An investigation into the origins of polar substituent effects upon 19F chemical shifts, using 4-substituted P ,P-difluorostyrenes

WILLIAM F. REYNOLDS, VICTORIA G. GIBB. A N D NICK PLAVAC Department ofchemistrj , Uriitersity of Toronto, Toronto, Ont., Canada M5S / A /

Received November 14, 1979

WILLIAM F. REYKOLDS, VICTOKIA G. GIBB, and NICK PLAVAC. Can. J. Chem. 58.839 (1980) IyF, I3C. and 'H chemical shifts have been determined for p .b-difluorostyrene and eight 4-substituted derivatives. The p-fluorine

chemical shift difference, A6,, is used to evaluate the constant in the Buckingham equation. A = 3.0 x lo-'' esu for C-F bonds which is in excellent agreement with the value derived by Adcock and Khor. This allows accurate estimates of direct field effect contributions to I9F chemical shifts in aryl fluorides. Substituent parameter correlations demonstrate that the primary polar effect on I9F chemical shifts is field-induced .rr polarization. Ab initio molecular orbital calculations confirm that the suhstituent-induced IyF chemical shifts reflect changes in fluorine rc electron density.

WILLIAM F. REYNOLDS. VICTORIA G. GIBB et NICK PLAVAC. Can. J . Chem. 58,839 (1980). On a determine les deplacements chimiques du I9F, du I3C et du IH des p ,P-difluorostyrenes et de 8 derives substitues en position

4. On utilise la difference de deplacement chimique, A&. d'un fluor en p pour evaluer la constante de I'equation de Buckingham. Cette constante a pour valeur A = 3.0 x 10-I' ues dans le cas des liaisons C-F ce qui est en parfait accord avec la valeur calculee par Adcock et Khor. Ceci permet une evaluation precise des contributions de l'effet de champ direct sur le deplacernent chimique du I9F dans !es fluorures d'aryles. Les correlations des parametres de substituant demontrent que I'effet polaire primaire. sur les dtplacements chimiques du IyF, est une polarisation .rr induite par le champ. Des calculs ab inirio des orbitales moleculaires confirment le fait que les deplacements chimiques du I9Finduits par le substituant refletent un changement de la densite des electrons n du fluor.

[Traduit par le journal]

Introduction where AE, (= E,(C-F(a)) - E,(C-F(b))) is the While in the past there has been considerable difference in electric field components acting along

controversy about the origins of polar substituent the two C-F bonds.2 In turn, A can be used in the effects upon I9F chemical shifts in aryl fluorides (1, estimation of field effects in other systems. 2), recent work on several model systems has dem- This approach has been tried (3), using previ- onstrated that the main polar effect (except at very ously reported data for I (12). However, the data close range) is field-induced polarization of the were not entirely satisfactory for this purpose since aromatic n electron system (3-8). However, there individual chemical shifts were only reported to is still uncertainty concerning the exact magnitude kO. 1 ppm, compared to a total range of A6,of only of the minor contributions to 19F chemical shifts 0.9 ppm. In addition, only one strongly polar sub- due to direct field effects (3-8). As previously stituent (CN) was included in the data set (12). pointed out (3), an excellent system for evaluating Adcock and Khor have independently estimated a direct field effects is a series of 4-substituted P ,P- value of A from data for phenylbicyclo[2.2.2]oc- difluorostyrenes, I: tanes, I1 (4):

/H

xec\c-F(a) X M F / I1

F(b) However, Ewing and Toyne have also investigated I I1 and concluded that direct field effects were neg-

Assuming identical through-bond contributions to ligible in this and similar systems (6, 8). Con-

6 ~ ( a j and 6 ~ ( h j (9), A6 , ( 6 F(a) - 6 F(bj) should provide sequently a careful reinvestigation of I seemed

a monitor of direct (through-space) field effects.' justified in order to compare estimated values of A This allows evaluation of the constant, A , in the from two different systems. It was felt that this

Buckingham equation (1 I): would remove the final uncertainty concerning

[ I ] h6,= A(AE3 ZThe Buckingham equation also contains a term in EZ (1 1). However this should make a neglible contribution at the dis- tances involved (9). In addition, the Buckingham equation was

IThis approach, when applied to the corresponding 4- derived for axially symmetric bonds. While strictly speaking this substituted styrenes (Y), has been cited as providing convincing is not true for the C-F bonds in both I and aryl fluorides, this proof for direct fieid effects on 'H chemical shifts (10). should not introduce any significant error.

