Long-range 13C,'H and 13C,19F coupling constants as indicators of the conformational properties of 4-fluoroanisole, 2,3,5,6-tetrafluoroanisole, and pentafluoroanisole

TED SCHAEFER, REINO L A A T ~ K A ~ N E N , ' TIMOTHY A . WILD MAN,^ JAMES PEELING,^ GLENN H. PENNER, JAMES BALEJA, AND KIRK MARAT

Department of Chemistry, University of Manitoba, Winnipeg, Man., Canada R3T 2N2

Received October 28, 1983

This paper is dedicated to Professor Gerald E . Dunn on the occasion of his 65th birthday

TED SCHAEFER, REINO LAATIKAINEN, TIMOTHY A. WILDMAN, JAMES PEELING, GLENN H. PENNER, JAMES BALEIA, and KIRK MARAT. Can. J . Chem. 62, 1592 (1984).

Long-range spin-spin coupling constants over six bonds between I3C nuclei in the methyl group and ring protons or "F nuclei in the para position are reported for 4-fluoroanisole, 2,3,5,6-tetrafluoroanisole, and pentafluoroanisole in solution. The couplings are a-.rr electron mediated, as indicated by INDO MO FPT computations and by measurements on anisole, 2,6-dibromoanisole, 2,6-dichloroanisole, 2,6-dibromo-4-fluoroanisole, and 2,6-dibromo-4-methylanisole. On the basis of the measured coupling magnitudes and a hindered rotor model, it is concluded that the barrier to internal rotation about the C,,,2-0 bond in 4-fluoroanisole lies near 6 kcal/mol, is nearly zero in the tetrafluoroanisole, and is somewhat less than L kcal/mol in the pentafluoroanisole. In the latter, the preferred conformation is that in which the rnethoxy group lies in a plane perpendicular to the pentafluorophenyl plane. Some inconclusive dynamic nmr experiments on anisole, including T I , mea- surements, are briefly discussed.

TED SCHAEFER, REINO LAATIKAINEN, TIMOTHY A. WILDMAN, JAMES PEELING, GLENN H. PENNER, JAMES BALEJA et KIRK MARAT. Can. J. Chem. 62, 1592 (1984).

On rapporte les constantes de couplage A longue distance A travers six liaisons entre le noyau I3C du groupe methyle et les protons du cycle ou les noyaux "Fen position para du fluoro-4 anisole, du tttrafluoro-2,3,5,6 anisole et du pentafluoroanisole en solution. Les couplages sont transmis par les tlectrons u-.rr comme l'indiquent (es calculs d'OM INDO FPT et des mesures sur l'anisole et sur les dibromo-2,6, dichloro-2,6, dibromo-2,6 fluoro-4 et dibromo-2,6 methyl-4 anisoles. En se basant sur I'intensitk des couplages mesures et sur un modkle rotatif encombre, on conclut que la barritre la rotation interne autour de la liaison C,2-0 dans le fluoro-4 anisole est voisine de 6 kcal/mol alors qu'elle est voisine de zero dans le cas du tttrafluoroanisole et quelque peu inferieure a I kcal/mol dans le cas du pentafluoroanisole. Dans ce dernier cas, la conformation prtftr te est celle ob le groupe mCthoxy se situe dans un plan perpendiculaire a celui du pentafluorophtnyle. On discute brikvement de certaines exptriences de rmn dynamique sur l'anisole, y compris des mesures de TI, , qui ne permettent toutefois pas de tirer de conclusions.

[Traduit par le journal]

Introduction Various physical techniques (1 -8) indicate4 coplanar heavy

atoms in the preferred conformation of anisole and imply that the barrier to internal rotation about the C,,Z-0 bond is strong- ly state dependent, and that in solution it lies between about 1.5 and 6 kcal/mol (1 cal = 4.184 J). In some treatments, notably those applied to the torsional mode corresponding to oscillation about the C,2-0 bond (10, 1 l ) , a twofold hindering potential is assumed. It is also usually assumed that 2 p - ~ r conjugation accounts for the preferred planarity and that repulsion between the methyl group and the ortho C-H bonds perhaps introduces a fourfold component (7) to the rotational potential (the rota- tional barrier in phenol is twofold (12)).

