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The effect of lanthanide shift reagents on the NMR spectra of molecules partially oriented in a nematic phase: Pyridine
JOURNAL
OF MAGNEl-lC
RESONANCE
13, 167-I
76 (19%)
The Effect of Lanthanide Shift Reagentson the NMR Spectra of MoleculesPartially Oriented in a Nematic Phase: Pyridine I. M. ARMITAGE,* E. E. BURNELL, M. l3. DUNN,? L. D. HALL,~ AND R. B. MALCOLM~ Department
of Chemistry,
University
of British
Columbia,
Vancouver
8, Canada
Received July 2,1973 Results of a study of pyridine oriented in a nematic phase in the presence of the lanthanide shift reagent, tris (dipivalomethanato) europium(III) are reported. The shift reagent does not appear to affect the pyridine geometry. The isotropic and anisotropic parts of the lanthanide induced shifts are reported. 1. INTRODUCTION
The study of NMR spectra of molecules dissolved and oriented in nematic phases is a well-known technique for the determination of relative molecular geometries and of anisotropies in chemical shifts (1-3). Pyridine (see Fig. 1) has been studied three times by this method (4-6). H3
FIG. 1. Pyridine.
Lanthanide shift reagents are also well known as a means of increasing the dispersion of isotropic high resolution NMR spectra (7, 8). Their use for the determination of molecular geometries of the lanthanide : substrate complex has also been reported (8), and again pyridine has been studied (9, 10). Here we report the combination of both of the above experimental techniques in an attempt to study any possible changes in the anisotropy of the proton chemical shifts of pyridine induced by a lanthanide shift reagent, and any change in the pyridine geometry caused by association with the lanthanide reagent. * Present address: Department of Chemistry, California Institute of Technology, Pasadena, California, 91109, U.S.A. t Present address: Science Research Council, State House, High Holborn, London WClR 4TA, England. $ Alfred P. Sloan Foundation Research Fellow, 1971-73. $ Present address: The College of New Caledonia, Prince George, British Columbia, Canada. Copyright 0 1974 by Academic Press, Inc. AI1 rights of reproduction in any form reserved. Printed in Great Britain
167
168
ARMITAGE
ET AL.
2. EXPERIMENTAL
Spectra of pyridine dissolved in the nematic phase of N-(p-ethoxybenzylidene)-p’ti-butylaniline (EBBA) were obtained using varying concentrations of pyridine and the shift reagent tris (dipivalomethanato)europium(III) (Eu(dpm),). Spectra were obtained at different temperatures with both spinning and nonspinning sample tubes on a Varian XL-100 spectrometer operated in both CW and FTmodes at a scale of 1 Hz = 0.2 mm and a linewidth of -7 Hz. Tetramethylsilane (TMS) was used to provide both a reference for chemical shift measurements and an internal lock signal for the CW spectra; and D,O was used as a lock signal for the FT spectra using a coaxial sample tube arrangement (II). Slow spinning of the sample tube has been used previously to decrease temperature and concentration gradients in the sample (12), and thus give narrower linewidths. It has also been used to change the orientation of the sample so as to obtain anisotropies of chemical shifts (13) from equations of the type: d( di’)/dS,, zz
= gag
hl! where (02) is the observed chemical shift of nucleus i in the oriented media with z the space-fixed magnetic field direction, oas Q) is the chemical shift in the p molecule fixed direction caused by a magnetic field in the CImolecule fixed direction for nucleus i, and S,, = 3/2(cos~,cos~,) - l/26,, is the orientation parameter where Bnis the angle between the molecular q axis and the magnetic field direction z. A repeated Greek sufTix denotes summation, and angle brackets a time average. With this method one can vary (ci2) and Sdlrwith spinning speed and can, in principle, calculate the components of ~22 (with an assumed value for Tr(o$)). The techniques of measuring the isotropic shift at raised temperatures, or the shifts at different nematic temperatures (both commonly used to determine the components of o$) are not used here because the isotropic lananthide induced chemical shifts are temperature dependent (8, 9, 14, 1.5) and hence it was desirable to perform all measurements at the same temperature. In all cases where spinning sample tubes were used (except case C, see tables) the ET technique was used to accumulate the entire spectrum at the same time. This method reduces the problem caused by slight changes in spinning speed which affects the line positions. The effect now shows up only in the form of slightly increased linewidth when time averaging experiments are performed. Table 1 gives the experimental conditions used for each spectrum. The spectra were similar to those previously published for pyridine dissolved in a nematic phase (4-6). 3. ANALYSIS
OF
THE
SPECTRA
The analysis was carried out with the iterative computer program LAOCOONOR (4) fitting about 40 observed lines. Statistics of the “fits” are included in Table 1. The values of the Jij used were from Ref. (16). The direct dipole-dipole coupling constants are defined by the equation (3) :
where rij is the distance between the two nuclei i and j, I, is the cosine of the angle between the i-j and y directions, and yk is the magnetogyric ratio of nucleus k. The
EFFECT
OF SHIFT
REAGENTS
TABLE
ON ORIENTED
169
PYRIDINE
1
EXPWMENTAL CONDITIONS”
Case A B c D* E” F’ G” H”
Mole % Eu(dpmh
Mole % pyridine
Spinning speed (Hz)
RMS error from LAOCOONOR (Hz)
0
9.2
9.1 9.1 9.1 4.0 4.0 4.0 4.0
0 0 8.0 0 0
0.8
0.3 0.3 0.3 0 0 0.4 0.4
6.5 0 6.0
0.6 1.3 0.7 0.4 0.9 0.5 0.5
a EBBA as solvent with -1 mole % TMS added as internal reference; temperature -40°C. * Temperature -20%. c Measured by FT. Cases B -+ D have [Eu(dpm)J[pyridine] = 0.033; cases 6, H have [Eu(dpm)J[pyridine] = 0.101.
spectral parameters obtained from the “fit” to each spectrum are in Table 2. Errors are at the 68 % confidence level and are calculated by a standard procedure throughout. The signs are absolute if pyridine orients with S,, negative. 4. MOLECULAR
STRUCTURE
The molecular structure of pyridine is well known (4, 5, 17). The iterative weighted least-squares computer program SHAPE (18) was used to calculate geometrical information and orientation parameters from the Dlj in Table 2. The values of a(2), b(2), and b(3) were chosen from microwave data (17) and were not varied. This was necessary because of the large covariance between S,, and b(3) when b(3) was also varied in the least-squares routine; in some cases the program did not converge. Since the purpose of the experiment is to determine any change in the relative proton coordinates caused by the addition of Eu(dpm),, and since this change might be expected to be largest for a(l) and b(l), it seemed reasonable to fix the other proton coordinates. The results obtained from these calculations are in Table 3 ; it will be noted that no significant geometrical changes are detected. The liquid crystal measurements give an average of both the bound and free pyridine. Since each molecule of Eu(dpm), can bind up to two molecules of pyridine (as well as the liquid crystal solventl) , ,520 % of the pyridine for cases G and H can be bound. Thus, the geometry of the bound pyridine is that of cases G and H (which are the same as the other cases) with at least five times the uncertainty in a(l) and b(l). For cases B and C the uncertainty is even larger. An experiment performed with a far greater 1 Lanthanide shift reagents associate to some extent with any organic molecule having oxygen or nitrogen donor atoms. Hence it is possible that the Eu(dpm), associates with EBBA; however, the relatively low solubility implies that although some such association likely occurs, it certainly is not overwhehelmingly strong.