0008-4042/80/080839-07$01.00/0 a1980 National Research Council of CanadalConseil national de recherches du Canada

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CAN J . CHEM. VOL. 58. 1980

TABLE 1. lYF. 13C, and vinyl IH chemical shifts for 4-substituted P.P-difluorostyrencs

Chemical shift

X

H N(CH3)2 OCH, CH3 F C1 CF3 CN NO*

"In oom reiatlve to internal CFCI, Hirh field shifts are neratlve - b~8 ; : 8 - CIn ppm relative to ~nterna l Si(CH,),. Loiv field shifts are positive 'Qverlapp~ng peaks near8 128.8. 'Not observed.

contributions of direct field effects to 19F chemical shifts.

There is also uncertainty about the relationship between field effects upon I9F chemical shifts and upon fluorine electron densities (3, 4). While CND012 molecular orbital calculations for I pro- ved some insight into this relationship. the calcu- lated changes in atomic charges were. in many cases, comparable to the uncertainties in the charges. Consequently, it was decided to carry out more accurate ah initio (STO-3G minimal basis set (13)) calculations on I since this type of calculation has already proved very useful for understanding 19F chemical shifts (14). Results of the experimen- tal investigation and calculations are reported below.

Results 19F chemical shifts and 19F-19F coupling con-

stants were determined by analysis of the AB I9F spectra obtained with 'H decoupling. 'H chemical shifts and 'H-19F coupling constants were deter- mined by ABX analysis of the 'H and I9F spectra for the difluorovinyl groups. 19F-I3C coupling con- stants were also determined by ABX analysis, using the I9F parameters from the AB analysis in conjunction with the 19F-I3C splittings observed in the 'H-decoupled 13C spectra. Chemical shifts are tabulated in Table 1 and coupling constants in Table 2.

Substituent-induced changes in calculated fluorine and vinyl carbon atomic charges as well as changes in o and n components of these charges are summarized in Table 3.

Discussion ( a ) Chewzical Shifts

Taft's 0, a n d ~ ~ s u b s t i t u e n t constants (15) can be used to separate polar and resonance contributions

TABLE 2. I9F-IyF. 'H-19F. and I3C-"F coupling constants (in Hz) for 4-substituted P,P-difluorostyrenes

Coupling constants

X F-F H-F(a) H-F(b) C-F(a) C-F(b)

H +32.4 f 3 . 8 N(CH,), +41.2 +4.0 OCH, +37.9 +3.7 CH3 f 3 4 . 7 +3.8 F +33.5 +3.7 C1 +30.3 +3.5 CF3 +25.5 +3.3 C N +22.3 +3.3 NO* f 2 0 . 2 +3.2

to chemical shifts. Results of these correlations are summarized in Table 4. In the case of the 19F

chemical shift correlations, significantly better cor- relations are obtained when the fluoro derivative is omitted (e.g. correlation coefficient, r., for 6F(b) is respectively 0.993 and 0.997 with and without the fluoro derivative). However, the p, and p, values change to an insignificant degree. While the origins of the abnormal behaviour are unclear, similar problems have been noted in previous investiga- tions, e.g. see ref. 9 where similar deviations were noted for vinyl lH chemical shifts in 4- fluorostyrene.

The correlations for and 6,,, show the ex- pected pattern of substituent effects. pR is very similar for each fluorine, confirming that through bond effects are nearly equal for the two fluorines. p, is 17% larger for 6 ,,, than for 6 F(bp (3 I provides a measure of the field effect of a substituent (16, 17). Since the field effect depends upon the component of the electric field acting along the C-F bond (1 1) and since this is greater for 6,(,, (3), the enhanced p, value for this fluorine can be attributed to direct field effects.

Although p, values are very similar for 6F(a) and

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REYNOLDS ET AL. 84 1

T ~ B L E 3. Net atomic charges ( x lo4) a n d o and x components of these charges ( x lo4) for the vinyl group of P.P-difluorostyrene and substituent-induced changes in these parameters ( x 10" for 4-substituted

fl ,P-difluorostyrenes

k'' q ,,eP

Parameter H NH, OH CH, F CF3 CN NOz

"Atomic charge relatlve to neutral atom ( x lo4). Negative sign corresponds to increased electron density. bSubstituent-induced changes in atomic charges ( x lo4). "Aq = qmai - iirri,~.