The most recent calculations on anisole (7, 9) at the ST0 3G MO and 4-3 1 G MO levels of theory imply a very shallow local minimum, if present at all, in the perpendicular conformation, but that a fourfold component does exist. The magnitude of internal barrier to rotation about the C,,?-0 bond is computed (7) as 1.9 kcal/mol, to be compared with a molecular mechan-

I Present address: Department of Chemistry, University of Kuopio, Kuopio 10, Finland.

*Present address: Division of Chemistry, National Research Coun- cil, Ottawa, Ont., Canada K1A 0R6.

Present address: Department of Chemistry, University of Petro- leum and Minerals, Dhahran, Saudi Arabia.

4The literature on anisole conformations and barrier has been re- viewed in detail (9) up to July, 1982, citing some 100 publications.

ics result of 1.9 kcallmol (8). In solution this barrier must be ~,

substantially higher, i s implied by the torsional data and by our coupling data below. In the solid the barrier magnitude is about 11.5 kcal/mol (1).

A technique not yet applied to this problem is the J method (13), in which a long-range nuclear spin-spin coupling with a known conformational dependence is related to the hindering potential and other parameters. This method has had some success in defining the low-energy conformation and the two- fold internal potential of sidechains in benzene derivatives, for example in such compounds as ethylbenzene (14) and thio- phenol (15, 16) derivatives. Such studies utilized couplings over four and six bonds between sidechain protons and ring protons or fluorine nuclei. More recently (17), the couplings over five bonds between sidechain protons and ring para car- bon nuclei, as well as over six bonds between p carbon nuclei and para ring protons have been examined and found to be useful in the J method.

Now, in anisole and its derivatives suitable coupling con- stants would involve a I3C nucleus in the methyl group, the methyl protons displaying no long-range couplings to para protons or fluorine nuclei. A series of experiments on anisole and its derivatives in this laboratory have yielded numerous coupling data for 'H, 19F, and I3C nuclei. These results have not been entirely explicable for anisole, where the fourfold com- ponent of the barrier to rotation about the Cs,,2--0 bond adds an additional complication. However, for the title compounds a reasonable explanation of these data is possible. The coupling

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SCHAEFER ET AL.

TABLE I. Relevant spin-spin coupling constants for some anisole derivatives at 305 K as degassed samples in 10-mm od tubes

6 C H S C H 4 C H J,,.

I C H J f , ' J,,,' J, '

Anisole <0.05"./ 0.1 16(2) <0.05 142.782(3) <0.05" 0.1 12(2) <0.05 143.593(3)

2,6-DiC1' (-)0.603(5) 0.265(7) - 145.441(5) 2,6-DiBr' (-)0.625(7) 0.264(4) - 145.476(7) 2,6-DiBr-4-Me' 0.637(20) 0.264(10) - 145.169(20)

6 C F(H1 5 C F 4 C F J,' J ,,,' J , '

4-F' 0.085(5)/ - - -

2,3,5,6-Tetra-p 0.331(8)*' 0.373(46) 3.843(6) - Penta-F" 0.963(23) 0.373(46) 3.444(15) -

4-F-2.6-diBr" 1.476(20) - - - -

"Enriched to 95% a-"C, 200 mg in 2.5 mL of a solvent consisting of 30 v/v% C,D12 in CS2.

"Enriched as above. approximately 5 mol% in acetone-d,; both samples run at 315 K in an attempt to reduce spin-lattice relaxation rates of a-"C. Diethylether as solvent did not appear to alter the long-range couplings.

'Samples were 50 v/v% in benzene-d,, 0.637 Hz being 'J:.~. "Samples were 50 v/v% in acetone-d6. "Samples were I I mol% in acetone-d,. 'Numbers in parentheses are estimated errors in the last one or two significant figures

for these parameters obtained from first-order splitting patterns; in 2,5-dichloro-, 3,5-dichloro-. and 2,3,5-dichloroanisoles no evidence for couplings to ortho or para protons larger than those in the table could be found.

