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199Hg spin-lattice relaxation and chemical shift anisotropy in diphenylmercury
JOURNAL
OF MAGNETIC
RESONANCE
42,420-428
(1981)
lggHg Spin-Lattice Relaxation and Chemical Shift Anisotropy in Diphenylmercury D. G. GILLIES,* Shell
L. P. BLAAUW,
Research
B.V.,
G. R. HAYS,
KoninklijkelShell-Laboratorium,
R. HUB, AND A. D. H. CLAGUE Amsterdam,
The Netherlands
Received May 27, 1980; revised August 28, 1980 lg9Hg spin-lattice relaxation times (T,) have been measured for diphenylmercury at magnetic fields of 2.35 and 7.05 T. T, was nine times shorter at the higher field (0.033 set at 310 K) than at the lower field (0.30 set), showing relaxation by chemical shift anisotropy (GA) to be the dominant mechanism even at the lower field. Variabletemperature 13C T, measurements at 6.35 T made it possible to determine a value for the correlation time for motion perpendicular to the axis of the molecule (50 psec at 300 K), which with the lg9Hg data allowed a value of 6800 k 680 ppm to be calculated for the mercury chemical shift anisotropy. The activation energy for rotational motion was 13.3 kJ.mote-l. Previous data on dimethylmercury have been reassessed and the importance of the CSA mechanism for ls9Hg at high fields is pointed out. INTRODUCTION
With the increasing interest in NMR studies of metal nuclei (1) it is important to understand the mechanisms by which these nuclei can relax. The large range of chemical shifts typically observed (several thousand parts per million) indicates that in nonsymmetric molecules the chemical shift anisotropy (CSA) may also be large. For nuclei with spin Z greater than l/2, relaxation will be dominated by the quadrupolar mechanism, but when Z is equal to l/2, relaxation by the CSA mechanism may predominate, particularly at high fields, because of the square law dependence of the relaxation time on the external magnetic field B,,. The experimentally measured spin-lattice relaxation time T1, or rate RPBS (= l/T,), includes contributions from chemical shift anisotropy (RFSA), intramolecular dipole-dipole (R ptra DD) and intermolecular dipole-dipole interactions (R I;nterDD),spin rotation (RSR), and scalar coupling (RfC). Thus &-PBS
=
RESA
+
Rpt’aDD
+
RyterDD
+
RSR
+
RfC.
[II
Magnetic shielding anisotropies and methods of measurements have been reviewed by Appleman and Dailey (2). Equation [2] gives the rate of relaxation by chemical shift anisotropy for a nucleus lying on the symmetry axis of a linear molecule under conditions of extreme narrowing (3), * Visiting scientist at KSLA, Egham, Surrey, TW 20 OEX.
March to September 1979. Present address: Royal Holloway College, 420
0022-2364/81/030420-09$02.00/O Copyright 0 1981 by Academic Press, Inc. AU rights of reproduction in any form reserved.
RELAXATION
IN
421
DIPHENYLMERCURY
R,CSA= (T,csA)-’ = ;
y%;(L\~r)~~,,
VI
where Acr = o,, - oI is the chemical shift anisotropy, r1 is a correlation time describing molecular reorientation about an axis perpendicular to the symmetry axis, and the other symbols have their usual meanings. Relaxation by the mechanism of scalar coupling (4) is the only other process that can give fielddependent relaxation (decreasing with increasing field) and is irrelevant in the present case. The measurements reported here were made at two fields, 2.35 and 7.05 T, and allowed calculation of both the CSA relaxation rate, RySA, and the relaxation rate arising from other, field-independent mechanisms, Rpther, from the following equations: ROBS l(2.35 T)
= R&T,
ROBS l(7.05 T)
= 9R5&T,
+ Rpther, + Ryther.
Pal WI
The remaining contributions to Rpther are the dipolar and spin rotation terms. NOE measurements allowed determination of the total dipolar rate, RPtra DD + R I;nterDD, leaving R sR as the only unknown field-independent term. lssHg nuclear magnetic resonance has recently been reviewed (5) and although there are quite a lot of data on chemical shifts and coupling constants, covering a range of chemical compounds, there are in contrast relatively few reports on relaxation processes (6 -8). Sens et al. (7) reported a small temperature dependence (-35 to +4O”C) for T, in neat dimethylmercury at a field of 2.35 T and explained this qualitatively in terms of nearly equal contributions from the CSA and SR mechanisms, with the former dominating at low temperature and the latter at high. Lassigne and Wells (B), working at the lower field of 1.41 T, reported the SR mechanism to be dominant even at low temperature. MEASUREMENT
OF CHEMICAL
SHIFT
ANISOTROPY
From Eqs. [2] and [3] it is clear that T, measurements at two fields allow calculation of Au as long as 71 is known. Diphenylmercury was selected for the present study since its linear geometry (9) allows TV to be determined from the proton-induced dipole-dipole relaxation rate for the para carbon. The C-H vector lies along the symmetry axis and is subjected to the same motional fluctuations as the shielding of mercury. The equation is RDD l(para)
=
3/2c&fi2~Ci~1.
r41
Wehrli (10) determined 13C shielding anisotropies for the cyano and C, carbons in benzonitrile by this method. Kennedy and McFarlane (11) have measured lssHg chemical shift anisotropies from studies in the nematic phases of liquid crystal solvent and reported values of +7475 ppm for dimethylmercury and about 5500 ppm for methylmercuric chloride, bromide, and iodide. The value of 4600 + 1000 ppm reported for dimethylmercury by Lassigne and Wells (8) should be regarded as being of the
GILLIES
422
ET AL.