TABLE 4. Substituent parameter correlations for experimental chemical shifts and coupling constants and calculated charge densities for 4-substituted P ,P-difluorostyrenes

Substituent Variable parameter PI(PF)' P R' r FC

CLWeighting factors in correlation equatlon vanable = p,o, + hoRO + C o r vanable = ppTF + prtTR0 + C. *Correlation coefficient. ' F test of variance. dFluoro derivative omitted from correlation (see text)

6,,,, there is a sufficient difference (ca. 4%) to standard geometries, except that the G(1)- result in a noticeable resonance contribution to A&, C(a)-C(P) angle was taken as 128.5" (see Details of (particularly for the N(CH,), derivative). Con- Calculations). Results are summarized in Table 5 . seauentlv AZjpis not a ~ r e c i s e measure of direct field A correlation of o,o, versus AE, vields: . - - - - efficts. h i e v e r , fieid effects for individual sub- stituents can be estimated from p l o t for the A6, [2] pto t= (3.01 + 0.12) x 10-"AE, (r = 0.996)

correlation. These can be compared with values of However, since the calculations are sensitive to the BE, estimated by classical electrostatic calcula- assumed geometry (3), a more reasonable estimate tions. E, values were estimated assuming that the of the uncertajnity in A is probably (3.0 + 0.5) x substituent acts as a point dipole and using R-X aliphatic group dipole moments for hydrocarbon 3The corresponding correlation of versus AE, glves A = solvents (18). Calculations were carried out using 3 . 3 x 10-l1 esu (r = 0.947), within these error limits.

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C A N . J . CHEM. VOL. 58, 1980

T ~ B L E 5 . Calculated electric field components used in evaluating A for Buckingham equation

"RX aliphatic group dipole moment (16). bAs\umed distance of point dipole (in A) along C(I)-X bond. 'Electric field component ( ~ n esu) acting along C-F(a) bond. "lectric field component (in esu) acting along C-F(b) bond. 'AE, = E,(A) - E,(B). %timated electric field component to A&, . p, = 0.85 (see Table 4). 'Along C(1)-C bond at midpoint ofC-F bonds p, taken as equal to dipole moment for CH,CF, (29) "1 midpoint of C-N bond.

lo-" This is in excellent agreement with the significantly alter the substituent effects in styrene estimate of Adcock and Khor of (2.5-3.1) x 10-l1 derivatives. However, the range 0 f6~ , , fo r I is very based on 19F chemical shifts for I1 (4). Therefore, it significantly reduced comparted to 4-substituted is felt that A is now sufficiently well defined for styrenes (9), although still proportional to S C ( ~ ) for fluorine attached to sp2 carbons that it is possible to the latter system: make nearly quantitative calculations ofdirect field contributions to I9F chemical shifts in arvl [4] 6 C(P)(I) = (0.185 L 0.036)6 cta,(styrene)

fluorides, thus resolving more than ten years bf (U = 0.996) controversy in this area.

The electrostatic calculations predict negligible field effects for since the field acts almost at right angles to C-F(b). However the o I ,oRO corre- lation gives a significant pI value for tiF(b) (Table 4). This must reflect the field-induced n polarization (the n polarization effect of a polar substituent is also proportional t o o ,(16)). In the absence of direct field effects, substituent-induced changes in carbon n electron density result in proportional changes in n electron density and chemical shift of the fluorine bonded to carbon (3). This must be true for 6,,b, since this parameter is directly proportional to 6c(P) (9) for the corresponding styrene derivatives:

Non-additive I3C chemical shifts are now well documented for para-disubstituted benzene de- rivatives (19-22) and to a lesser extent for 4,P- disubstituted styrenes (23). However, eq. [4] illus- trates what is apparently the most extreme example of non-additivity yet found (perfect additivity would require unit slope). In 4,P-disubstituted styrenes, non-additivity has been attributed to variable electron demand by the P-substituted vinyl group (23). However, this cannot be the case for I since this should lead to a non-linear relationship for eq. [4], while the actual correlation shows no evidence of non-linearity. Rather, the fluorines must alter the shift-to-charge ratio for I3C chemical

[3] (0.95 f 0.03)6C(e,(styrene) (r = 0.997) shifts (22). This point is further discussed in part c.