YThe value of 0.331 Hz is equal to 'J:'~ and was indistinguishable in magnitude from 5 C F J.; . The "C nmr spectrum was acquired with the INEPT pulse sequence.

mechanisms for 6 ~ ; ' H and 6 ~ ; ' F are investigated and the ob- spectral width of about 400 Hz and up to 1000 scans were employed. served numbers are used to show that the barrier in 4-fluoro- The I3C nmr spectra of the fluorine derivatives were also acquired anisole must indeed be rather large, that it is very small or zero under conditions of broadband 'H decoupling. 'The solvents and con-

in the tetrafluoroanisole, and that in the pentafluoroanisole the centrations are indicated in Table I . Variable temperature experiments

preferred conformation is perpendicular. were performed using the temperature controller, previously cali- brated with a thermocouple.

Experimental Materials

A n i ~ o l e - a - ' ~ C was prepared by the standard method (18) in which methyl iodide containing 95 at.% I3c (Merck, Sharp and Dohme) was added to a mixture of zone-refined phenol (Aldrich) and anhydrous K2C03 in acetone. The product was identified by 'H nmr and mass spectrometry. The 2,6-dichloro- and 2,6-dibromoanisoles came from Aldrich, as did the 4-methyl-2,6-dibromophenol used to prepare the corresponding anisole derivative using unenriched methyl iodide. The 4-fluoroanisole and the pentafluoroanisole were purchased from Al- drich, while 2,3,5,6-tetrafluoroanisole was a PCR, Inc. product.

The 2,6-dibromo-4-fluoroanisole was synthesized as follows: 8.96 g of 4-fluorophenol (Pfaltz and Bauer, 0.08 mol) were dissolved in 60 mL CS, containing 10 mL ether in a 250-mL round bottom flask, and 25.6 g of Br, (0.082 mol, ca. 8.5 mL) in 20 mL CS2 were added dropwise at room temperature over a 0.5-h period. The mixture was stirred for 1 day and then refluxed for 3 h, reduced in volume by rotary evaporation under reduced pressure, taken up in ether, washed twice with water, dried over MgSO,, and rotary-evaporated to yield 17.28 g of a crystalline solid (80%). Spectral data confirmed product identi- ty. This 2,6-dibromo-4-fluorophenol was methylated as above (18) and the anisole derivative was submitted to mass spectral and 'H nmr analysis.

Spectral data The spectral data employed in this paper were mainly accumulated

on a WH90 FT nmr spectrometer at a probe temperature maintained at 305 K, unless otherwise stated. The "C nmr spectra of the anisole derivatives were acquired using the INEPT (19) pulse sequence. A typical experiment had a 12-ps PW pulse for carbon, a 43-ps Pw pulse for protons, 1/45 = 1.8 ms, and a 5-s equilibrium delay time. A

Molecular orbital calculations These were carried out on an Amdahl470/V8 computer system and

employed the INDO MO FPT programs (20, 21). The geometry of anisole was that recently described in another connection (22), in which the C 2 C 1 0 angle is given by 120" + 5.0 cos 0 and the COC angle by 112" + 3.0" lcos 0 ) . Here 0 is the angle of twist about the C,2-0 bond, zero corresponding to a heavy-atom coplanar geome- try. Similar calculations on 4-fluoroanisole employed a standard ge- ometry (23). Some computations were performed in which certain off-diagonal Fock matrix elements were forced to vanish, in the man- ner of Barfield (24), thereby providing indications of coupling path- ways.

Results and discussion Conformational dependence of 6 ~ i ' H and 6~i 'F.

In anisole, 6 ~ ; 2 H could not be observed, as illustrated in Fig. 1, and is clearly <0.05 Hz. In the presence of two ortho chlorine or bromine substituents the coupling becomes 0.603(5) and 0.625(7) Hz, respectively. In 4-methyl-2,6-di- bromoanisole, 7 ~ ; . H is 0.637(20) Hz. In the presence of two sizeable ortho substituents, X-ray diffraction data indicate a perpendicular conformation of the methoxy group (25-27). For a u-T mechanism, in which the coupling is transmitted via the T orbitals of the benzene ring, it is expected that 6~i 'H = - 7 ~ ; . H (13, 28) in the dibromoanisole and its 4-methyl deriva- tive. The signs of these two couplings are unknown. However, our INDO MO FPT computations in Table 2 clearly show that 6 C . H . J , is negative and that it has a sin2 0 dependence as expected for a u-T mechanism. In particular, a calculational series in

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SCHAEFER ET AL. 1595

TABLE 2. INDO MO FPT values for 6 C H .

