geometrical factor (similar to Eq. [ 11) relating the reorientation axis to the principalaxis system of the efg. If the jumps are of lower symmetry, then an axially asymmetric spectrum may result whose magnitude and asymmetry depend upon similar geometrical factors. The motion which is most commonly observed to yield spectra of an axially asymmetric nature is the two-site flip-flop (25-2 7). As the rate of the motion (K) slows to the order of the quadrupolar interaction, the NMR spectrum will collapse due to a short T2 (because the quadrupolar frequency is no longer well defined). In the situation of anisotropic motion, this spectral collapse will be anisotropic, producing characteristic spectra from which we can learn more about both the type of motion and the rate at which it is occurring. As discussed by Spiess and Sillescu ( 25 ) , the quadrupolar echo lineshape of a powder in this coalescence region will change as the pulse spacing T is varied due to anisotropic T2 effects.
'd: (b) 0
6
x 0 0 00
0
x
0
x
‘< 2 ‘$1
x
0
0
x
x
x
3
4
(c)
c
IO’-: t=%
G
0
-
L
0 0
lO-0 0
0 Oo
I
I 2
I
fl
0
1
3
I
I
I
'009qo!,0
)
1
1
4
FIG. 3. *H Tr data for selectively deuterated salts of choline. In each case the arrow indicates the hightemperature phase transition point of the nondeuterated material ( 12). For the data taken above 293 K (1000/T less than 3.4) the Larmor frequency was 35.55 MHz, while below this temperature (1000/T greater than 3.4) the Larmor frequency was 41.442 MHz. (a) Choline-C.& chloride TI data; the transition temperature is reported to be 35 1 K. (b) Choline bromide T, data for the Cd-d, (0) and C,-dr (X) salts. The transition temperature is reported to be 363 K. (c) Choline-C.+-dr iodide T, data; the transition temperature is reported to be 367 K. 356
MOTION
IN CHOLINE
SALTS
357
In order to take advantage of these facts, lineshapes must be calculated for various motional models at various rates, and compared with experimental data. The calculations of the theoretical spectra presented here were performed on an Amdahl470-V8 computer at the University of British Columbia using an exchange matrix formulation (25 ) . The effects of finite pulse length were taken into account by considering evolution under a static first-order quadrupolar interaction (22). This has been shown to lead to small errors in iineshape calculations in the intermediate motion limit due to stochastic evolution during the RF pulses (28)) and we must consider our neglect of this effect to be a possible source of error. The nature of the motion. The deuterium NMR spectra of all three C,-d2 salts at room temperature are presented in Fig. 4a. Assuming the static *H efg in these compounds to be approximately axial, we observe in the chloride and iodide salts an apparent effect of 180” flip-flop motion (25, 26) on the intermediate (K - e*qQ/ h) and fast (K 9 e*qQ/ h) time scales. While the choline bromide spectrum appears to be rigid, the 2H T, is much shorter, and the observed e*qQ/ h of 140 kHz is somewhat lower than the 170 kHz we would expect for a rigid C-D bond (29). As the temperature is raised (Fig. 5), all three salts show the growth of an isotropic (chloride. bromide) or nearly isotropic (iodide, in which a small residual coupling remains) peak in the spectrum. Above the respective phase transitions, the *H spectra of these salts are dominated by this feature. These results are completely in agreement with those of Wemmer et al. (6). In Fig. 4b the room temperature spectra of the three C,-d2 salts are presented. The primary point of interest here is that the bromide salt shows a fairly large asymmetry parameter (9 = 0.84), while the chloride and iodide spectra are very similar to those of the G-d2 salts. Assuming that there are no large differences in the cation conftguration between the isotopically labeled compounds, this observation can only be taken to mean that the orientation of the C-D bond vectors on carbons 4 and 5 with respect to the axis of motion must be approximately the same (there are different but
FIG.4. Quadrupole echo 2H NMR spectra of the three C4-dz (a) and C,-d, (b) choline salts taken at 293 K. The quadrupole coupling parameters derived from these spectra are (a) Br - &Q/h = 140 kHz, VJ= 0.05, I - e2qQ/h = 76 kHz, g = 0.93; and (b) Br - e’qQ/h = 73 kHz, v = 0.84,1e’gQ/h = 76 kHz, 7 = 0.96. In each case, the chloride spectrum is distorted by slow motions, and no quadrapole coupling parameters may be assigned to it.