If p1 for 6,(b, is assigned entirely to n polarization, then the p1 value for 6,,,, indicates that the polar effect for this probe is about 85% (4.9015.75) due to n polarization and ca. 15% (0.8515.75) due to direct field effects. This is consistent with previous con- clusions about the relative importance of field and n polarization effects in similar systems (3-8), al- though indicating a slightly greater contribution from field effects than suggested by Toyne and Ewing (6).

6 ell,, 6 ,,,,, and 6. for I give substituent parameter correlations (Table 4) which are very similar to previously reported correlations for styrenes (9, 19), suggesting that the two fluorines do not

4To convert A to SI units, this value should be multiplied by 0.33 x after which its units are mV-'.

(6) 19F-19F, 'H-I9F, and 13C-I9F Coupling Con- stants

The geminal (19F-"F) and uicinal ('H-19F) vinyl coupling constants for I cIosely parallel the corre- sponding 'H-'H coupling constants in styrene (9) in that the ge~ninal coupling constant is much more sensitive to substituent effects than either cicinal coupling constant (see Table 2). 2J,-,is sensitive to both polar and resonance effects (Table 4). It is not possible to unambiguously assign the two I J C p F

coupling constants to F(a) and F(b) since the 13C satellite peaks in the 19F spectra were too weak to be observed. However, the two coupling constants show a similar pattern of substituent effects as 6,,,, and 6F(b) (similar pR values but a larger p, value for one coupling). In view of this parallel, it seems

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REYNOLDS ET AL. 843

logical to assign the coupling which is more sensi- tive to polar effects as 'Jc-,(,). This is consistent with the corresponding 13C-lH coupling constants in A-substituted styrenes (24). 'Jc-,,,, should be more sensitive to field-induced o bond polarization (24).

(c ) Calculated Electron Densities and Cotnpurisorz usith 19F and 13C Che~nical Shifts

The ab initio molecular orbital calculations for P,P-difluorostyrene (Table 3) indicate that the difluorovinyl group acts as a weak n donor to the phenyl group (total n electron transfer of 0.0088 electrons) and a weak o acceptor (total o electron transfer of 0.0134 electrons). These competing ef- fects result in a minimal overall effect for the difluorovinyl group.

The most interesting result concerns the re- lationship between 6, and the fluorine n charge density q,n . Figure 1 shows that there is an excel- lent linear relationship between these parameters. The most impressive feature of Fig. 1 is that it involves a plot of actual chemical shifts for two different fluorines in the same molecule against their actual calculated n charge density rather than a plot of substituent-induced changes in chemical shifts and n charge densities. This is possible be- cause the calculations give a n charge density dif- ference for the parent compound (-0.01 lo), corre- sponding to a calculated chemical shift difference, A6, (- 1.63 ppm). which is in excellent agreement with the experimental value for this compound (A6, = - 1.78 ppm). We believe that Fig. 1 represents the clearest evidence yet obtained for a linear rela- tion between lYF chemical shifts and fluorine n charge densities in a system where the fluorine is bonded to an sp2 carbon.

By contrast, the fluorine total atomic charges do not predict A6,for the parent compound, since the difference in atomic charge between F(a) and F(b) has the wrong sign ($0.00322). Consequently, one can only plot substituent-induced chemical shifts versus substituent-induced changes in atomic charges (i.e. defining 6,(,), 6,(b), qF(,), and qF(b) all as zero for X = H). When this is done, it is found that these are different slopes for 6,(,, (758 + 44 ppmlunit charge) and 8F(b, (890 + 45 ppmlunit charge). The o charge densities in Table 3 suggest that two slopes are obtained because the calcula- tions indicate field-induced o-bond polarization which is relatively much greater than the observed field effects upon chemical shifts. This can be confirmed by correlating q,o and q,n with the theoretical field, T,, and resonance, TRO substituent

FIG. 1. Plot of 19F chemical shifts (6,) for !3 ,p-difluorostyrenes versus calculated n components of net atomic charges for fluorine ( q ~ ) ( x lo4). Positive sign of q,,n corresponds to de- creased electron density relative to a neutral fluorine atom.