- J,,' In anisole as a function of the tor- sion angle, 0

0 (deg)"." A" B" C'

0 0.009 0.010 0.027 7.5 0.016 0.020 0.025

15 0.039 0.049 0.024 30 0.122 0.158 0.021 45 0.243 0.310 0.016 60 0.355 0.469 0.018 75 0.443 0.594 0.016 90 0.474 0.636 0.016

"The geometry is described in the text and was the same as that used in a satisfactory calculation Of b5~:,1. Clf3 (22).

0 is the angle of twist about the C,,?-0 bond, zero corresponding to a planar form.

' No twisting of CH3 about its 0-C bond, being staggered about the ortho C-H bond for 0 = 0.

he methyl group twists so as to keep two H-H,,,,,,,, distances equal (correlated motion).

"All Fock matrix elements involving oxygen p,, methyl carbon p,, and methyl hydrogen I s orbitals are kept at zero.

fore qualitative support of the conclusion above. Calculations on 2-fluoroanisole, not reproduced here, sug-

gest that 6 ~ : . H changes by 0.01 Hz, at most, in the presence of the ortho fluorine substituent, implying that intrinsic ring sub- stituent perturbations of this coupling are small or negligible. Consequently, 6 ~ : ; H in anisole and its derivatives will be taken as 0.63 + 0.01 Hz in magnitude. Of course, the methoxy group in the 2,6-dibromo derivative is not rigidly held at 0 = 90'. However, if the barrier to internal rotation in this molecule is greater than about 6 kcal/mol, then the quoted value for 6 ~ : o H

is acceptable. Turning to 6 ~ ; H , INDO MO FPT calculations substantiate its

sin' 0 dependence in 4-fluoroanisole but estimate its maximum value as only 0.7 Hz. For 2,6-dibromo-4-fluoroanisole the observed value is 1.476(20) Hz. An underestimate of the mag- nitude of 6 ~ L in 4-fluorotoluene is also found (28b). The methyl group replacement technique (25) is inapplicable as a test of the u-IT mechanism but, in line with the computations for 6 ~ L and 6 ~ : F , the latter will also be taken as a u-IT coupling. The presence of a small contribution to 6 ~ i ' from a mechanism other than u-IT cannot be ruled out, nor can an intrinsic ring substituent perturbation of its magnitude. Yet the discussion below is self-consistent if these assumptions are made, namely that 6 ~ : . F = 1.48 (sin2 0), where (sin' 0) is the expectation value of sin' 0 for the hindered motion.

Conformational deductions (i) 4-Fluoroanisole On the assumption that 6 ~ i . F is a u-IT coupling, its mag-

nitude being 0.085 + 0.005 Hz, one has (sin' 0) as (0.085 t 0.005)/(1.476 t 0.020) = 0.058 2 0.005, where the error estimate depends on the measurement errors only. For a two- fold barrier, tables of (sin' 0) as a function of temperature, reduced moment of inertia, and the barrier are available (29). In the present instance they yield the barrier as 5.6 t 0.5 kcal/mol. If harmonic oscillator instead of free rotor functions are used as a basis, the barrier becomes 5.2 + 0.5 kcal/mol.

'The torsional modes for liquid 4-fluoroanisole gave 6.1

kcal/mol (error not given (1 I)), assuming a harmonic oscillator model and a twofold barrier. If, as indicated by molecular orbital calculations, the fourfold component is a significant part of the barrier in anisole and therefore no doubt also for 4-fluoroanisole, then the present results are only an approxi- mation to the true barrier. According to the torsional fre- quencies, the barrier is markedly state dependent, being only about 3 kcal/mol in the gas phase. Microwave spectra confirm the preferred planar conformation of 4-fluoroanisole (30) and place a lower limit of 1.1 kcal/mol on the barrier in the gas phase and are not compatible with a perpendicular conformer of appreciable stability.