358
PRATUM
do
:
Frequency
AND KLEIN
-lb0
IkHzl
364O I
II
III IO0
1 0 c _^-.._-^..
III ,L”-I
1 -100
II
FIG. 5. Choline-C4-dz chloride (a), bromide (b), and iodide (c) 2H quadrupole echo NMR spectra at temperatures 293 K and above.
indistinguishable orientations, as will be described later) in the chloride and iodide salts, while the bromide salt shows a significant difference. The room temperature spectra of the three choline- d9 salts are observed to be much narrower, and completely consistent with static efg’s which have been twice averaged by threefold jump motion, once by rapid methyl group rotation, and then by trimethylammonium group rotation. The spectra are all of intermediate motion character, and the rates which we may crudely estimate from these spectra are consistent with what may be assessed from the proton second moment study (i.e., approximately 350 kHz for Br, 100 kHz for I, and 70 kHz for Cl). As the temperature dependence of these spectra was found to be dominated by rather uninteresting threefold rotational motions, no further data are presented on these compounds. Interpretation in terms of 180” reorientations. Assuming that a simple 180” tlip-flop motion is dominant, we may interpret our observed average of efg’s in terms of a static efg, and the Euler angles (Yand ,6 which describe the orientation of the molecular flipping axis with respect to the principal-axis system of the efg (see Fig. 6). An equation which expresses a second-rank tensor interaction in terms of fixed angle planar rotational jumps about the axis z” in Figure 6 has previously been presented by Mehring (30). We may use this expression to determine the average quadrupolar fre-
MOTION
IN CHOLINE
359
SALTS
FIG. 6. The definition of the angles a and j3 which relate the axis of planar jumps (z”) to the principle axis of the electric field gradient (efg) tensor.
quency observed in the rapid motion limit for arbitrary orientations of the molecular llipping axis with respect to H,, (described by the angles 8 and 4), ~Q=0.75e2q~/h[~2(cQse)~2(COs~)-0.75((~~-~2)sin28sin28cos~ - sin*P sin28 cos 24) + 0.5q(P2(cos @sin28 cos 2a + (pl - p2)sin B sin 26( cos p cos 2cu cos r#~- sin 2a sin $) +sin28(0.5(1
+cos2p)cos2ff
cos2$ -cospsin2asin2&))],
[5]
where P2(x) = (3x2 - 1)/2, and pI and p2 are the normalized a priori occupation probabilities of the two sites which are in exchange. When pI and p2 are equal, this expression reduces to vQ = o.75e2qQ/h[P2(cos + 0.75
Sill’fl
Sin28
,d)&(cos 6) COS
24 + 0.5~{
pz(COS
8)SiI12/?
COS
2a
+ sin28(0.5( 1 + cos2/3)cos 2c~cos 24 - cos p sin ICYsin 24) }I.
[6]
For this situation, one of the principal values of the averaged efg will appear when the axis of reorientation is aligned with Ho (i.e., when fl= 0” and 4 = 0”) and is given by V,, = 0.75e2qQ/h{P2(cos@)+0.5~
sin2/3cos2a}.
VI The other two principal values will occur when 8 = 90” and 4 = $‘, I$‘, where 14 - 4” 1 = 90”. These other values may be obtained by finding the maximum and minimum values of Eq. [ 61 subject to 6 = 90“; one of these will occur at 4’ = 0.5 atan{ -(2 q cos /3 sin 2a)/( 3 sin26 + v( 1 + cos*~)cos 2cy)1,
VI
and the other will be related to this by 90”. Note that if 17is small, or cyis near 0” or 90”,$’ = 0” and 4” = 90”. In this case v,, = -0.75eZqQ/h(l
+ ?j cos 2a)/2,
]91
and V,, = -VI,-
V,, = -0.75e2qQ/h{P2(cosp)-
1 -0.5~cos2acos2~}.