TABLE ls9Hg RELAXATION Temperature
zysa
WI
(set)
273 300 313 323 338
0.136 0.244 0.306 0.358 0.452
1
DATA
AT
17.90 MHz TPDC (=I
0.22 0.10 0.07 0.04
1.72 7.18 17.0 32.3
,p”“DDd (se4
0.148 0.253 0.312 0.459
’ Errors estimated to be of the order of 5%. * Nuclear Overhauser enhancement: qmax = 2.79 for ls9Hg - {‘H}. ’ (l/T,)” = (q/q,,,)(l/T,)oBs; (lIT,)O= = (l/T#‘” + (l/T,~DO. d Ty”DD = TfSA in this case; see text.
correct order of magnitude. It was derived indirectly from the spin-rotation contribution and the error limits are almost certainly larger than those quoted. EXPERIMENTAL
A solution of diphenylmercury (Merck) in a 50/50 mixture by volume of 1,1,2,2tetrachlorodeuteroethane (Merck) and carbon tetrachloride was prepared (ca. 0.4 M, the maximum concentration which precluded crystallization at the lowest temperature of measurement) and degassed by passing dry nitrogen through it for several minutes. All the spectra were run with internal field-frequency lock to the deuterium resonance of the solvent. Proton-decoupled lggHg spectra at 17.90 MHz were run in a 12-mm tube on a Varian XL-loo-15 spectrometer using the multinuclear gyrocode observe facility. Temperatures were measured before and after each run using a thermometer immersed in an ethylbenzene sample and were estimated to be accurate to within 1°C. Proton-decoupled lggHg spectra at 53.69 MHz were run in a lo-mm tube in a 20-mm probe on a Bruker CXP-300 spectrometer at ambient temperature. Proton-decoupled 13Cspectra at 67.89 MHz were run in a lo-mm tube on a Bruker WH-270 spectrometer. Variable temperature studies were performed using the standard Bruker temperature controller and temperatures were estimated to be accurate to within 2°C. TI measurements were made by the inversion-recovery method and values calculated by least squares from semilogarithmic plots. The sample was confined to the volume of the coil by a vortex plug. NOE enhancement factors (lggHg at 17.90 MHz and 13C at 67.89 MHz) were determined by the gated decoupling technique using recycle times of at least 10 x T,. RESULTS
Mercury
Relaxation
AND DISCUSSION
Times
The lggHg T, and nuclear Overhauser enhancement (7) data are shown in Table 1, together with the derived values for the dipolar ( TyD) and nondipolar
RELAXATION
IN
423
DIPHENYLMERCURY
0.25
0 20
0 15 3.0
FIG. 1. Too
3.2
3.4
3.6 1000 K
for ls9Hg as a function of inverse temperature
at 17.90 MHz.
CTyonDD) relaxation times. A plot of In TTonDD versus inverse temperature is shown in Fig. 1. The straight line plot with an activation energy of 13.3 + 0.1 kJ * mole-l indicates that spin rotation, which would have the opposite temperature dependence, makes an insignificant contribution to the relaxation. This behavior suggests that CSA is the mechanism responsible. This is confirmed by the measurement at 57.69 MHz and 309.5 K of a T1 value of 33.3 msec, which is one-ninth the value of 0.30 set calculated by least-squares analysis from the data shown in Fig. 1. This means that conDo in Table 1 is equal to TFSAand that relaxation by the CSA mechanism is the dominant mechanism even at 2.35 T, the contribution varying from 92% at 273 K to 98.5% at 338 K. If no allowance is made for the dipolar contribution, an activation energy of 14.2 2 0.2 kJ.mole-’ is found for TPBS, implying a higher activation energy for dipolar relaxation than for CSA. A plot of In TFD versus inverse temperature is shown in Fig. 2 and gives an activation energy of 35.5 2 2.7 kJ.moleel. Relaxation by the intramolecular dipole-dipole mechanism via the ortho protons cannot be responsible since the Hg-H distance (0.3 1 nm) is too great. This points to relaxation via an intermolecular dipole-dipole mechanism.
424
GILLIES
ET AL.
3.2
3.4
0
3.0
3.6 F K
FIG. 2. TPD for ls9Hg as a function of inverse temperature at 17.90 MHz.