a See Table
-296.4 -265.8 -194.0 -281.9 -349.1 -275.9 -259.6 -202.1
0.2 0.1 0.3 0.1 0.1 0.2 0.1 0.1
-53.6 -48.9 -35.8 -52.1 -63.8 -49.8 -50.0 -38.8
I for experimental
i: + i + + + + +
DJ2
+ rf: L+ k + + 5
013
conditions.
0.3 0.2 0.4 0.2 0.2 0.4 0.2 0.2
-30.6 -29.7 -21.7 -31.5 -37.5 -29.4 -33.8 -26.6 Values
+ f + 2 & + 2 k
D14
100 MHz
0.2 0.1 0.3 0.1 0.1 0.2 0.1 0.1
-46.9 -46.7 -34.7 -50.0 -58.5 -46.2 -57.2 -44.0
rt: k 5 & f + + +
0.3 0.2 0.4 0.3 0.2 0.4 0.1 0.2
PARAMETERS
-228.9 -222.1 -162.6 -238.1 -280.9 -222.8 -254.6 -196.6
(IN Hz)
f 0.2 rt 0.2 ri 0.3 IL- 0.2 + 0.1 + 0.3 2 0.1 ir 0.1
OF PARTIALLY
2
-40.1 + 0.3 -41 .o + 0.2 -29.5 * 0.5 -43.8 + 0.2 -51.3 2 0.2 -40.4 i- 0.4 -49.9 + 0.2 -38.8 & 0.2
D 24
ORIENTED
-836.9 -936.5 -944.6 -959.4 -829.2 -835.2 -1080.2 -1090.8
~~~
+t(u’l’) zz
PYRIDINE”
for Jij used are (16) Jlz = 4.9, J13 = 1.9, J14 = 1.0, x15 = -0.1,
NMR
TABLE
k + + * 2 + k Ifr
=
-698.6 -729.3 -731.5 -744.2 -696.1 -697.9 -777.8 -778.8 7.7, ~~~ = 1.3.
0.6 0.5 0.7 0.5 0.5 0.6 0.5 0.5
f + * rt + + I: +
0.6 0.5 0.7 0.5 0.5 0.7 0.5 0.5
+ -735.8 -765.0 -766.0 -779.5 -733.9 -736.3 -810.9 -811.1
f. + + + f + i. +
0.6 0.5 0.7 0.6 0.5 0.8 0.5 0.5
;i
.
=;
EFFECT
OF SHIFT
REAGENTS
ON ORIENTED
TABLE GEOMETRIC
Case A B c D E F G W Pb Q’ g
41) -2.057 -2.066 -2.055 -2.069 -2.063 -2.074 -2.063 -2.068 -2.068 -2.056 -2.0557 -2.056
k i. rt + + & rt +
0.005 i% 0.004 0.009 0.004 0.003 0.007 0.002 0.003
3
AND ORIENTATION
PARAMETERS
b(l)
s cm
4 + + + + + k k
0.010 A 0.008 0.023 0.009 0.006 0.016 0.007 0.010
+ f * + rt k f rt
0.0002 0.0001 0.0002 0.0001 0.0001 0.0002 0.0001 0.0001
OF PYRIDINE’+
s cc -0.0656 -0.0614 -0.0453 -0.0662 -0.0788 -0.0623 -0.0671 -0.0515 -0.0748 -0.0868 -0.1018
from
r.m.5. error SHAPE (18) 0.22 Hz 0.10 0.25 0.20 0.14 0.31 0.27 0.09 0.03 0.01 0.03
a The following geometrical values from Ref. (17) are assumed for the calculation b(2) = 0, a(3) = 0, and b(3) = 1.287 A. b Calculated from 60 MHz data in Ref. (4). c Calculated from data in Ref. (5). Q is from 100 MHz spectrum and R is from d Microwave values (17). e See Table I for experimental conditions.
a(2) = -2.1525
L- 0.0014
0.0270 0.0274 0.0199 0.0293 0.0341 0.0272 0.0334 0.0259 0.0270 0.0319 0.0389
171
+ 0.0003 & 0.0002 F 0.0005 + 0.0003 + 0.0002 + 0.0005 k 0.0002 _+ 0.0002
+ 0.001
-2,499 -2.489 -2.501 -2.504 -2.482 -2.481 -2.497 -2.474 -2.481 -2.482 -2.4816 -2.481
PYRIDINE
220 MHz
A,
spectrum.
amount of bound pyridine could decrease this uncertainty. Unfortunately, the lanthanide shift reagent is not very soluble in the liquid crystal, and the use of lower pyridine concentrations would give weak signals, which would require longer time averaging. In particular, experiments using spinning sample tubes would be d&cult. Attempts to take advantage of the stronger binding affinity (19) of t&(2,2-dimethyl6,6,7,7,8,8,8-heptafluoro-3,5-octanedianato) europium(II1) (Eu(FOD),) were unsuccessful because the pyridine-Eu(FOD), complex appeared to crystallize from solution. 5. ANISOTROPY
OF
CHEMICAL
SHIFT
Observed Shift
We wish to relate the observed chemical shifts in Table 2 with molecular parameters. The observed isotropic-nematic shift, (02) - oCi) (using TMS as internal reference), arises from several factors: Eq. [3] indicates how this term may be partitioned into several components : (53
-
5(i)
= dog,
+ do;;;
$ dog.
PI
Aa:& is the observed isotropic-nematic shift for nucleus i arising from intramolecular contributions, and is in general given by AGO&= 2/35$(mol)
Sap.
141
In pyridine, because of the C,, symmetry, only the diagonal elements of o$(mol) and S,, are nonzero.
172
ARMITAGE ET AL.