[lo]
360
PRATUM
AND
KLEIN
Equations [ 71, [ 91, and [lo], for the situation in which cy= O”, have previously been derived by Soda and Chiba (26). When q is large, and LYis not close to 0” or 90”, one has no choice but to substitute Eq. [ 81 into Eq. [ 61. If 7 is small enough to be neglected, which should be an adequate approximation for aliphatic deuterons, then (Y has no meaning, and the observed e’qQ/ h and 17take on a particularly simple form: 0"
v,, = v,
(e’qQlh)obs = PZ(COSP)(e2qQ/h)static t&s
=
(1
-
P,(cos
P))I~2(cos
[Ill 1121
P),
35.3” < ,LI< 45”; I’,, = V,,, Vi, = V,,, V,, = V, and 45” < ,f3< 54.7”; V2, = V,, V33 = v,, v,, = v,
(e2qQlhb = (e2qQlh)sd2 %bs=
54.7"
1131
16cos*P-31,
[I41
v,, = v,
(e2&?lh>obs = (10 - p2(cosB))(e2sQlh),tic %bs
=
(l/2
+
p2(cos
8))/t1
/2
-
p2(c0s
P)).
[I61
Equations [ 1 I]- [ 161 imply that the observed e*qQ/h and 71values for 45” < /? < 90” are a reflection of those for 0” < B < 45” (e.g., p = lo” values are equivalent to those with 0 = 80”); therefore only a 45” range of @ will result in distinguishable powder spectra, and there will be several values of /I which could correspond to our observed 2H NMR spectra. Cholineiodide.Both C, and C5 compounds show approximately q= 1 .Olineshapes at room temperature (Cd rl = 0.93, Cs 7 = 0.95), so ,f3must be near 35.3” or 54.7”. The exact values would be Cd: 34.5” or 55.4, Cg: 34.8” or 55.2” for a static v = 0.0; or Cs: 33.8” or 56.2”, Cs: 34.0” or 56.0“ for a static q= 0.05 and LY= 0”. The only way to choose between the degenerate values of P would be through geometrical arguments. To determine the activation energy, we have obtained spectra at several lower temperatures (see Fig. 7) and compared these with calculated spectra (see Fig. 8; note that the exact choice of p is not an important factor in determining rates). Comparison of Figs. 7 and 8 yields the following rates (Key; ~~~ = K~,): 253 K w 5 X lo6 Hz, 233 K x 2.5 X lo6 Hz, 213 K a 650 X lo3 Hz, 193 K = 150 X lo3 Hz. The subscripts of K refer to the site labels; thus ~~2 defines the rate of flipping from site 1 to site 2. To these rates we have fitted an Arrhenius equation with an activation energy of5.8 + 1 kcal/mol. Cholinechloride.Here we do not obtain a rapid motion limit lineshape due to the onset of isotropic reorientation below the phase transition (see Fig. 5a). Nonetheless, the fact that we see no spectral intensity beyond l/2 V,, and we observe significant intensity in the center of these motionally distorted spectra, indicate that reorientation must occur through at least an angle of 70.5” (this comment applies if @is 90”; if /3 is other than 90”, the angle must be even larger). Therefore, the amplitude determined from the anisotropic thermal analysis is far too small to explain our spectra,
MOTION
-
100
IN CHOLINE
0 Frequency
SALTS
361
-100 IkHzI
FIG. 7. Quadrupole echo 2H NMR spectra of choline-C5-d2 iodide at temperatures (A) 293 K, (B) 253 K, (C) 233 K, (D) 2 13 K, (E) 193 K. The symmetric structure which permeates these lineshapes is due to a sample which was not finely divided enough to obtain a completely isotropic distribution of orientations. Very similar spectra are obtained for the Cd-d2 salt.