Carbon Relaxation
Times
The carbon spectrum was measured and assigned previously (22). The 13C T1 data are shown in Table 2. x is the ratio of the average T, for the ortho and meta carbons to the T, for the para carbon and 71 was calculated from T,(para) using Eq. [4]. This was justified since the NOE factors determined at several temperatures were found to be maximum within experimental error and also the CSA contribution (3.7% at 67.89 MHz with AU = 180 ppm) was considered to be negligible. Figure 3 shows the T, data plotted against inverse temperature. The data for ortho and meta carbons have been taken together and yield an activation energy of 15.1 + 0.3 kJ .mole-‘, which is significantly higher than that for the para carbon, which is 13.0 ? 0.5 kJ*mole-‘. The latter figure is in excellent agreement with the value of 13.3 +- 0.1 kJ*mole-’ derived from the lssHg CSA data and confirms that the lssHg relaxation by the CSA mechanism is governed by the same motional parameters as the intramolecular DD mechanism for the para carbon. Levy et al. (13) have shown how internal rotation of a phenyl group leads to an increase in the relaxation times for the ortho and meta carbons relative to the para. Our data, with T, values being some 2.0 to 2.6 times higher for the ortho and meta
RELAXATION
IN
TABLE 13C RELAXATION ph”
Temperature
peta
(set)
(se4
(se4
273 288 300 303 315 330 343
1.12 1.65 2.18 2.26 2.89 3.56 4.35
1.12 1.66 2.17 2.18 2.72 3.48 4.55
0.55 0.71 0.93 0.91 1.21 1.50 1.70
T,(ortho,meta)lT,(para). from R,(para) = 2.149
2
DATA
AT 67.89
MHz b
para
W
” x = Mean * Calculated
425
DIPHENYLMERCURY
(pkc)
XU
2.04 2.33 2.39 2.44 2.32 2.35 2.62
x 1O+1o~L using
Eq.
[4] with
D
Rd
(nsec-I)
(nsec-‘)
84.6 65.5 50.0 51.1 38.5 31.0 27.4
1.97 2.54 3.33 3.26 4.33 5.36 6.08
rCmH = 0.109
nm.
7.32 12.54 17.26 17.60 21.14 26.82 37.46
carbons than for the para carbon, show that the effective correlation times are indeed shorter for ortho and meta carbons, indicating overall motional anisotropy. The higher activation energy supports the conclusion that the relaxation is T .s .o b -
‘.5
8.0.5.o2 5-
.o-
FIG.
3. T, for
13C as a function
of inverse
temperature
at 67.89
MHz.
426
GILLIES
ET
AL.
TABLE lssHg Molecule
SHIFTS
(ppm)
AND ANISOTROPIES
AU
&Me,
3
8”
+7475 +7325
k +
+6800
* 680
-742
+ 5300
MeHgCl
+5535
k
80
-811
+4500
(II)
MeHgBr
+5455
? 100
-910
+4500
(II)
MeHgI
+5480
k 300
- 1085
+4700
(II)
WC&
12
” Positive Ref. (7).
values
indicate
80 55
Reference
shifts
0 0
to high
+5000 +4880
frequency
(low
(11) (19) This
work
cr); taken
from
dependent on different motional properties. It is not possible, however, to separate the effects of internal phenyl rotation from overall rotation of the molecule around its axis, although we feel that the former effect probably dominates for this case. Assuming isotropic overall motion and a diffusion model (24) for internal rotation, one may calculate the internal rotation coefficient Rd from x and the isotropic diffusion constant D (13, 15-l 7). D is obtained from the correlation time for the para carbon (D = 1/6~~). Values for D and Rd are included in Table 2 and yield a value of 17.0 & 0.7 kJ*mole-’ for the activation energy. Chemical
Shift Anisotropy
The chemical shift anisotropy was calculated as 6800 + 680 ppm from Eqs. [2] and [4], using the data at 300 K. The 13C T, value of 0.93 set was taken directly from Table 2 and the lggHg value of 0.25 set was calculated by least-squares analysis from the data shown in Fig. 1. TABLE
4
laeHg RELAXATION
Molecule Hg(GH, HgMeZ”
1.4 T La
1.48* 0.219d
(100%) (29%)
DATA
AT 27°C
2.35 T 4.10 0.6086
a The small intermolecular contribution zero for HgMe, (Ref. (7)). b Calculated from the measured value at c Immeasurably small, assumed zero. * Calculated using AU = 7325 ppm, 71 the measured T, value of 0.87 set (Ref. RySA value.