parameters which have been derived for use with calculated charge densities (16, 17). The correla- tions (Table 4) show a much greater polar effect for q,(,,o relative to qF(b)o ( P , ( ~ ) / P ~ ( ~ ) = 2.19) compared to the corresponding chemical shift correlations ( P ~ ( ~ ) / P ~ ( ~ ) = 1.17). However, the correlations for q,n closely parallel the chemical shift correlations. This confirms that lYF chemical shifts respond to n rather than total o r o charge density changes (14). It also provides an answer for an apparent dichotomy in the literature (3,4). It appears that the main field effect upon fluorine atomic charges is due to field- induced o bond polarization while the main field effect upon the lYF chemical shifts is due to the weaker polarization of the partial C-F n bond (as previously suggested (4)).

q,,,n gives an extremely precise linear relation- ship with q,(p,n :

This confirms that, as suggested in part ( a ) , 6F(b) simply reflects substituent-induced changes in car- bon n electron density and concomitant changes in fluorine n- electron density.

Correlation of q,,,,n for P versus the corre- sponding parameter for styrene (16) yields:

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844 C.4N. .I. CHEM. VOL. 58. 1980

[6] q,,p,n(I) = (0.78 $. 0.02)qC,,,n (styrene)

This demonstrates that the decreased substituent dependence for tic(@) shown in eq. [4] is only partly due to decreased substituent effects on q,,,,n. Cor- relations of chemical shifts with n charge densities shown:

The former correlation gives a typical value for the shift-to-charge ratio (25, 26) while eq. [8] shows an abnormally small value. While the theoretical rea- sons for substituent-induced variations in the em- pirical shift-to-charge ratio are as yet unclear (21, 22). eq. [7] strongly suggests that it is the decrease in this parameter which is mainly responsible for the non-additive C(P) chemical shifts in I.

Summary and Conclusion Consideration of field effects on the 19F chemical

shift difference, 86, . for 4-substituted P ,P-difluoro- styrenes yields a value of the constant in the Buck- ingham equation, A = 3.0 x lo-" esu. in excellent agreement with a previous independent estimate from another system (4). Therefore it is now possi- ble to make near quantitative estimates of direct field effects on 19F chemical shifts in aryl fluorides. Clear evidence is presented for a linear relationship between 19F chemical shifts and fluorine n electron density and further evidence is obtained that non- additive lSC chemical shifts are mainly due to changes in the shift-to-charge ratio.

Exgerimenhl and Details of Calculations 4-Substituted P.P-difluorostyrenes were generally synthe-

sized by the method of Naae (27) which involves a Wittig reac- tion of (C,H,),P plus CF2Br2 with the appropriate Csubstituted benzaldehyde. Dry CH2CI, was used as a solvent. The reaction mixture was dumped into rapidly stirred pentane. The pentane layer was separated and the pentane was evaporated off. The p .P-difluorostyrene was then purified. either by distillation (X = H, CH,. OCH,, F. Cl. CF,) or by thin layer chromatography (X = CN. NO2. N(CH,),). [(CH,),N],P was used in place of tri- phenylphosphine for the synthesis of the N(CH,), derivative. using glyme as solvent.

The compounds which, with the exception of the CF, deriva- tive. were known compounds were characterized by their 'H and I9F spectra which showed characteristic ABX spectra for the difluorovinyl groups and AA'BB' spectra for the paru- disubstituted benzene functions. Purities were confirmed by "C nmr spectra.

IyF and 'H spectra were obtained on a Varian XL- 100 operat- ing in continuous wave mode at respective frequencies of 94.19 and 100.06 MHz and at 31°C. Solutions contained 1 mol% com- pound in C,D,, with 1% CFCI, and 0.1% Si(CH,), as reference

peaks. Peak positions were obtained as averages of several measurements with a precision of 0.001 ppm. Further dilution of the parent compound and NO, and N(CH,), derivatives indi- cated that the observed chemical shifts differed from infinite dilution values by no more than 0.01 ppm. "C spectra were obtained on the same spectrometer operating in pulsed Fourier Transform mode. Samples were 0.4 .W solutions of the com- pound in CCI, containing 8% C,D,, as a lock signal.

Ah ir~itio (STO-3G minimal basis set (13)) molecular orbital calculations were carried out on a SEL 32/75 computer using the program SUPERGAUSS 78, a modified version of GAUSSIAN 76 (28). developed by M. R. Peterson and R . A. Poirier. A partial energy minimization gave a value of the C(l)--C(cc&C(P) angle of 128.5". Other geometries were identical to those given previ- ously (16, 17). Planar NH, and OH were used in place of N(CH,), and OCH, for reasons of computational economy.