and we will henceforth assume that 180” jumps are occurring. Taking this assumption into account, the presence of the peak in the center indicates that p must be nearly 35.3” or 54.7”. This type of motion is expected to produce a very sharp feature in the center of the spectrum (see, for example, Ref. (25)). From a two-site exchange
FIG. 8. Simulated quadrupole echo spectra for a spin- I nucleus whose electric field gradient undergoing 180” tlips using the following parameters: a = o”, j3 = 56.2”, e*qQ/h = 152 kHz, 7 = 0.05, r = 40 es, and K,~ = K~, = (A) 10 X 103kHz, (B) 7 X 103kHz, (C) 5 X lO’kHz, (D) 2.5 X lo3 kHz, (E) 1 x 103kHz, (F) 650 kHz, (G) 150 kHz. All of these spectra have had 500 Hz of isotropic Lorentzian line broadening applied and have been multiplied by a function to account for the effect of finite duration pulses (22). The small amplitude wiggles are the result of truncation of the calculated half echoes.
362
PRATUM
AND KLEIN
model, the broad feature in the center of our spectrum could have two origins: (a) there is a distribution of jump angles 6, centered about some particular value, or (b) the angle /3 defining the molecular reorientation axis is distributed, or possibly diffuses over a range of values. These two possibilities were tested by adding an anisotropic exponential decay ( Tza) to each component of the powder lineshape, where (a) T;i cc (dvQ/dS)AS,
[I71
(b) T;i’ cc (+/4-3A/A
[I81
with A6 and A/? representing the assumed breadth of the distribution in 6 and & respectively. Although this treatment is quantitatively incorrect for either a static distribution or the effect of small-amplitude Brownian motion (21, .?I), it will give us a qualitatively correct feeling for the possible effects of (a) and (b). We have found that (a) will not reproduce the observed anisotropic line broadening unless 6 is substantially different from 180”. This is easily understood when one realizes that the averaged efg principle values are fairly insensitive to jump angle when this is nearly 180”. On the other hand, (b) will reproduce the observed anisotropic broadening in a qualitative sense (see Fig. 10). Unfortunately, we cannot mimic the exact shape of the central feature in Fig. 9 using this method. It is possible that the broad central feature would be better simulated using a very anisotropic rotational diffusional model (32)) but the sharpness of the edges of these spectra indicate that the character of the motion is strongly jump-like. Therefore, we have only a very rough idea as to the reorientation rates (K,~; K12 = Key): 293 K = 800 X lo3 Hz, 273 K N 150 X lo3 Hz, 253 K = 20 X lo3 Hz, 233 K x 5 X lo3 Hz. The value we estimate at 233 K is primarily based upon our measured Tze ( = 200 PSfor the C4 and Cs deuterated salts), which should approximately equal K ;i in the very slow motional region (33, 34). The fit to an Arrhenius equation yields an activation energy of 11 + 1.5 kcal/ mol.
0 Frequency
-100 IkHzI
RG. 9. Quadrupole echo ‘H NMR spectra of choline-C4-dz chloride at temperatures (A) 293 K, (B) 273 K, (C) 253 K, (D) 233 K. The spectrum at 233 K appears to have two barely resolved quadrupolar splittings, and will approximately fit to a sum of e’qQ/h = 160.0 kHz, q = 0.07, and e*qQ/h = 155.5 kHz, 7 = 0.0. Very similar spectra are obtained for the C,-d2 salt.
MOTION
i,
’
I
I
I
IN CHOLINE
II
160
I 6
Frequency
363
SALTS
I
I-
R I
,--ri -160
IkHzI
FIG. 10. Simulated quadrupole echo spectra for 180” jump motion using the following parameters: (Y = 90”, fl = 53.6”, A@ = 1” (A, B), Afl= o’(C, D, E), e’qQ/h = 156 kHz, n = 0.05, r = 40 us, and K,~ = K?, =(A)800kHz,(B)150WIz,(C)20kHz,(D)lOkHz,(E)5kHz.Thesespectrahavebeenmultipiiedby a function to account for finite duration pulses (22). The isotropic Lorentzian linewidth was assumed to be 500 Hz. The inset shows a simulation using the parameters of (A), but with A/3 = 0”.