7.05
(100%) (53%)
has not
T
36.9 (100%) 5.47d (91%) been
included
for
Hg(C,H,),
RTR oc 0.541 and is
2.35 T. = 6.7 x 10-l* sec. RfR (7)) at 2.35 T together
was calculated using with the calculated
RELAXATION
IN
DIPHENYLMERCURY
427
For a linear molecule the average screening observed in solution is given by uiso
= 1/3(a,,,, + 20,)
[51
and hence values for op.,, and uI may be calculated if AU is known. Mercury shifts are dominated by the paramagnetic term, up. This term is zero for the free atom since precession of the electrons in the applied magnetic field is unhindered by the spatial limitations arising from bonding. This is also true for linear and symmetric top molecules (2) when the field is applied along the symmetry axis, i.e., up,,, is zero. Determinations of (+P,llhence indicate the shift zero. This method was discussed previously for mercury (5, I1 ) and all the known data are presented in Table 3, where the neat dimethylmercury has been taken as the shift zero. The value of +5300 ppm for up,,, reported here is in good agreement with the previous values. The mean value of +4800 ppm agrees well with the value of +4686 ppm reported for the free atom (18). Signi$cance
of CSA Relaxation
for lggHg
The demonstration of total dominance of CSA relaxation for diphenylmercury, even at 2.35 T, means that the general importance of the CSA mechanism for lggHg may be assessed. Molecules will differ in their AU values, correlation times, and the contribution from other mechanisms, which in the case of mercury may be taken as spin rotation. CSA relaxation is more efficient in diphenylmercury than in dimethylmercury despite the fact that the anisotropy is smaller. Lassigne and Wells (8) reported a correlation time of 6.7 psec at 27°C for the dimethyl compound, seven times shorter than our value of 50 psec for the diphenyl compound. Our calculated TFSA for dimethylmercury at 2.35 T is 1.65 sec. Previous calculations (7,ZZ) of 2 set were in error, since a correlation time of 100 psec was assumed which if correct would lead to a value of 0.1 sec. The importance of the CSA mechanism for the two molecules at three fields is shown in Table 4. The calculated 53% CSA contribution at 27°C is not inconsistent with the T, temperature dependence found by Sens et al. (7), who reported a maximum T, value around 18°C. Further, our calculated value of 1.32 set for the total relaxation time at 1.4 T is in agreement with the experimental value of 1.18 set at 23°C reported by Lassigne and Wells (8). The CSA rates for dimethylmercury have been calculated from the latest vibrationally corrected value for Au (7325 ? 55 ppm) obtained by Jokisaari and Diehl (19) and measured in a smectic liquid crystal solvent, a method which does not require the assumption of an isotropic shift value. It seems that anisotropies measured in liquid crystal solvents may be used for calculation of CSA relaxation ratios in isotropic liquids. CONCLUSIONS
Our conclusion is that the effects of CSA relaxation are important for lggHg NMR in general. At the high fields now available for multinuclear studies the effects become more obvious, causing line broadening. The increased relaxation rate also affects coupled nuclei. Thus the lggHg satellites in the 13C spectrum of diphenyl-
428
GILLIES
ET AL.
mercury are broader at 67.89 MHz than at 25.1 MHz. The CSA mechanism may be important even for the mercuric ion by analogy with the work of Hinton anh Ladner (20) on the thallous ion. REFERENCES 2. R. K. HARRIS AND B. E. MANN, “NMR and the Periodic Table,” Academic Press, London-New York, 1978. 2. B. R. APPLEMAN AND B. P. DAILEY, Advan. Mugn. Reson. 7, 231 (1974). 3. T. C. FARRAR AND E. D. BECKER, “Pulse and Fourier Transform Nuclear Magnetic Resonance,” Chap. 4, Academic Press, New York, 1971. 4. A. ABRAGAM, “The Principles of Nuclear Magnetism,” Chap. VIII, Oxford Univ. Press, New York/London, l%l. 5. R. G. KIDD AND R. J. GOODFELLOW, in “NMR and the Periodic Table” (R. K. Harris and B. E. Mann, Eds.), Chap. 8, Academic Press, London/New York, 1978. 6. G. E. MACIEL AND M. BORZO, .I. Mngn. Reson. 10, 388 (1973). 7. M. E. SENS, N. K. WILSON, P. D. ELLIS, AND J. D. ODOM, J. Magn. Reson. 19, 323 (1975). 8. C. R. LASSIGNE AND E. J. WELLS, Can. J. Chem. 55, 1303 (1977). 9. B. ZIOLKOVSKA, R. M. MYASNIKOVA, AND A. I. KITAIGORODSKII, J. Struct. Chem. USSR 5,678 (1964). 10. F. W. WEHRLI, J. Magn. Reson. 32, 451 (1978). II. J. D. KENNEDY AND W. MCFARLANE, J. Chem. Sot. Faraday Trans. II 72, 1653 (1976). 12. N. K. WILSON, R. D. ZEHR, AND P. D. ELLIS, J. Magn. Reson. 21, 437 (1976). 13. G. C. LEVY, J. D. CARGIOLI, AND F. A. L. ANET, J. Am. Chem. Sot. 95, 1527 (1973). 14. N. BLOEMBERGEN, Phys. Rev. 109, 1542 (1956). 15. D. E. WOESSNER, J. Chem. Phys. 36, 1 (1962). 16. D. E. WOESSNER, B. J. SNOWDON, AND G. H. MEYER,J. Chem. Phys. 50, 719 (1969). 17. R. K. HARRIS AND B. J. KIMBER, Advan. Mol. Relaxation Processes 8, 23 (1976). 18. H. KRUGER, 0. LUTZ, A. NOLLE, AND A. SCHWENK, Z. Phys. A 273, 325 (1975). 19. J. JOKISAARI AND P. DIEHL, Org. Magn. Reson. 13, 359 (1980). 20. J. F. HINTON AND K. H. LADNER, J. Magn. Reson. 32, 303 (1978).