The term dG,‘iL is the isotropic-nematic shift caused by the presence of the Ianthanide shift reagent. Thus we have, AG~$ = (~2 (Ian)} - G,(ii),, PI where (G$~(lan)) and ~1:; are the induced shifts in the nematic and isotropic phases. We note that ~1':: is commonly expressed as Av/v, or AH/H, in publications on shift reagents (15, 20, 21). If, as is commonly assumed (d), the lanthanide induced shift is pseudocontact in origin, then for most theoretical cases discussed in the literature> = (2/3) Lo TBySE,+ (l/3) Ls Tap, 161 where the first term is proportional to AG$~&and the second (isotropic) term is proportional to GI:~. The components of tas are functions of either the g tensor (for McConnell’s and Robertson’s (20) usual case in solution & = gas; for their case for the solid, if the g tensor is diagonal, tas = g$), or the magnetic susceptibility (for Bleaney’s theory (15), I& = xlxs)of the complex with respect to the rx, /3 pyridine axes. T,, is the dipole-dipole coupling tensor, :71 Tas = (1,‘9)(3~,~0 -&a”), where Pis the distance between the lanthanide ion and nucleus i and rn is the y component of Y. The quantity Aojii is the isotropic-nematic shift caused by different changes of the local environment of the solute molecules, and may easily be 0.1-0.2 ppm (22, 23). Thus, only observed shifts ((~22) - G(~)) 2 O. l-0.2ppmcan be reliably used to determine AGO& and AG$:~,.Note that this limit is for isotropic-nematic shifts, and will be attenuated by a factor of roughly (SC, - S~.,)/S,, for shifts obtained using differing orientation parameters. Determination of From Eq. [4]
@(mol)
(mol)) - of& = 2/3(G,, - 1/2(G,, + Gbb))i& SC, + 1/3(G~ - GM>%I 6, - %d. I?1 In principle three experiments with pyridine oriented in a nematic phase with no shift reagent present are sufficient to determine G~,,~, (‘) 2/3(0,, - 1/2(a,, + G&))$!~ and l/3(Gau G&)2&. Values from experiments A, E, and F are reported in Table 4. The neglect of dGg&
=
(GFi
TABLE CHEMICAL
1 2 3
SHIFT
RESULTS
-8.59 * 0.04 -7.06 + 0.04 -7.45 + 0.04
4
FROM SAMPLES REAGENT"
WI'I~I
-5.1 + 0.6 -1.6 i 0.5 -1.4 rk 0.5
NO LANTHANIDE
SHIFT
9+3 3+3 O&3
a Calculations are based on cases A, E and F. Results are in ppm, and TMS is taken as (I = 0.
EFFECT
OF SHIFT
REAGENTS
ON ORIENTED
PYRIDINE
173
might be expected to increase the uncertainty at least fourfold. Experiments with larger variation in S,, might lead to more accurate values for the above parameters. These experiments might include temperature, solvent and concentration studies of both the nematic and isotropic phases. However, for the present study, we are only concerned with the lanthanide induced shifts at a constant temperature in EBBA as solvent. We shall also assume that the above parameters are a measure of don& + dcrj:t, that is, that there is no change in the local solvent effect upon addition of the shift reagent. This may be roughly valid for TMS and the unbound pyridine, but will certainly be false for the 520 % pyridine bound to the lanthanide ion. Thus, upon addition of the shift reagent, we might expect a change in the local solvent effect 50.2 times that expected for all pyridine bound to the lanthanide ion. This might easily be larger than the uncertainty in the measurement. One further assumption should be pointed out; that is, that the local solvent shift is the same for case A as for cases E and F, which have different pyridine concentrations. AG$$
The Lanthanide Induced Shift We now wish to extract values for AC& G;$, and hence Ao,‘i’,/o,‘ih for both of the two lanthanide concentrations used. We note that for both of the two sets of experiments (B and C) and (G and H) that S,,fS,, differs by only -1 % within a given set, i.e., between spectra from spinning and nonspinning sample tubes. Thus, roughly within experimental error, the orientation parameters for one experiment equals the product of those for the other experiment multiplied by a constant factor. This factor is ((3/2)cos2a - l/2) if, as suggested by Yarmoni (IJ), the effect of spinning the sample tube is to twist the direction of orientation of the nematic phase from the magnetic field direction by an angle 6. Hence, for any set of experiments, the change in any component of S,, is proportional to the change in S,,. Since (Gg) - G(i) = +G$ .&, then {GE) - oci’ a SC,, or d(p$)/dS,, is constant for a given set of experiments. From two experiments (spinning and nonspinning sample tube) one can determine d(og)/dS,, and CT(~), and hence (c$ - &) for each experiment. rs,‘f’,is now easily obtained by subtraction. The results are given in Table 5. The contribution to the shift from Aa$i, can be calculated for experiments B, C, 6, and H using Eq. [8] and the corresponding orientation parameters. Assuming that Aa:& includes contributions from do,‘& ACT& and hence da,(fh/a& are obtainable by substitution into Eq. [3]. Values are in Table 5. The quantities Aoj$ and A&/G:~!, are purely experimental observables related to the space-fixed coordinate system, and their values vary with SGs.Relation of these results to the molecular parameters tas would require the use of Eq. [6], but this calculation has not been attempted at the present time for several reasons. First of all, the Sz, reported are an average for the free and bound pyridine; the value of S,, for the bound complex is required for Eq. [6] and is almost certainly different from the average.
174
ARMITAGE
ET AL.