Cholinebromide.For both of the isotopically labeled compounds a T, minimum is observed at nearly the same temperature as the iodide salt, but a slow or intermediate motion spectrum is not observed above 193 K (see Figs. 11 and 12 ) . Interestingly, the apparent asymmetry parameter of the C5 compound drops rapidly below 273 K, approaching zero in a more or less exponential manner as the temperature is lowered
FIG. 11. Quadrupole echo ‘H NMR spectra ofcholine-G-d2 bromide. The temperatures and associated quadrnpole coupling parameters are (A) 273 K, e’qQ/h = 141 kHz, 11z=0.03; (B) 253 K, e*qQ/h = 147 kHz, n = 0.0; (C) 233 K, e*qQ/h = 152 kHz, n = 0.036, and e*qQ/h = 152 kHz, 1) = 0.0; (D) 213 K, e*qQ/ h = 156 kHz, n = 0.058, and e2qQ/ h = 158 kHz, n = 0.028. The out of phase channel was zeroed prior to Fourier transformation of(B) , (C) , and (D) .
364
PRATUM
I ,
AND KLEIN
R I
I 100
I
I Frequency
I IkHzl
I
I
I
I
-100
FIG. 12. Quadrupole echo ‘H NMR spectra of choline-Cs-dz bromide. The temperatures and best fit coupling parameters (for which there are considerable errors, as explained in the text) are (A) 293 K, e’qQ/h = 73 kHz, 7 = 0.84; (B) 273 K, e*qQ/h = 75 kHz, 7 = 0.86; (C) 263 K, e*qQ/h = 103 kHz, 9 = 0.47; (D) 253 K, e*qQ/h = 112 kHz, 7 = 0.35; (E) 233 K, e*qQ/h = 133 kHz, q = 0.16; (F) 213 K, e*qQ/h = 146 kHz, v = 0.09; (G) 193 K, e*qQ/h = 154 kHz, T = 0.06.
(Fig. 13), while the C, salt shows nearly axial spectra of slightly reduced quadrupole coupling throughout this temperature range (Fig. 1 I). Assuming that twofold jumps are occurring in the C, salt, then the reorientation axis must lie nearly along one of the twofold axes of the efg (i.e., p is near 0” or 90”). Examination of the C5 spectra taken below 273 K shows that the turning points corresponding to V, and V,, take on a distinctly washed out appearance, while V, remains fairly sharp. A plot of V,, V,,, and V,, as a function of temperature (Fig. 13) indicates that V,, and V,, switch places just below 273 K, and VW then shows a small approximately linear temperature dependence (70 Hz/K), while V, and V,, increase exponentially as the temperature is lowered. Because the rate of change of V, and V,, with temperature is so large, even a small temperature gradient, 3°C in our case, will broaden the spectra to a significant extent. The interpretation of these results follows from the observation by Lemmon et al. of a very small peak at about 270 K in their thermogram of choline bromide ( 12). This is approximately the same temperature at which our spectra begin to change, so we may conclude that a low enthalpy phase transition is responsible for our observations. The fact that V, and V,, behave approximately as would an order parameter shows this phase transition to have considerable second-order character. There are two possible phase-transition mechanisms which are consistent with our observa-
MOTION
IN CHOLINE
SALTS
365
Temp (OK)
FIG. 13.FQinciplevaluesofthe2Hefg(~V,j~)derivedfromFig. 12; V,(O), P’,(x),andV,(*).The solid line through V,, below 273 K follows Vz, = 56.4 1 + I 15.4. R”.53, while that through V, follows V,, = 4.34 + 102.4. R”.56, where R represents the reduced temperature ((273 - T)/273).