OF MAGNETIC
RESONANCE
42,420-428
(1981)
lggHg Spin-Lattice Relaxation and Chemical Shift Anisotropy in Diphenylmercury D. G. GILLIES,* Shell
L. P. BLAAUW,
Research
B.V.,
G. R. HAYS,
KoninklijkelShell-Laboratorium,
R. HUB, AND A. D. H. CLAGUE Amsterdam,
The Netherlands
Received May 27, 1980; revised August 28, 1980 lg9Hg spin-lattice relaxation times (T,) have been measured for diphenylmercury at magnetic fields of 2.35 and 7.05 T. T, was nine times shorter at the higher field (0.033 set at 310 K) than at the lower field (0.30 set), showing relaxation by chemical shift anisotropy (GA) to be the dominant mechanism even at the lower field. Variabletemperature 13C T, measurements at 6.35 T made it possible to determine a value for the correlation time for motion perpendicular to the axis of the molecule (50 psec at 300 K), which with the lg9Hg data allowed a value of 6800 k 680 ppm to be calculated for the mercury chemical shift anisotropy. The activation energy for rotational motion was 13.3 kJ.mote-l. Previous data on dimethylmercury have been reassessed and the importance of the CSA mechanism for ls9Hg at high fields is pointed out. INTRODUCTION
With the increasing interest in NMR studies of metal nuclei (1) it is important to understand the mechanisms by which these nuclei can relax. The large range of chemical shifts typically observed (several thousand parts per million) indicates that in nonsymmetric molecules the chemical shift anisotropy (CSA) may also be large. For nuclei with spin Z greater than l/2, relaxation will be dominated by the quadrupolar mechanism, but when Z is equal to l/2, relaxation by the CSA mechanism may predominate, particularly at high fields, because of the square law dependence of the relaxation time on the external magnetic field B,,. The experimentally measured spin-lattice relaxation time T1, or rate RPBS (= l/T,), includes contributions from chemical shift anisotropy (RFSA), intramolecular dipole-dipole (R ptra DD) and intermolecular dipole-dipole interactions (R I;nterDD),spin rotation (RSR), and scalar coupling (RfC). Thus &-PBS
=
RESA
+
Rpt’aDD
+
RyterDD
+
RSR
+
RfC.
[II
Magnetic shielding anisotropies and methods of measurements have been reviewed by Appleman and Dailey (2). Equation [2] gives the rate of relaxation by chemical shift anisotropy for a nucleus lying on the symmetry axis of a linear molecule under conditions of extreme narrowing (3), * Visiting scientist at KSLA, Egham, Surrey, TW 20 OEX.
March to September 1979. Present address: Royal Holloway College, 420
0022-2364/81/030420-09$02.00/O Copyright 0 1981 by Academic Press, Inc. AU rights of reproduction in any form reserved.
RELAXATION
IN
421
DIPHENYLMERCURY
R,CSA= (T,csA)-’ = ;
y%;(L\~r)~~,,
VI
where Acr = o,, - oI is the chemical shift anisotropy, r1 is a correlation time describing molecular reorientation about an axis perpendicular to the symmetry axis, and the other symbols have their usual meanings. Relaxation by the mechanism of scalar coupling (4) is the only other process that can give fielddependent relaxation (decreasing with increasing field) and is irrelevant in the present case. The measurements reported here were made at two fields, 2.35 and 7.05 T, and allowed calculation of both the CSA relaxation rate, RySA, and the relaxation rate arising from other, field-independent mechanisms, Rpther, from the following equations: ROBS l(2.35 T)
= R&T,
ROBS l(7.05 T)
= 9R5&T,
+ Rpther, + Ryther.
Pal WI
The remaining contributions to Rpther are the dipolar and spin rotation terms. NOE measurements allowed determination of the total dipolar rate, RPtra DD + R I;nterDD, leaving R sR as the only unknown field-independent term. lssHg nuclear magnetic resonance has recently been reviewed (5) and although there are quite a lot of data on chemical shifts and coupling constants, covering a range of chemical compounds, there are in contrast relatively few reports on relaxation processes (6 -8). Sens et al. (7) reported a small temperature dependence (-35 to +4O”C) for T, in neat dimethylmercury at a field of 2.35 T and explained this qualitatively in terms of nearly equal contributions from the CSA and SR mechanisms, with the former dominating at low temperature and the latter at high. Lassigne and Wells (B), working at the lower field of 1.41 T, reported the SR mechanism to be dominant even at low temperature. MEASUREMENT
OF CHEMICAL
SHIFT
ANISOTROPY
From Eqs. [2] and [3] it is clear that T, measurements at two fields allow calculation of Au as long as 71 is known. Diphenylmercury was selected for the present study since its linear geometry (9) allows TV to be determined from the proton-induced dipole-dipole relaxation rate for the para carbon. The C-H vector lies along the symmetry axis and is subjected to the same motional fluctuations as the shielding of mercury. The equation is RDD l(para)
=
3/2c&fi2~Ci~1.
r41
Wehrli (10) determined 13C shielding anisotropies for the cyano and C, carbons in benzonitrile by this method. Kennedy and McFarlane (11) have measured lssHg chemical shift anisotropies from studies in the nematic phases of liquid crystal solvent and reported values of +7475 ppm for dimethylmercury and about 5500 ppm for methylmercuric chloride, bromide, and iodide. The value of 4600 + 1000 ppm reported for dimethylmercury by Lassigne and Wells (8) should be regarded as being of the
GILLIES
422
ET AL.