TABLE
5
CHEMICALSHIFT RESULTS FROMSAMPLESWITHLANTHANLDESHIFTREAGENTADDED d(u”‘> Dz Case
Nucleus 1
[Eu(f~+l
[pyrrdlne]
*“’
d&c
@pm>
(PPm)
(1) *Ian
AC$‘,
6mm)
hm-4
B 0.033
-4.9
+ 0.5
-9.67
i 0.03
-1.07
G -6.8
rf: 0.5
-11.26
i: 0.03
-2.66
B 0.033
-1.3
f 0.5
-7.37
k 0.03
-0.32
G -0.6
k 0.5
-7.82
i: 0.03
-0.76
B 0.033
-0.6
f 0.6
-7.69
2 0.03
-0.23
G H
-0.1
* 0.5
-8.12
+ 0.03
-0.67
0.04 + 0.04
-0.04
rt 0.04
0.12 * 0.05
-0.05
+ 0.02
0.09 If: 0.04
-0.03
* 0.02
0.003
+ 0.04
-0.01
+ 0.7
0.004
zk 0.03
-0.01
& 0.5
-0.07
rt 0.05
0.09 IO.09
-0.06
5 0.04
0.07 rt 0.07
-0.05
+ 0.05
0.2 + 0.8
-0.04
* 0.03
-0.09
+ 0.05
0.2 + 0.6 0.13 f 0.12
-0.07
Ik 0.04
0.10 * 0.09
2 0.05
C 0.101
_+ 0.05
+ 0.05
H 3
-0.05
+ 0.05
C
0.101
0.06 rir 0.05
+ 0.05
H 2
(1) *1an
* 0.05
c
0.101
ALT$$
+ 0.05
In addition, as pointed out by X-ray crystallographic work (24), the pyridine molecule is not on a symmetry axis in the solid complex, and the ELI-N direction makes an angle of N 8” with the plane of the pyridine ring. Recent work on pyridine (and other compounds) dissolved in carbon disulphide in the presence of Eu(dpm), at low temperatures yields two ortho and two meta resonances, demonstrating the asymmetry of the complex (25). Both the crystallographic (24) and shift reagent (25) work discussed above point out the incorrect use of a simple P,(cosO) relationship (20) for the interpretation of lanthanide induced shifts in isotropic phases. These studies indicate that more than two components of S,, are needed to describe the orientation of the complex dissolved in a nematic phase. Unfortunately, only two components of S,, innuendo the observed spectra, which are an average of rapidly exchanging free and bound pyridine; the DIj between the pyridine and the Eu(dpm), are not observed. Therefore, the other components of S,, needed for use in Eq. [6] are not available from the present study. A further problem is the necessity of making assumptions about: (1) the geometry of the pyridine-Eu(dpm), complex (which would give the components of the Tea tensor in Eq. [6]), (2) the symmetry and direction characteristics of the (FblBtensor, and (3) the magnitude of any contact interaction. Because of the problems discussed above, trial calculations that have been performed
EFFECT
OF SHIFT
REAGENTS
ON ORIENTED
PYRIDlNE
175
(26) are not reported. The only significant result is that do~jlh/ol& is of similar order of magnitude as S,, in both sets of experiments. Hence the anisotropy in the lanthanide induced shift is small. 6. CONCLUSIONS
In isotropic phases, the large chemical shifts induced by the addition of Eu(dpm~~ can be used to increase the dispersion of proton spectra so as to give simple, first order type spectra (7). This was not the case with pyridine dissolved in EBBA. In the NM spectra of molecules oriented in liquid crystalline solvents, the dipole-dipole couplings are normally of the order of kHz. Thus, we do not envisage shift reagents being of much practical use in solving the NMR spectra of oriented molecules, except perhaps in some special cases combined with the use of high field spectrometers. Furthermore, even if one did succeed in separating out the resonances from the different protons, one would only measure 12Dij + Jjjj (27). That is, one would automatically lose valuable information about the isotropic spin-spin coupling constants. To relate the determined value of d~j-~~/~f~~ to meaningful molecular information, many assumptions have to be made. Preliminary studies which have been performed yield no useful information (26). Nonetheless AG~~~/G(~) lan is of similar order of magnitude as S,,. Hence, the anisotro in cr,,(lan) is nbt significantly greater than its isotropic value. Any change in geometry caused by binding to the shift reagent is not detected in t.he present experiments, but should be checked with a larger value of the ratio [lanthanide]/[pyridine], or in another molecule where the expected effects might be larger. ACKNOWLEDGMENTS We wish to thank the following for very helpful discussions, ideas and support: Professors P. Diehl, F. G. Herring, C. A. McDowell, and A. G. Marshall, Drs. W. Niederberger and C. S. Yannoni and Mr. J. Vogt ; we also wish to thank a referee for helpful comments. The financial support of the National Research Council of Canada is gratefully acknowledged. REFERENCES
1. A. SAUPE AP\?) G. ENGLERT, 2. 3. 4. 5. 6. 7. 8. 9. 10.
IJ. 12. 13.
Whys. Rev. Lett. 11,462 (1963). A.D.BUCKINGHAMAND K. A. MCLAUCHLAN, Prog.NMR Spectrosc.2,63 (1967). P. DIEHL AND C. L. KHETRAPAL, “NMR-Basic Principles and Progress,” Vol. 1, Springer-Verlag, New York, 1969. P. DI~HL, C. L. KHETRAPAL, AND H. P. KELLERHALS, MO/. Phys. 15,333 (1968). E. E. BURNELL AND C. A. DELANGE, Mol. Phys. 16,95 (1969). C. A. VERACINI, M. LONGERI, AND P. L. BARILI, Chem. Phys. Lett. 19,592 (1973). C. C. HINCKLEY, J. Am. Chem. Sot. 91,516O (1969). JACQUES REUBEN, in “Prog. NMR Spect.” (J. W. Emsley, J. Feeney, and L. H. Sutcliffe, Eds.), Pergamon Press, New York, Vol. 9, p. 1 (1973). 1, M. ARMITAGE, Ph.D. Thesis, University of British Columbia, Vancouver 8, Canada, 1973. (a) C. BEAUTY, Z. W. WOLKOWSKI, ANLI N. THOAI, Tetrahedron Lett. 817 (1971); (b) J. REUBEN AND J. S. LEIGH, JR., J. Am. Chem. Sot. 94,2789 (1972); (c) R. A. FLETTON, G. F. H. GREEEN, AND J. E. PAGE, Chem. Znd. (London) 167 (1972); (d) 0. A. GANSOW, P. A. LOEPFLER, R. E. DAVIS, M. R. WILLCOTT III, AND R. E. LENKINSKI, J. Am. Chem. Sot. 95,339O (1973). G. ENGLERT, Z. Naturforsch 27a, 715 (1972). A. D. BUCKINGHAM, E. E. BURNELL AND C. A. DELANGE, Mol. Phys. 15,285 (1968). C. S. YANNONI, Z.B.M. J. Rex Develop. 15,59 (1971).
176 14. 1.5. 16. 17. 18. 19, 20. 21. 22. 23. 24. 2.5. 26. 27.
ARMITAGE
ET AL.
C. BEAUTY, S. CORNUEL, D. LELANDAIS, N. THOAI, AND Z. W. WOLKOWSKI, Tetrahedron Left. 1099 (1972). B. BLEANEY, J. Magn. Resonance 8,91 (1972). S. CASTELLANO, C. SUN, AND R. KOSTELNIK, J. Chem. Phyx 46,327 (1967). B. BAK, L. HANSEN-NYGAARD, AND J. RASTRUP-ANDERSEN, J. Mol. Spectrosc. 2,361 (1958). P. DIEHL, P. M. HENRICHS, AND W. NIEDERBERGER, Mol. Phys. 20,139 (1971). I. M. ARMITAGE, G. DUNSMORE, L. D. HALL, AND A. G. MARSHALL, Can. J. Chem. 50,2119 (1972)). H. M. MCCONNELL, AND R. E. ROBERTSON, J. Chem. Phys. 29,136l (1958). B. BLEANEY, C. M. DOBSON, B. A. LEVINE, R. B. MARTIN, R. J. P. WELIAMS, AND A. V. XAVLFR, J.C.S. Chem. Comm. 791 (1972). A. D. BUCKINGHAM, E. E. BURNELL, AND C. A. DELANGE, J. Am. Chem. Soe. 90,2972 (19683, and references therein. A. D. BUCKINGHAM, E. E. BURNELL, AND C. A. DELANGE, J. Chem. Phys. 54,3242 (1971). R. E. CRAMER AND K. SEFF, J.C.S. Chem. Comm. 400 (1972). R. E. CRAMER AND R. DUBOIS, J. Am. Chem. Sot. 95,3801(1973). E. E. BURNELL, unpublished results. E. E. BURNELL AND P. DIEHL, Can. J. Chem. 50,3566 (1972).