The coupling data were measured in acetone-d6 solution and confirm the rather high barrier found from the torsional data in the liquid. The dipole moment of anisole calculated by S T 0 3G (9) increases by 0.15 D on going to the perpendicular con- former, from 1.18 to 1.33 D. The experimental value is 1.30 + 0.03 D (31). Consequently a polar solvent should stabilize the perpendicular form (32), contrary to experiment.

It seems unequivocal that the barrier to rotation about the C,z-0 bond in 4-fluoroanisole is substantial. Whatever the true shape of the barrier, a small value would entail a much larger 6 ~ : . F than the observed value of c 0 . 1 Hz.

(ii) 2,3,5,6-Tetrafluoroanisole 6 C . H . J p is 0.331(8) Hz and, as before, (sin2 0) is (0.331 *

0.008)/(0.630 + 0.010) or 0.525 _t 0.020, where the 6 ~ : ; H is taken as discussed above. If the 0-CH, bond lay rigidly in a plane perpendicular to the benzene plane, then (sin2 0) would equal unity and would vanish if this bond were fixed in the molecular plane. Free rotation about the C,?-0 bond entails a value of 0.5 for (sin2 0). Consequently, a value of 0.525 suggests a very small barrier of 0.1 f 0.1 kcal/mol (from (sin2 0) versus twofold barrier tables) with a slightly preferred per- pendicular conformation. Whether the latter is indeed of lower energy depends rather crucially on the true value of 6 ~ : ; H . The safe conclusion. whether or not there exists a fourfold com- ponent, is that the internal barrier is effectively zero.

Support for this conclusion can be adduced in three ways. First, the high resolution nmr spectra of 2,3,5,6-tetrafluoro- anisole partially oriented in a nematic phase were consistent with either essentially free rotation or interconversion between 2"+ 1 equivalent positions (n = 1,2,3, . . .) of the methoxy group (33). This result can be taken as support for the conclu- sions reached above on the basis of 6 ~ ; H . Second, if a 4-fluoro substituent reduces the 2 p . . . IT conjugation of the methoxy group, pentafluoroanisole might well display an unmistakable - - -

preference for the perpendicular conformation. This aspect is discussed in the next section. Third, confirmatory evidence might be sought in the temperature dependence of S ~ r . C H ' , as can be seen in the following way.

S ~ : C H ' is a proximate (34) coupling, as shown in unpublished work in this laboratory. Its value is +0.2 to +0.3 Hz when the methoxy group is oriented in the plane of the ring with the methyl group lying trans to an orthoc-F bond. As the methyl group approaches this C-F bond it becomes rather larger than $3 Hz. For a 4.7 mol% solution of the tetrafluoroanisole in acetone-d,, 5 ~ : . C H ' increases from 1.30 Hz at 305 K to 1.45 Hz at 192 K. Because this coupling is largest when the methyl group approaches the ortho C-F bond most closely, this small increase of about Hz/deg implies that the perpendicular

- -

conformer is not the stable conformer. ow ever.-it is con- ceivable that a decrease in temperature stabilizes the planar or

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1596 CAN. J. CHEM. I

a near planar conformation as the density of the polar solution increases. Note that the planar conformation of 4-fluoroanisole is apparently strongly stabilized in solution.

(iii) Pentafluoroarzisole The value of (sin' 0) is (0.963 -+ 0.023)/( 1.476 5 0.020) or

0.652 * 0.025. It seems definite that the low-energy con- formation is perpendicular. In terms of a twofold barrier, its magnitude is therefore 0.7* 2 0. l 4 kcal/mol as obtained from tables of (sin2 0) as a function of the barrier, the temperature, and the reduced moment of inertia. The torsional frequencies in the gas phase yield a twofold barrier of 0.86 kcal/mol, but do not assign the low-energy confromation, nor do photoelectron data (35). The chemical shift of I3C-4 indicates a nonplanar conformation (36). Because apara fluorine substituent reduces the conjugative barrier in N-methylaniline (37) and thiophenol (15, 16), the present result is reasonable in comparison to 2,3,5,6-tetrafluoroanisole. Parenthetically, a para amino sub-

~ ~

stituent in thiophenol induces a perpendicular low-energy con- formation of the sulfhydryl group in thiophenol (16).