TABLE ls9Hg RELAXATION Temperature
zysa
WI
(set)
273 300 313 323 338
0.136 0.244 0.306 0.358 0.452
1
DATA
AT
17.90 MHz TPDC (=I
0.22 0.10 0.07 0.04
1.72 7.18 17.0 32.3
,p”“DDd (se4
0.148 0.253 0.312 0.459
’ Errors estimated to be of the order of 5%. * Nuclear Overhauser enhancement: qmax = 2.79 for ls9Hg - {‘H}. ’ (l/T,)” = (q/q,,,)(l/T,)oBs; (lIT,)O= = (l/T#‘” + (l/T,~DO. d Ty”DD = TfSA in this case; see text.
correct order of magnitude. It was derived indirectly from the spin-rotation contribution and the error limits are almost certainly larger than those quoted. EXPERIMENTAL
A solution of diphenylmercury (Merck) in a 50/50 mixture by volume of 1,1,2,2tetrachlorodeuteroethane (Merck) and carbon tetrachloride was prepared (ca. 0.4 M, the maximum concentration which precluded crystallization at the lowest temperature of measurement) and degassed by passing dry nitrogen through it for several minutes. All the spectra were run with internal field-frequency lock to the deuterium resonance of the solvent. Proton-decoupled lggHg spectra at 17.90 MHz were run in a 12-mm tube on a Varian XL-loo-15 spectrometer using the multinuclear gyrocode observe facility. Temperatures were measured before and after each run using a thermometer immersed in an ethylbenzene sample and were estimated to be accurate to within 1°C. Proton-decoupled lggHg spectra at 53.69 MHz were run in a lo-mm tube in a 20-mm probe on a Bruker CXP-300 spectrometer at ambient temperature. Proton-decoupled 13Cspectra at 67.89 MHz were run in a lo-mm tube on a Bruker WH-270 spectrometer. Variable temperature studies were performed using the standard Bruker temperature controller and temperatures were estimated to be accurate to within 2°C. TI measurements were made by the inversion-recovery method and values calculated by least squares from semilogarithmic plots. The sample was confined to the volume of the coil by a vortex plug. NOE enhancement factors (lggHg at 17.90 MHz and 13C at 67.89 MHz) were determined by the gated decoupling technique using recycle times of at least 10 x T,. RESULTS
Mercury
Relaxation
AND DISCUSSION
Times
The lggHg T, and nuclear Overhauser enhancement (7) data are shown in Table 1, together with the derived values for the dipolar ( TyD) and nondipolar
RELAXATION
IN
423
DIPHENYLMERCURY
0.25
0 20
0 15 3.0
FIG. 1. Too
3.2
3.4
3.6 1000 K
for ls9Hg as a function of inverse temperature
at 17.90 MHz.
CTyonDD) relaxation times. A plot of In TTonDD versus inverse temperature is shown in Fig. 1. The straight line plot with an activation energy of 13.3 + 0.1 kJ * mole-l indicates that spin rotation, which would have the opposite temperature dependence, makes an insignificant contribution to the relaxation. This behavior suggests that CSA is the mechanism responsible. This is confirmed by the measurement at 57.69 MHz and 309.5 K of a T1 value of 33.3 msec, which is one-ninth the value of 0.30 set calculated by least-squares analysis from the data shown in Fig. 1. This means that conDo in Table 1 is equal to TFSAand that relaxation by the CSA mechanism is the dominant mechanism even at 2.35 T, the contribution varying from 92% at 273 K to 98.5% at 338 K. If no allowance is made for the dipolar contribution, an activation energy of 14.2 2 0.2 kJ.mole-’ is found for TPBS, implying a higher activation energy for dipolar relaxation than for CSA. A plot of In TFD versus inverse temperature is shown in Fig. 2 and gives an activation energy of 35.5 2 2.7 kJ.moleel. Relaxation by the intramolecular dipole-dipole mechanism via the ortho protons cannot be responsible since the Hg-H distance (0.3 1 nm) is too great. This points to relaxation via an intermolecular dipole-dipole mechanism.
424
GILLIES
ET AL.
3.2
3.4
0
3.0
3.6 F K
FIG. 2. TPD for ls9Hg as a function of inverse temperature at 17.90 MHz.