OF MAGNEl-lC
RESONANCE
13, 167-I
76 (19%)
The Effect of Lanthanide Shift Reagentson the NMR Spectra of MoleculesPartially Oriented in a Nematic Phase: Pyridine I. M. ARMITAGE,* E. E. BURNELL, M. l3. DUNN,? L. D. HALL,~ AND R. B. MALCOLM~ Department
of Chemistry,
University
of British
Columbia,
Vancouver
8, Canada
Received July 2,1973 Results of a study of pyridine oriented in a nematic phase in the presence of the lanthanide shift reagent, tris (dipivalomethanato) europium(III) are reported. The shift reagent does not appear to affect the pyridine geometry. The isotropic and anisotropic parts of the lanthanide induced shifts are reported. 1. INTRODUCTION
The study of NMR spectra of molecules dissolved and oriented in nematic phases is a well-known technique for the determination of relative molecular geometries and of anisotropies in chemical shifts (1-3). Pyridine (see Fig. 1) has been studied three times by this method (4-6). H3
FIG. 1. Pyridine.
Lanthanide shift reagents are also well known as a means of increasing the dispersion of isotropic high resolution NMR spectra (7, 8). Their use for the determination of molecular geometries of the lanthanide : substrate complex has also been reported (8), and again pyridine has been studied (9, 10). Here we report the combination of both of the above experimental techniques in an attempt to study any possible changes in the anisotropy of the proton chemical shifts of pyridine induced by a lanthanide shift reagent, and any change in the pyridine geometry caused by association with the lanthanide reagent. * Present address: Department of Chemistry, California Institute of Technology, Pasadena, California, 91109, U.S.A. t Present address: Science Research Council, State House, High Holborn, London WClR 4TA, England. $ Alfred P. Sloan Foundation Research Fellow, 1971-73. $ Present address: The College of New Caledonia, Prince George, British Columbia, Canada. Copyright 0 1974 by Academic Press, Inc. AI1 rights of reproduction in any form reserved. Printed in Great Britain
167
168
ARMITAGE
ET AL.
2. EXPERIMENTAL
Spectra of pyridine dissolved in the nematic phase of N-(p-ethoxybenzylidene)-p’ti-butylaniline (EBBA) were obtained using varying concentrations of pyridine and the shift reagent tris (dipivalomethanato)europium(III) (Eu(dpm),). Spectra were obtained at different temperatures with both spinning and nonspinning sample tubes on a Varian XL-100 spectrometer operated in both CW and FTmodes at a scale of 1 Hz = 0.2 mm and a linewidth of -7 Hz. Tetramethylsilane (TMS) was used to provide both a reference for chemical shift measurements and an internal lock signal for the CW spectra; and D,O was used as a lock signal for the FT spectra using a coaxial sample tube arrangement (II). Slow spinning of the sample tube has been used previously to decrease temperature and concentration gradients in the sample (12), and thus give narrower linewidths. It has also been used to change the orientation of the sample so as to obtain anisotropies of chemical shifts (13) from equations of the type: d( di’)/dS,, zz
= gag
hl! where (02) is the observed chemical shift of nucleus i in the oriented media with z the space-fixed magnetic field direction, oas Q) is the chemical shift in the p molecule fixed direction caused by a magnetic field in the CImolecule fixed direction for nucleus i, and S,, = 3/2(cos~,cos~,) - l/26,, is the orientation parameter where Bnis the angle between the molecular q axis and the magnetic field direction z. A repeated Greek sufTix denotes summation, and angle brackets a time average. With this method one can vary (ci2) and Sdlrwith spinning speed and can, in principle, calculate the components of ~22 (with an assumed value for Tr(o$)). The techniques of measuring the isotropic shift at raised temperatures, or the shifts at different nematic temperatures (both commonly used to determine the components of o$) are not used here because the isotropic lananthide induced chemical shifts are temperature dependent (8, 9, 14, 1.5) and hence it was desirable to perform all measurements at the same temperature. In all cases where spinning sample tubes were used (except case C, see tables) the ET technique was used to accumulate the entire spectrum at the same time. This method reduces the problem caused by slight changes in spinning speed which affects the line positions. The effect now shows up only in the form of slightly increased linewidth when time averaging experiments are performed. Table 1 gives the experimental conditions used for each spectrum. The spectra were similar to those previously published for pyridine dissolved in a nematic phase (4-6). 3. ANALYSIS
OF
THE
SPECTRA
The analysis was carried out with the iterative computer program LAOCOONOR (4) fitting about 40 observed lines. Statistics of the “fits” are included in Table 1. The values of the Jij used were from Ref. (16). The direct dipole-dipole coupling constants are defined by the equation (3) :
where rij is the distance between the two nuclei i and j, I, is the cosine of the angle between the i-j and y directions, and yk is the magnetogyric ratio of nucleus k. The
EFFECT
OF SHIFT
REAGENTS
TABLE
ON ORIENTED
169
PYRIDINE
1
EXPWMENTAL CONDITIONS”
Case A B c D* E” F’ G” H”
Mole % Eu(dpmh
Mole % pyridine
Spinning speed (Hz)
RMS error from LAOCOONOR (Hz)
0
9.2
9.1 9.1 9.1 4.0 4.0 4.0 4.0
0 0 8.0 0 0
0.8
0.3 0.3 0.3 0 0 0.4 0.4
6.5 0 6.0
0.6 1.3 0.7 0.4 0.9 0.5 0.5
a EBBA as solvent with -1 mole % TMS added as internal reference; temperature -40°C. * Temperature -20%. c Measured by FT. Cases B -+ D have [Eu(dpm)J[pyridine] = 0.033; cases 6, H have [Eu(dpm)J[pyridine] = 0.101.