tions: (a) the angle 6 suddenly begins to change below the phase transition temperature ( T,), or (b) the occupation probabilities of the two sites involved in the flip-flop (P, and p2) suddenly begin to change below T, . For a discussion of (a), assume that 0 is initially 36.87” (although it could just as easily be 53.13”, as both of these values lead to the observed 17of 0.84 when the static Q = 0.0). We then calculate /3 at each temperature from our observed T)using Eqs. [ 121 and [ 141; the result is plotted in Fig. 14. We find that A/3 = 36.87 - fi fits quite well to an exponential equation in the reduced temperature (R = ( T, - T)/ T,) with a critical exponent of 0.5 1 -+ 0.05. Because the spectra of the C4 deuterons change very slightly through the phase transition, we must conclude that this mechanism would only involve a change in j3 at Cs , a highly improbable event. As the probability of occupation of the two sites varies, the time-averaged efg tensor tilts toward the site of highest probability, so this becomes equivalent to changing the angle fi. Assuming /I?is given, the new efg may be calculated numerically for given values of p1 using Eq. [ 5 ] and related to our observed r]. The value of p1 is equated to a value of residual disorder M = pI - p2 and plotted against temperature in Fig. 14. These data fit an exponential equation in R with a critical exponent of 0.29 St 0.05. This type of phase transition, where the cations are disordered between the two sites above T, and become ordered below, is completely consistent with what is observed in the C4 salt. The spectra at the C4 position show only a regular increase in Vz,, and a growing inequivalence of the two deuterons as the temperature is lowered, as evidenced by the splitting of the perpendicular edge. As the likely mechanism here is order-disorder, we will never expect to see a truly
366
PRATUM
AND KLEIN I
60
0
II
III 200
1
III
210
( 280
III
1
I
320
Temp PK) FIG. 14. Interpretation of the temperature dependence of the choline bromide-C5-d2 spectra in terms of a change in the angle @(X ), and a change in the relative occupation probabilities of the two sites (p, - pz = aP; (0)). The error bars represent a liberal estimate of the error which results from deriving these parameters from the spectra presented in Fig. 12. The solid line which originates on the right-hand side follows j3 = 36.87 - 49.11. R".5', while that which originates on the left follows AP = 1.35. R'.", where R represents the reduced temperature (( 273-T)/273).
slow or intermediate motion spectrum since the reorientation will be frozen out by the variation ofp, , and the rates will be determined by the condition of equilibrium. It is rather surprising that a discontinuity is not observed in the 2H T, data near T,; it may exist, but be too small for our crude measurements to reveal. The unambiguous characterization of the reorientation axis (z” in Fig. 6) in relation to the molecular geometry presented in Fig. 1 is not straightforwardly possible from our 2H data. This is because we observe only powder spectra and therefore cannot discriminate between orientations which yield equivalent principal axis values. Consider the following example, which follows very closely our observations at C4 and C5 in the chloride and iodide salts: assume that our observed 7 = 1.0, our static 3 = 0.0, and our observed e’qQ/h is half its static value for both deuterons on a CD2 moiety. Here, /3’s of 3X3”, 54.7”, 125.3”, and 144.7” all lead to equivalent powder spectra. If we assume that the principal axis of the 2H efg lies along the C-D bond, and that the geometry of the CD2 group is tetrahedral, then the simplest conclusion would be that z” bisects the two C-D bond vectors (yielding B = 54.7“ for both), or it lies in the plane of the bond vectors but is perpendicular to the CD2 bisector (yielding p, = 35.3” and a2 = 144.7”). Neither of these possibilities is physically reasonable because they are each nearly perpendicular to the long axis of the choline ion as presented in Fig. 1. It is more reasonable to assume that z” must be nearly perpendicular to the plane of the CD2 group, and thus be more nearly parallel to the long axis of the choline ion. This results in 0 = 90” for both deuterons and is not in agreement with our spectra. However, a slight tilt of z” relative to the CD2 plane yields PI = 35.3” and & = 125.3”, which is in general agreement with our chloride and