Carbon Relaxation
Times
The carbon spectrum was measured and assigned previously (22). The 13C T1 data are shown in Table 2. x is the ratio of the average T, for the ortho and meta carbons to the T, for the para carbon and 71 was calculated from T,(para) using Eq. [4]. This was justified since the NOE factors determined at several temperatures were found to be maximum within experimental error and also the CSA contribution (3.7% at 67.89 MHz with AU = 180 ppm) was considered to be negligible. Figure 3 shows the T, data plotted against inverse temperature. The data for ortho and meta carbons have been taken together and yield an activation energy of 15.1 + 0.3 kJ .mole-‘, which is significantly higher than that for the para carbon, which is 13.0 ? 0.5 kJ*mole-‘. The latter figure is in excellent agreement with the value of 13.3 +- 0.1 kJ*mole-’ derived from the lssHg CSA data and confirms that the lssHg relaxation by the CSA mechanism is governed by the same motional parameters as the intramolecular DD mechanism for the para carbon. Levy et al. (13) have shown how internal rotation of a phenyl group leads to an increase in the relaxation times for the ortho and meta carbons relative to the para. Our data, with T, values being some 2.0 to 2.6 times higher for the ortho and meta
RELAXATION
IN
TABLE 13C RELAXATION ph”
Temperature
peta
(set)
(se4
(se4
273 288 300 303 315 330 343
1.12 1.65 2.18 2.26 2.89 3.56 4.35
1.12 1.66 2.17 2.18 2.72 3.48 4.55
0.55 0.71 0.93 0.91 1.21 1.50 1.70
T,(ortho,meta)lT,(para). from R,(para) = 2.149
2
DATA
AT 67.89
MHz b
para
W
” x = Mean * Calculated
425
DIPHENYLMERCURY
(pkc)
XU
2.04 2.33 2.39 2.44 2.32 2.35 2.62
x 1O+1o~L using
Eq.
[4] with
D
Rd
(nsec-I)
(nsec-‘)
84.6 65.5 50.0 51.1 38.5 31.0 27.4
1.97 2.54 3.33 3.26 4.33 5.36 6.08
rCmH = 0.109
nm.
7.32 12.54 17.26 17.60 21.14 26.82 37.46
carbons than for the para carbon, show that the effective correlation times are indeed shorter for ortho and meta carbons, indicating overall motional anisotropy. The higher activation energy supports the conclusion that the relaxation is T .s .o b -
‘.5
8.0.5.o2 5-
.o-
FIG.
3. T, for
13C as a function
of inverse
temperature
at 67.89
MHz.
426
GILLIES
ET
AL.
TABLE lssHg Molecule
SHIFTS
(ppm)
AND ANISOTROPIES
AU
&Me,
3
8”
+7475 +7325
k +
+6800
* 680
-742
+ 5300
MeHgCl
+5535
k
80
-811
+4500
(II)
MeHgBr
+5455
? 100
-910
+4500
(II)
MeHgI
+5480
k 300
- 1085
+4700
(II)
WC&
12
” Positive Ref. (7).
values
indicate
80 55
Reference
shifts
0 0
to high
+5000 +4880
frequency
(low
(11) (19) This
work
cr); taken
from
dependent on different motional properties. It is not possible, however, to separate the effects of internal phenyl rotation from overall rotation of the molecule around its axis, although we feel that the former effect probably dominates for this case. Assuming isotropic overall motion and a diffusion model (24) for internal rotation, one may calculate the internal rotation coefficient Rd from x and the isotropic diffusion constant D (13, 15-l 7). D is obtained from the correlation time for the para carbon (D = 1/6~~). Values for D and Rd are included in Table 2 and yield a value of 17.0 & 0.7 kJ*mole-’ for the activation energy. Chemical
Shift Anisotropy
The chemical shift anisotropy was calculated as 6800 + 680 ppm from Eqs. [2] and [4], using the data at 300 K. The 13C T, value of 0.93 set was taken directly from Table 2 and the lggHg value of 0.25 set was calculated by least-squares analysis from the data shown in Fig. 1. TABLE
4
laeHg RELAXATION
Molecule Hg(GH, HgMeZ”
1.4 T La
1.48* 0.219d
(100%) (29%)
DATA
AT 27°C
2.35 T 4.10 0.6086
a The small intermolecular contribution zero for HgMe, (Ref. (7)). b Calculated from the measured value at c Immeasurably small, assumed zero. * Calculated using AU = 7325 ppm, 71 the measured T, value of 0.87 set (Ref. RySA value.