spectral parameters obtained from the “fit” to each spectrum are in Table 2. Errors are at the 68 % confidence level and are calculated by a standard procedure throughout. The signs are absolute if pyridine orients with S,, negative. 4. MOLECULAR
STRUCTURE
The molecular structure of pyridine is well known (4, 5, 17). The iterative weighted least-squares computer program SHAPE (18) was used to calculate geometrical information and orientation parameters from the Dlj in Table 2. The values of a(2), b(2), and b(3) were chosen from microwave data (17) and were not varied. This was necessary because of the large covariance between S,, and b(3) when b(3) was also varied in the least-squares routine; in some cases the program did not converge. Since the purpose of the experiment is to determine any change in the relative proton coordinates caused by the addition of Eu(dpm),, and since this change might be expected to be largest for a(l) and b(l), it seemed reasonable to fix the other proton coordinates. The results obtained from these calculations are in Table 3 ; it will be noted that no significant geometrical changes are detected. The liquid crystal measurements give an average of both the bound and free pyridine. Since each molecule of Eu(dpm), can bind up to two molecules of pyridine (as well as the liquid crystal solventl) , ,520 % of the pyridine for cases G and H can be bound. Thus, the geometry of the bound pyridine is that of cases G and H (which are the same as the other cases) with at least five times the uncertainty in a(l) and b(l). For cases B and C the uncertainty is even larger. An experiment performed with a far greater 1 Lanthanide shift reagents associate to some extent with any organic molecule having oxygen or nitrogen donor atoms. Hence it is possible that the Eu(dpm), associates with EBBA; however, the relatively low solubility implies that although some such association likely occurs, it certainly is not overwhehelmingly strong.
a See Table
-296.4 -265.8 -194.0 -281.9 -349.1 -275.9 -259.6 -202.1
0.2 0.1 0.3 0.1 0.1 0.2 0.1 0.1
-53.6 -48.9 -35.8 -52.1 -63.8 -49.8 -50.0 -38.8
I for experimental
i: + i + + + + +
DJ2
+ rf: L+ k + + 5
013
conditions.
0.3 0.2 0.4 0.2 0.2 0.4 0.2 0.2
-30.6 -29.7 -21.7 -31.5 -37.5 -29.4 -33.8 -26.6 Values
+ f + 2 & + 2 k
D14
100 MHz
0.2 0.1 0.3 0.1 0.1 0.2 0.1 0.1
-46.9 -46.7 -34.7 -50.0 -58.5 -46.2 -57.2 -44.0
rt: k 5 & f + + +
0.3 0.2 0.4 0.3 0.2 0.4 0.1 0.2
PARAMETERS
-228.9 -222.1 -162.6 -238.1 -280.9 -222.8 -254.6 -196.6
(IN Hz)
f 0.2 rt 0.2 ri 0.3 IL- 0.2 + 0.1 + 0.3 2 0.1 ir 0.1
OF PARTIALLY
2
-40.1 + 0.3 -41 .o + 0.2 -29.5 * 0.5 -43.8 + 0.2 -51.3 2 0.2 -40.4 i- 0.4 -49.9 + 0.2 -38.8 & 0.2
D 24
ORIENTED
-836.9 -936.5 -944.6 -959.4 -829.2 -835.2 -1080.2 -1090.8
~~~
+t(u’l’) zz
PYRIDINE”
for Jij used are (16) Jlz = 4.9, J13 = 1.9, J14 = 1.0, x15 = -0.1,
NMR
TABLE
k + + * 2 + k Ifr
=
-698.6 -729.3 -731.5 -744.2 -696.1 -697.9 -777.8 -778.8 7.7, ~~~ = 1.3.
0.6 0.5 0.7 0.5 0.5 0.6 0.5 0.5
f + * rt + + I: +
0.6 0.5 0.7 0.5 0.5 0.7 0.5 0.5
+
f. + + + f + i. +
0.6 0.5 0.7 0.6 0.5 0.8 0.5 0.5
;i
.
=;
EFFECT
OF SHIFT
REAGENTS
ON ORIENTED
TABLE GEOMETRIC
Case A B c D E F G W Pb Q’ g
41) -2.057 -2.066 -2.055 -2.069 -2.063 -2.074 -2.063 -2.068 -2.068 -2.056 -2.0557 -2.056
k i. rt + + & rt +
0.005 i% 0.004 0.009 0.004 0.003 0.007 0.002 0.003
3
AND ORIENTATION
PARAMETERS
b(l)
s cm
4 + + + + + k k
0.010 A 0.008 0.023 0.009 0.006 0.016 0.007 0.010
+ f * + rt k f rt
0.0002 0.0001 0.0002 0.0001 0.0001 0.0002 0.0001 0.0001
OF PYRIDINE’+
s cc -0.0656 -0.0614 -0.0453 -0.0662 -0.0788 -0.0623 -0.0671 -0.0515 -0.0748 -0.0868 -0.1018
from
r.m.5. error SHAPE (18) 0.22 Hz 0.10 0.25 0.20 0.14 0.31 0.27 0.09 0.03 0.01 0.03
a The following geometrical values from Ref. (17) are assumed for the calculation b(2) = 0, a(3) = 0, and b(3) = 1.287 A. b Calculated from 60 MHz data in Ref. (4). c Calculated from data in Ref. (5). Q is from 100 MHz spectrum and R is from d Microwave values (17). e See Table I for experimental conditions.
a(2) = -2.1525
L- 0.0014
0.0270 0.0274 0.0199 0.0293 0.0341 0.0272 0.0334 0.0259 0.0270 0.0319 0.0389
171
+ 0.0003 & 0.0002 F 0.0005 + 0.0003 + 0.0002 + 0.0005 k 0.0002 _+ 0.0002
+ 0.001
-2,499 -2.489 -2.501 -2.504 -2.482 -2.481 -2.497 -2.474 -2.481 -2.482 -2.4816 -2.481
PYRIDINE
220 MHz
A,
spectrum.
amount of bound pyridine could decrease this uncertainty. Unfortunately, the lanthanide shift reagent is not very soluble in the liquid crystal, and the use of lower pyridine concentrations would give weak signals, which would require longer time averaging. In particular, experiments using spinning sample tubes would be d&cult. Attempts to take advantage of the stronger binding affinity (19) of t&(2,2-dimethyl6,6,7,7,8,8,8-heptafluoro-3,5-octanedianato) europium(II1) (Eu(FOD),) were unsuccessful because the pyridine-Eu(FOD), complex appeared to crystallize from solution. 5. ANISOTROPY
OF
CHEMICAL
SHIFT
Observed Shift
We wish to relate the observed chemical shifts in Table 2 with molecular parameters. The observed isotropic-nematic shift, (02) - oCi) (using TMS as internal reference), arises from several factors: Eq. [3] indicates how this term may be partitioned into several components : (53
-
5(i)
= dog,
+ do;;;
$ dog.
PI
Aa:& is the observed isotropic-nematic shift for nucleus i arising from intramolecular contributions, and is in general given by AGO&= 2/35$(mol)
Sap.