MOTION
IN CHOLINE
SALTS
367
iodide spectra. This assignment is not unique, and there are other orientations which give similar values of fi and maintain consistency with the axes plotted in Fig. 1. In the bromide salt our *H NMR data indicate that the orientation of z” relative to the choline cation differs very much from that present in the other two salts, but the lack of X-ray crystallographic data allows us to say no more than this. ‘4 N data. Deviations from rigid lattice lineshapes are observed in the chloride and iodide salts, but not for the bromide salt (16). For the chloride salt we observe a collapse of the central portion of the line (between -t I’,,,,) with increasing 7 (Fig. 16 ) , while for the iodide salt we observe a collapse of the entire lineshape, save the turning points (i.e., spikes form at V,, VW, and I’,,; see Fig. 15 ) , Because the 14N efg in these tetraalkylammonium salts is likely to be axially asymmetric ( 16)) two angles (Yand /3 (see Fig. 6) will generally need to be specified in order to describe the relationship between the molecular flipping axis and the static efg. For the iodide salt, the following parameters fit many of the observed spectral features: LY= 90”, p = 25”, 7 = 0.5, and e*qQ/h = 67 kI-Iz. The sharp features of the experimental spectra are reproduced in the simulations, but the apparent filling in of the lineshape between +-V, is not (see Fig. 15 ). This may be because 17is not equal
A
(bl
1
T=-40°C
'
FIG. 15. (a) The effect of the pulse spacing 7 on the appearance of choline iodide quad-pole echo 14N NMR spectra at 233 K. (b) The calculated effect of 7 using the parameters a = 90”, @= 25”, e*qQfh = 67 kHz, VJ= 0.50, and setting ~~2 = ~2, = 3 X lo3 kHz. Note that this is approximately the reorientation rate determined from the *H NMR data at this temperature.
368
PRATUM
AND KLEIN
T =48”C
T (psec)
III
1
III
(
III
1
III
c= 3r106Hz
FIG. 16. (a) The effect of the pulse spacing 7 on the appearance of choline chloride quadrupole echo 14N NMR spectra at 321 K. (b) Calculated effect of 7 using the parameters e*qQ/h = 57.3 kHz, 9 = 0.20, (Y = 50”, /3 = 25”, A@ = I”, and setting K,~ = K*, = 3 X lo3 kHz. Note that this is about the rate we would expect at 32 1 K based upon an extrapolation of the Arrhenius equation fit to our ‘H NMR data for this salt.
to 0.5, and CYis different from 90”, or it could represent the freezing out of an additional motion at low temperature. The agreement between the rates derived from i4N and ‘H data shows that our assignment of i4N parameters must be approximately correct. The choline chloride 14N NMR results are difficult to analyze because of the very washed out appearance of the spectra and a very long Ti . In order to simulate our results we assumed ,8 equal to that of the iodide salt, but found that LYhad to be closer to 0“ in order to reproduce the experimentally observed collapse between k V,. The value of 50” for CYwas chosen to account for the apparent change in 9 between the low- and high-temperature spectra. This may not be necessary, as the asymmetry parameter could have a temperature dependence owing to some other process (16, 19). Here we merely indicate that, while fi may be the same for both the chloride and the iodide salts, czis clearly different. The poorly defined nature of the edges of these spectra is consistent with the conclusion from the deuterium results that a range of ,fYsexists, or the molecular flipping axis is diffusing over a small range of angles. We see, in Fig. 16, that our model parameters give reasonable agreement between calcu-
MOTION
IN
CHOLINE
SALTS
369