Additional evidence for a perpendicular conformer in penta- fluoroanisole comes from the temperature dependence of 5~r .CH' . For a 5 mol% solution in acetone-d6, this coupling decreases from 1.085(2) Hz at 305 K to 1.003(8) Hz at 180 K. For the reasons discussed in the ~revious section. this decrease is consistent with a preferred p&pendicular conformer whose amplitude of oscillation about the C,:-0 bond decreases as the temperature of the sample drops.

(iv) Concerning the barrier inagnit~ide The large decrease in the barrier to rotation about the

C,--O bond in the presence of the two ortho C-F bonds is probably a composite of steric and conjugative interactions. That two ortho C-F bonds can raise a barrier by steric forces is clear from 2,6-difluoroethylbenzene (38). In this molecule the twofold barrier is most likely mainly steric and is 4 to 5 kcal/mol higher than in ethylbenzene itself. Of course, in the ethylbenzene the stable conformer is perpendicular, so that the barrier increases as the ortho C-F bonds introduce increased repulsion with the methyl group in a planar conformer during the rotation about the C,2--CVz bond. In the anisole deriva- tives, the planar conformation is destabilized relative to the nonconjugative perpendicular conformer by the steric re- pulsions between the C-F bonds and the methyl group. As well, because fluorine is a T donor, the ortho C-F bonds will decrease the double bond character of the C,1-0 bond and therefore cause a further destabilization o f t h e conjugative ground state. The net result is a nearly zero internal barrier in the tetrafluoroanisole. Addition of a para fluorine substituent to the latter further decreases the 2p-T conjugation of the methoxy group, leading to a perpendicular ground state in pentafluoroanisole. Of course, the coupling data in Table I and X-ray structure analysis show that ortho substituents larger than fluorine, such as chlorine or bromine, cause very stable perpendicular conformers in anisole.

(v) Reinarks on the barrier in anisole The discussion in previous paragraphs implies a higher bar-

rier in anisole than in 4-fluoroanisole. The torsional data for the two liquids do not support this implication, giving 6.05 and 6.10 kcal/mol for the former and latter, respectively (1 1). The

sin' 0 dependence. The "C nmr spectrum of the methyl group in Fig. 1 suggests that 6 ~ ; ' H can hardly be larger than 0.036 in magnitude. Therefore our coupling data for anisole and 4-fluoroanisole are consistent with a higher barrier in the former, the barrier in the latter being estimated above as 5.6 2 0.5 kcal/mol.

However, a barrier as high as 6 kcal/mol should have led to a positive DNMR result using I3C nmr spectra. Yet, for a dimethyl ether solution at 120 K not even a relative broadening of the C-2 and C-6 peaks was observable, and similarly for the 3,5-dichloro derivative. On the basis of known substituent ef- fects on I3C shifts (39, 40), the difference in chemical shifts between C-2 and C-6 in planar anisole at slow exchange can be hardly less than 6 ppm. It is then a simple matter to show that the free energy of activation for rotation about the C,z-0 bond must be less than 4.7 kcal/mol.

If the small 6 ~ ; ' H for anisole is to be consistent with this result, the potential barrier cannot be purely twofold. The po- tential must increase rather rapidly as the methoxy group leaves the ring plane if 6 ~ ; ' H is to be as small as observed. A rapid rise in potential can be provided by a substantial fourfold com- ponent. A coupling of -0.030 Hz can be fit with V2 and V., values of 2.7 2 0.9 and 2.2 2 0.5 kcal/mol, respectively, yielding a maximum in the hindering potential of 3.8 kcal/mol. 'This number, while consistent with the DNMR result, is incon- sistent with the torsional data in solution. It is noteworthy, however, that in the gas phase (1) the methoxy torsion fre- quency can be reproduced exactly by a V2 of 4.8 kcal/mol and a minor contribution from V4. Even if, as suggested ( l ) , the V4 component becomes more important in solution, the evidence for an increased magnitude of the barrier in solution suggests the inadequacy of the fit to the coupling data. Of course, the -0.03 Hz is only an estimate of 6~; 'H. It may actually be smaller in magnitude (see Fig. 1).