141
In pyridine, because of the C,, symmetry, only the diagonal elements of o$(mol) and S,, are nonzero.
172
ARMITAGE ET AL.
The term dG,‘iL is the isotropic-nematic shift caused by the presence of the Ianthanide shift reagent. Thus we have, AG~$ = (~2 (Ian)} - G,(ii),, PI where (G$~(lan)) and ~1:; are the induced shifts in the nematic and isotropic phases. We note that ~1':: is commonly expressed as Av/v, or AH/H, in publications on shift reagents (15, 20, 21). If, as is commonly assumed (d), the lanthanide induced shift is pseudocontact in origin, then for most theoretical cases discussed in the literature
@(mol)
(mol)) - of& = 2/3(G,, - 1/2(G,, + Gbb))i& SC, + 1/3(G~ - GM>%I 6, - %d. I?1 In principle three experiments with pyridine oriented in a nematic phase with no shift reagent present are sufficient to determine G~,,~, (‘) 2/3(0,, - 1/2(a,, + G&))$!~ and l/3(Gau G&)2&. Values from experiments A, E, and F are reported in Table 4. The neglect of dGg&
=
(GFi
TABLE CHEMICAL
1 2 3
SHIFT
RESULTS
-8.59 * 0.04 -7.06 + 0.04 -7.45 + 0.04
4
FROM SAMPLES REAGENT"
WI'I~I
-5.1 + 0.6 -1.6 i 0.5 -1.4 rk 0.5
NO LANTHANIDE
SHIFT
9+3 3+3 O&3
a Calculations are based on cases A, E and F. Results are in ppm, and TMS is taken as (I = 0.
EFFECT
OF SHIFT
REAGENTS
ON ORIENTED
PYRIDINE
173
might be expected to increase the uncertainty at least fourfold. Experiments with larger variation in S,, might lead to more accurate values for the above parameters. These experiments might include temperature, solvent and concentration studies of both the nematic and isotropic phases. However, for the present study, we are only concerned with the lanthanide induced shifts at a constant temperature in EBBA as solvent. We shall also assume that the above parameters are a measure of don& + dcrj:t, that is, that there is no change in the local solvent effect upon addition of the shift reagent. This may be roughly valid for TMS and the unbound pyridine, but will certainly be false for the 520 % pyridine bound to the lanthanide ion. Thus, upon addition of the shift reagent, we might expect a change in the local solvent effect 50.2 times that expected for all pyridine bound to the lanthanide ion. This might easily be larger than the uncertainty in the measurement. One further assumption should be pointed out; that is, that the local solvent shift is the same for case A as for cases E and F, which have different pyridine concentrations. AG$$
The Lanthanide Induced Shift We now wish to extract values for AC& G;$, and hence Ao,‘i’,/o,‘ih for both of the two lanthanide concentrations used. We note that for both of the two sets of experiments (B and C) and (G and H) that S,,fS,, differs by only -1 % within a given set, i.e., between spectra from spinning and nonspinning sample tubes. Thus, roughly within experimental error, the orientation parameters for one experiment equals the product of those for the other experiment multiplied by a constant factor. This factor is ((3/2)cos2a - l/2) if, as suggested by Yarmoni (IJ), the effect of spinning the sample tube is to twist the direction of orientation of the nematic phase from the magnetic field direction by an angle 6. Hence, for any set of experiments, the change in any component of S,, is proportional to the change in S,,. Since (Gg) - G(i) = +G$ .&, then {GE) - oci’ a SC,, or d(p$)/dS,, is constant for a given set of experiments. From two experiments (spinning and nonspinning sample tube) one can determine d(og)/dS,, and CT(~), and hence (c$ - &) for each experiment. rs,‘f’,is now easily obtained by subtraction. The results are given in Table 5. The contribution to the shift from Aa$i, can be calculated for experiments B, C, 6, and H using Eq. [8] and the corresponding orientation parameters. Assuming that Aa:& includes contributions from do,‘& ACT& and hence da,(fh/a& are obtainable by substitution into Eq. [3]. Values are in Table 5. The quantities Aoj$ and A&/G:~!, are purely experimental observables related to the space-fixed coordinate system, and their values vary with SGs.Relation of these results to the molecular parameters tas would require the use of Eq. [6], but this calculation has not been attempted at the present time for several reasons. First of all, the Sz, reported are an average for the free and bound pyridine; the value of S,, for the bound complex is required for Eq. [6] and is almost certainly different from the average.
174
ARMITAGE
ET AL.
TABLE
5
CHEMICALSHIFT RESULTS FROMSAMPLESWITHLANTHANLDESHIFTREAGENTADDED d(u”‘> Dz Case
Nucleus 1
[Eu(f~+l
[pyrrdlne]
*“’
d&c
@pm>
(PPm)
(1) *Ian
AC$‘,
6mm)
hm-4
B 0.033
-4.9
+ 0.5
-9.67
i 0.03
-1.07
G -6.8
rf: 0.5
-11.26
i: 0.03
-2.66
B 0.033
-1.3
f 0.5
-7.37
k 0.03
-0.32
G -0.6
k 0.5
-7.82
i: 0.03
-0.76
B 0.033
-0.6
f 0.6
-7.69
2 0.03
-0.23
G H
-0.1
* 0.5
-8.12
+ 0.03
-0.67
0.04 + 0.04
-0.04
rt 0.04
0.12 * 0.05
-0.05
+ 0.02
0.09 If: 0.04
-0.03
* 0.02
0.003
+ 0.04
-0.01
+ 0.7
0.004
zk 0.03
-0.01
& 0.5
-0.07
rt 0.05
0.09 IO.09
-0.06
5 0.04
0.07 rt 0.07
-0.05
+ 0.05
0.2 + 0.8
-0.04
* 0.03
-0.09
+ 0.05
0.2 + 0.6 0.13 f 0.12
-0.07
Ik 0.04
0.10 * 0.09
2 0.05
C 0.101
_+ 0.05
+ 0.05
H 3
-0.05
+ 0.05
C
0.101
0.06 rir 0.05
+ 0.05
H 2
(1) *1an
* 0.05
c
0.101
ALT$$
+ 0.05