lated and experimental spectra as a function of 7 at fixed temperature using a nipping rate extrapolated from ‘H data. The correspondence between the 14N and 2H results can be considered good, but this may only be because the 14N S/N is so poor. In surveying these data, one realizes that the 14N efg is sensitive to 180” reorientations of the choline cation, yet insensitive to threefold trimethylammonium group rotation. The lack of observed effect of threefold ammonium group rotation on the 14N efg in glycine has been rationalized as being due to relaxation of the local electronic environment to that determined by the lattice in a time short compared to the correlation time for the molecular motion (35). It seems reasonable to invoke the same explanation for the absence of threefold rotational effects in the 14N NMR of the choline salts. The reason that 180” reorientations do have such a large effect is undoubtedly because the entire choline ion flips, not just the substituents on the nitrogen. In this situation it is unreasonable to assume that the electronic structure would relax to its previous value between jumps, and so the orientation of the efg changes with each jump. The relationship between the 14N efg and the axis of reorientation differs slightly between the chloride and the iodide salts, while for the bromide salt it remains unknown. Since the orientation of the 14N efg tensor is not known for any tetraalkylammonium salt, we can make no statements as to whether our values of (Yand p make any sense in relation to Fig. 1. The absence of motional effects in the 14N spectra of the bromide salt is consistent with the 2H NMR results which show a rapid flip-flop motion at all temperatures investigated. Finally we note that the temperature dependence of the bromide efg parameters, from previous work ( 16, 19)) shows no effect of the phase transition at 270 K. CONCLUSIONS
We have confirmed the existence of 180” cation flip-flop motions in the chloride, bromide, and iodide salts of choline. This motion has previously been proposed on the basis of X-ray data in the iodide, but has not been proposed for the chloride or bromide. The activation energies are about 6 kcal for the iodide and 11 kcal for the chloride. These activation energies are similar to those observed for reorienting water molecules in crystalline hydrates (26). As the reorientation in the chloride salt is not obvious in the X-ray results, it is possible that it is associated with the 21 screw axis. The reorientation in the bromide salt seems to be rapid at all temperatures investigated and is associated with an order-disorder phase transition close to 0°C. The flipflop motion in this salt is probably connected with the C2” unit cell symmetry inferred from infrared measurements ( I1 ). The presence of the phase transition, and the rather interesting 14N efg parameters, makes this salt a particularly attractive candidate for a future X-ray structure determination. The fact that very rapid motion is occurring in the iodide salt may be the reason that it lacks radiation-sensitive behavior. Unfortunately, this conclusion cannot be extended to the bromide salt where rapid reorientation is occurring, yet radiation sensitivity is observed. It is possible that the flip-flop motion is necessary for chain propagation to occur, and the lack of radiation sensitivity in choline iodide is related to some other crystallographic parameter, such as a larger distance between choline ions. If this is the case, the low-temperature onset of the radiation-sensitive behavior
370
PRATUM
AND
KLEIN
indicates that other thermal activity is also important since this onset does not correlate well to the initiation of 180” reorientations, particularly in the bromide salt. ACKNOWLEDGMENTS We thank Professor G. S. Bates, Professor E. E. Bumell, Professor Myer Bloom, Dr. A. L. MacKay, Dr. D. E. Wemmer, Dr. R. M. Lemmon, Dr. S. R. Holbrook, and Dr. T. S. Young for fruitful discussions. We also thank Dr. Lemmon for his permission to reproduce Fig. 1 b from Ref. (6). This work has heen supported by the Office of Energy Research, Office of Health and Environmental Research, Health Effects Research Division ofthe U.S. Department of Energy under Contract DE-AC03-76SF00098, and the Natural Sciences and Engineering Research Council of Canada. T.P. thanks the University of Washington Chemistry Department for their continuing support during the preparation of this manuscript. REFERENCES 1. V. PETROULEAS, A. NATH, AND R. M. LEMMON, Radiut. Phys. Chem. 16,113 (1980). 2. R. M. LEMMON, P. K. GORDON, M. A. PARSONS, AND F. MAZZETTI, J. Am. Chem. Sot. 80,273O (1958). 3. M. E. SENKO AND D. H. TEMPLETON, Acta Crystallogr. 13,28 1 (1960). 4. J. HJORTAS AND H. SORUM, Actu CrystuZ/ogr. B 27, 1320 ( 197 1). 5. V. PETROULEAS AND R. M. LEMMON, J. Chern. Phys. 68,2243 (1978). 6. D. WEMMER, V. PETROULEAS, N. PANAGLOTOFQULOS, S. E. FILLIPPAKIS, AND R. M, LEMMON, J.
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