7.05
(100%) (53%)
has not
T
36.9 (100%) 5.47d (91%) been
included
for
Hg(C,H,),
RTR oc 0.541 and is
2.35 T. = 6.7 x 10-l* sec. RfR (7)) at 2.35 T together
was calculated using with the calculated
RELAXATION
IN
DIPHENYLMERCURY
427
For a linear molecule the average screening observed in solution is given by uiso
= 1/3(a,,,, + 20,)
[51
and hence values for op.,, and uI may be calculated if AU is known. Mercury shifts are dominated by the paramagnetic term, up. This term is zero for the free atom since precession of the electrons in the applied magnetic field is unhindered by the spatial limitations arising from bonding. This is also true for linear and symmetric top molecules (2) when the field is applied along the symmetry axis, i.e., up,,, is zero. Determinations of (+P,llhence indicate the shift zero. This method was discussed previously for mercury (5, I1 ) and all the known data are presented in Table 3, where the neat dimethylmercury has been taken as the shift zero. The value of +5300 ppm for up,,, reported here is in good agreement with the previous values. The mean value of +4800 ppm agrees well with the value of +4686 ppm reported for the free atom (18). Signi$cance
of CSA Relaxation
for lggHg
The demonstration of total dominance of CSA relaxation for diphenylmercury, even at 2.35 T, means that the general importance of the CSA mechanism for lggHg may be assessed. Molecules will differ in their AU values, correlation times, and the contribution from other mechanisms, which in the case of mercury may be taken as spin rotation. CSA relaxation is more efficient in diphenylmercury than in dimethylmercury despite the fact that the anisotropy is smaller. Lassigne and Wells (8) reported a correlation time of 6.7 psec at 27°C for the dimethyl compound, seven times shorter than our value of 50 psec for the diphenyl compound. Our calculated TFSA for dimethylmercury at 2.35 T is 1.65 sec. Previous calculations (7,ZZ) of 2 set were in error, since a correlation time of 100 psec was assumed which if correct would lead to a value of 0.1 sec. The importance of the CSA mechanism for the two molecules at three fields is shown in Table 4. The calculated 53% CSA contribution at 27°C is not inconsistent with the T, temperature dependence found by Sens et al. (7), who reported a maximum T, value around 18°C. Further, our calculated value of 1.32 set for the total relaxation time at 1.4 T is in agreement with the experimental value of 1.18 set at 23°C reported by Lassigne and Wells (8). The CSA rates for dimethylmercury have been calculated from the latest vibrationally corrected value for Au (7325 ? 55 ppm) obtained by Jokisaari and Diehl (19) and measured in a smectic liquid crystal solvent, a method which does not require the assumption of an isotropic shift value. It seems that anisotropies measured in liquid crystal solvents may be used for calculation of CSA relaxation ratios in isotropic liquids. CONCLUSIONS
Our conclusion is that the effects of CSA relaxation are important for lggHg NMR in general. At the high fields now available for multinuclear studies the effects become more obvious, causing line broadening. The increased relaxation rate also affects coupled nuclei. Thus the lggHg satellites in the 13C spectrum of diphenyl-
428
GILLIES
ET AL.
mercury are broader at 67.89 MHz than at 25.1 MHz. The CSA mechanism may be important even for the mercuric ion by analogy with the work of Hinton anh Ladner (20) on the thallous ion. REFERENCES 2. R. K. HARRIS AND B. E. MANN, “NMR and the Periodic Table,” Academic Press, London-New York, 1978. 2. B. R. APPLEMAN AND B. P. DAILEY, Advan. Mugn. Reson. 7, 231 (1974). 3. T. C. FARRAR AND E. D. BECKER, “Pulse and Fourier Transform Nuclear Magnetic Resonance,” Chap. 4, Academic Press, New York, 1971. 4. A. ABRAGAM, “The Principles of Nuclear Magnetism,” Chap. VIII, Oxford Univ. Press, New York/London, l%l. 5. R. G. KIDD AND R. J. GOODFELLOW, in “NMR and the Periodic Table” (R. K. Harris and B. E. Mann, Eds.), Chap. 8, Academic Press, London/New York, 1978. 6. G. E. MACIEL AND M. BORZO, .I. Mngn. Reson. 10, 388 (1973). 7. M. E. SENS, N. K. WILSON, P. D. ELLIS, AND J. D. ODOM, J. Magn. Reson. 19, 323 (1975). 8. C. R. LASSIGNE AND E. J. WELLS, Can. J. Chem. 55, 1303 (1977). 9. B. ZIOLKOVSKA, R. M. MYASNIKOVA, AND A. I. KITAIGORODSKII, J. Struct. Chem. USSR 5,678 (1964). 10. F. W. WEHRLI, J. Magn. Reson. 32, 451 (1978). II. J. D. KENNEDY AND W. MCFARLANE, J. Chem. Sot. Faraday Trans. II 72, 1653 (1976). 12. N. K. WILSON, R. D. ZEHR, AND P. D. ELLIS, J. Magn. Reson. 21, 437 (1976). 13. G. C. LEVY, J. D. CARGIOLI, AND F. A. L. ANET, J. Am. Chem. Sot. 95, 1527 (1973). 14. N. BLOEMBERGEN, Phys. Rev. 109, 1542 (1956). 15. D. E. WOESSNER, J. Chem. Phys. 36, 1 (1962). 16. D. E. WOESSNER, B. J. SNOWDON, AND G. H. MEYER,J. Chem. Phys. 50, 719 (1969). 17. R. K. HARRIS AND B. J. KIMBER, Advan. Mol. Relaxation Processes 8, 23 (1976). 18. H. KRUGER, 0. LUTZ, A. NOLLE, AND A. SCHWENK, Z. Phys. A 273, 325 (1975). 19. J. JOKISAARI AND P. DIEHL, Org. Magn. Reson. 13, 359 (1980). 20. J. F. HINTON AND K. H. LADNER, J. Magn. Reson. 32, 303 (1978).