In addition, as pointed out by X-ray crystallographic work (24), the pyridine molecule is not on a symmetry axis in the solid complex, and the ELI-N direction makes an angle of N 8” with the plane of the pyridine ring. Recent work on pyridine (and other compounds) dissolved in carbon disulphide in the presence of Eu(dpm), at low temperatures yields two ortho and two meta resonances, demonstrating the asymmetry of the complex (25). Both the crystallographic (24) and shift reagent (25) work discussed above point out the incorrect use of a simple P,(cosO) relationship (20) for the interpretation of lanthanide induced shifts in isotropic phases. These studies indicate that more than two components of S,, are needed to describe the orientation of the complex dissolved in a nematic phase. Unfortunately, only two components of S,, innuendo the observed spectra, which are an average of rapidly exchanging free and bound pyridine; the DIj between the pyridine and the Eu(dpm), are not observed. Therefore, the other components of S,, needed for use in Eq. [6] are not available from the present study. A further problem is the necessity of making assumptions about: (1) the geometry of the pyridine-Eu(dpm), complex (which would give the components of the Tea tensor in Eq. [6]), (2) the symmetry and direction characteristics of the (FblBtensor, and (3) the magnitude of any contact interaction. Because of the problems discussed above, trial calculations that have been performed
EFFECT
OF SHIFT
REAGENTS
ON ORIENTED
PYRIDlNE
175
(26) are not reported. The only significant result is that do~jlh/ol& is of similar order of magnitude as S,, in both sets of experiments. Hence the anisotropy in the lanthanide induced shift is small. 6. CONCLUSIONS
In isotropic phases, the large chemical shifts induced by the addition of Eu(dpm~~ can be used to increase the dispersion of proton spectra so as to give simple, first order type spectra (7). This was not the case with pyridine dissolved in EBBA. In the NM spectra of molecules oriented in liquid crystalline solvents, the dipole-dipole couplings are normally of the order of kHz. Thus, we do not envisage shift reagents being of much practical use in solving the NMR spectra of oriented molecules, except perhaps in some special cases combined with the use of high field spectrometers. Furthermore, even if one did succeed in separating out the resonances from the different protons, one would only measure 12Dij + Jjjj (27). That is, one would automatically lose valuable information about the isotropic spin-spin coupling constants. To relate the determined value of d~j-~~/~f~~ to meaningful molecular information, many assumptions have to be made. Preliminary studies which have been performed yield no useful information (26). Nonetheless AG~~~/G(~) lan is of similar order of magnitude as S,,. Hence, the anisotro in cr,,(lan) is nbt significantly greater than its isotropic value. Any change in geometry caused by binding to the shift reagent is not detected in t.he present experiments, but should be checked with a larger value of the ratio [lanthanide]/[pyridine], or in another molecule where the expected effects might be larger. ACKNOWLEDGMENTS We wish to thank the following for very helpful discussions, ideas and support: Professors P. Diehl, F. G. Herring, C. A. McDowell, and A. G. Marshall, Drs. W. Niederberger and C. S. Yannoni and Mr. J. Vogt ; we also wish to thank a referee for helpful comments. The financial support of the National Research Council of Canada is gratefully acknowledged. REFERENCES
1. A. SAUPE AP\?) G. ENGLERT, 2. 3. 4. 5. 6. 7. 8. 9. 10.
IJ. 12. 13.
Whys. Rev. Lett. 11,462 (1963). A.D.BUCKINGHAMAND K. A. MCLAUCHLAN, Prog.NMR Spectrosc.2,63 (1967). P. DIEHL AND C. L. KHETRAPAL, “NMR-Basic Principles and Progress,” Vol. 1, Springer-Verlag, New York, 1969. P. DI~HL, C. L. KHETRAPAL, AND H. P. KELLERHALS, MO/. Phys. 15,333 (1968). E. E. BURNELL AND C. A. DELANGE, Mol. Phys. 16,95 (1969). C. A. VERACINI, M. LONGERI, AND P. L. BARILI, Chem. Phys. Lett. 19,592 (1973). C. C. HINCKLEY, J. Am. Chem. Sot. 91,516O (1969). JACQUES REUBEN, in “Prog. NMR Spect.” (J. W. Emsley, J. Feeney, and L. H. Sutcliffe, Eds.), Pergamon Press, New York, Vol. 9, p. 1 (1973). 1, M. ARMITAGE, Ph.D. Thesis, University of British Columbia, Vancouver 8, Canada, 1973. (a) C. BEAUTY, Z. W. WOLKOWSKI, ANLI N. THOAI, Tetrahedron Lett. 817 (1971); (b) J. REUBEN AND J. S. LEIGH, JR., J. Am. Chem. Sot. 94,2789 (1972); (c) R. A. FLETTON, G. F. H. GREEEN, AND J. E. PAGE, Chem. Znd. (London) 167 (1972); (d) 0. A. GANSOW, P. A. LOEPFLER, R. E. DAVIS, M. R. WILLCOTT III, AND R. E. LENKINSKI, J. Am. Chem. Sot. 95,339O (1973). G. ENGLERT, Z. Naturforsch 27a, 715 (1972). A. D. BUCKINGHAM, E. E. BURNELL AND C. A. DELANGE, Mol. Phys. 15,285 (1968). C. S. YANNONI, Z.B.M. J. Rex Develop. 15,59 (1971).
176 14. 1.5. 16. 17. 18. 19, 20. 21. 22. 23. 24. 2.5. 26. 27.
ARMITAGE
ET AL.
C. BEAUTY, S. CORNUEL, D. LELANDAIS, N. THOAI, AND Z. W. WOLKOWSKI, Tetrahedron Left. 1099 (1972). B. BLEANEY, J. Magn. Resonance 8,91 (1972). S. CASTELLANO, C. SUN, AND R. KOSTELNIK, J. Chem. Phyx 46,327 (1967). B. BAK, L. HANSEN-NYGAARD, AND J. RASTRUP-ANDERSEN, J. Mol. Spectrosc. 2,361 (1958). P. DIEHL, P. M. HENRICHS, AND W. NIEDERBERGER, Mol. Phys. 20,139 (1971). I. M. ARMITAGE, G. DUNSMORE, L. D. HALL, AND A. G. MARSHALL, Can. J. Chem. 50,2119 (1972)). H. M. MCCONNELL, AND R. E. ROBERTSON, J. Chem. Phys. 29,136l (1958). B. BLEANEY, C. M. DOBSON, B. A. LEVINE, R. B. MARTIN, R. J. P. WELIAMS, AND A. V. XAVLFR, J.C.S. Chem. Comm. 791 (1972). A. D. BUCKINGHAM, E. E. BURNELL, AND C. A. DELANGE, J. Am. Chem. Soe. 90,2972 (19683, and references therein. A. D. BUCKINGHAM, E. E. BURNELL, AND C. A. DELANGE, J. Chem. Phys. 54,3242 (1971). R. E. CRAMER AND K. SEFF, J.C.S. Chem. Comm. 400 (1972). R. E. CRAMER AND R. DUBOIS, J. Am. Chem. Sot. 95,3801(1973). E. E. BURNELL, unpublished results. E. E. BURNELL AND P. DIEHL, Can. J. Chem. 50,3566 (1972).