Acknowledgements Financial support from the Natural Sciences and

Engineering Research Council of Canada is grate- fully acknowledged. We thank Professor P. Yates for helpful discussions concerning synthesis, M. R. Peterson and R. A . Poirier for help with the calcu- lations, and Professor R. W. Taft for supplying a preprint prior to publication.

1. J . FLJKUNAGA and R. W. TAFT. J . Am. Chem. Soc. 97.1612 (1975).

2. W. ADCOCK and B. 0 . GUPTA. J . Am. Chem. Soc. 97.6871 (1975).

3. W. F. R ~ Y X O L D S and G . K. HAMER. J. Am. Chem. Soc. 98, 7296 (1976).

4. (a) W. ADCOCK and T. C. KHOR. Tetrahedron Lett. 2063 (1976); (b) W . A ~ c o c ~ a n d T. C. KHOR. J . Am. Chem. Soc. 100,7799 (1978).

5. W. F. REYNOLDS. Tetrahedron Lett. 675 (1977). 6. (a) D. F. E\VIZG, S. SOTHEESWARAN. and K. J . TOYNE.

Tetrahedron Lett. 2041 (1977); (b) D. F . EWING and K . J . TOYNE. J . Chem. Soc. Perkin 11.243 (1979).

7. W. KITCHING, M. BULLPITT. D. GARTSHORE, W. ADCOCK. T. C. KHOR. D. DODDRELL, and I. D. RAE. J . Org. Chem. 42.241 1 (1977).

8. D. F. EWING. Org. Magn. Reson. 12.499 (1979). 9. 6 . K. HAMER. I. R. PEAT, and W. F. REYNOLDS. Can. J .

Chem. 51.897 (1973); 51,915 (1973). 10. W. T. RAYNES. I n The chemical society specialist reports

on nuclear magnetic resonance. Vol. 3. Edited by R. K. Harris. The Chemical Society, London. 1974. p. 16.

11. A. D. BUCKINGHA~I . Can. J . Chem. 38.300(1960). 12. I . D. RAE and L. K. S a i l - r ~ . Aust. J. Chem. 25, I465 (1972). 13. W. J . HEHRE. R. F. STEWART, and J . A. POPLE. J. Chem.

Phys. 51,2657 (1969). 14. W. J . HEHRE. R. W. TAFT. and R. D. TOPSOM. Prog. Phys.

Org. Chem. 12. 159(1976). 15. S. EHRENSON. R. T. C. BROWXLEE. andR. W. TAFT. Prog.

Phys. Org. Chem. 10. l(1973). 16. W. F. REYNOLDS. P. G. M E ~ F Y . and G. K. HAMER. Can. J.

Chem. 55, 522 (1977). 17. P. G. MEZEY and W. F. REYNOLDS. Can. J. Chem. 55, 1567

1 1977). 18. C. W. N . CUXIPER. Tetrahedron, 25. 3131 (1969). 19. R. A. MCCLELLAND and W. F. REYNOLDS. Can. J . Chem.

55,536 (1977).

Can

. J. C

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REYNOLDS ET AL. 845

20. .I. BRO~IILOW. R. T. C. BROWSLEE, R. D. TOPSOM. and R. W. TAFT. J . Am. Chem. Soc. 98.2020(1976).

21. B. M. LYYCH. Can. .I. Chem. 55,541 (1977). 22. R. W . TAFT. Private communication. 23. D. A. R. HAPPER and G. J. WRIGHT. J. Chem. Soc. Perkin

11, 694(1979). 24. W. F . REYNOI-DS, I. R. PEAT. and G. K. HAIIER. Can. J .

Chem. 52.3415 (1974). 25. P. C. LAUTERBUR. J . Am. Chem. Soc. 83. 1838 (1961).

26. H. SPIESECKE and W. G. SCHNEIDER. Tetrahedron Lett. 468 (1961).

27. D. N A ~ E . Ph.D. Thesis, University of Iowa, IowaCity, IA. 1972.

28. GAUSSIAN 76. Quantum Chemistry Program Exchange Program No. 168. University of Indiana, Bloomington, I N .

29. A. L. MCCLELLAN. Tables of experimental dipole mo- ments. W. H. Freeman and Co.. San Francisco. CA. 1963.

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