Because TIP measurements can give estimates of site ex- change rates rather higher than those obtainable from line- shapes, an attempt to resolve the unsatisfactory situation de- scribed above involved TIP measurements (41, 42). The result of an extensive set of measurements was that the free energy of activation is less than 4.4 kcal/mol on the assumption that the shift difference between C-2 and C-6 is 6 ppm, again a negative result. Attempts at measuring this chemical shift difference in solid anisole in another laboratory have been unsuccessful so far. In the solid state, this shift is 6.0 ppm for an ethylpheny1 ether derivative (43).

Because our DNMR and TIP measurements did not give positive results, they are not described here in detail.

As far as coupling constants are concerned, the anisole bar- rier problem remains unsatisfactory. In future, other couplings, including l3C,I3C interactions, will be studied. Perhaps a tem- perature dependence of some couplings will be found and may allow a fit to a two-term potential function.

Acknowledgements For financial help we are grateful to the Natural Sciences and

Engineering Research Council of Canada, the University of Petroleum and Minerals, and to the Research Council for the Natural Sciences of the Academy of Finland.

coupling data in Table 1 indicate a substantial barrier for anis- 1. H, K ~ ~ ~ ~ ~ ~ ~ , G , T ~ ~ ~ ~ , and C. GRUNDFELT-FORS~~~, J , ~ ~ 1 , ole. In fact, a classical averaging procedure gave the result that Struct. 77, 51 (1981), and references therein. V, is 6.0 kcal/mol and V4 is very nearly zero if 6 ~ ; ' H is as large 2. K. S. DHAMI and J . B. STOTHERS. Can. J . Chern. 44, 2855 as 0.036 Hz in magnitude and 6~:iH is -0.640 Hz with a strict (1966).

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3. P. F. OESPER, C. P. SMYTH, and M. S. KHARASCH. J. Am. Chem. Soc. 64, 937 (1942).

4. S. K. GARG and C. P. SMYTH. J. Chem. Phys. 46, 373 (1967). 5. D. G. LISTER and N. L. OWEN. J. Chem. Soc. Faraday Trans. 2,

69, 1304 (1973). 6. H. M. SEIP and R. SEIP. Acta Chem. Scand. 27, 4024 (1973). 7. M. KLESSINGER and A. ZYWIETZ. J. Mol. Struct. 90, 341 (1982),

Theochem, and references therein. 8. H. DODZIUK, H. V. VOITHENBERG, and N. L.ALLINGER. Tet-

rahedron, 38, 28 1 1 (1981), and references therein. 9. T. A. WILDMAN. Ph.D. Thesis, 1982. University of Manitoba,

Winnipeg, Canada. 10. N. L. OWEN and R. E. HESTER. Spectrochim. Acta, Part A, 25,

343 (1969). I I. G. ALLEN and S. FEWSTER. 111 Internal rotation in molecules.

Editedby W. J. Orville-Thomas. Wiley and Sons, London. 1974. Chapt. 8.

12. N. W. LARSEN and F. M. NICOLAISEN. J . Mol. Struct. 22, 29 (1974).

13. W. J. E. PARR and T. SCHAEFER. ACC. Chem. Res. 13, 400 (1980).

14. T . SCHAEFER, L. KRUCZYNSKI, and W. NIEMCZURA. Chem. Phys. Lett. 38, 498 (1976).

15. T. SCHAEFER and W. J . E. PARR. Can. J . Chem. 55,552 (1977). 16. T. SCHAEFER and T. A. WILDMAN. Chem. Phys. Lett. 80, 280

(1981). 17. T. SCHAEFER, J. PEELING and G. H. PENNER. Can. J . Chem. 61,

2773 (1983). 18. L. F. FIESER and M. FIESER. Reagents in organic synthesis. Wiley

and Sons, New York. 1967. pp. 682f. 19. G. MORRIS and R. FREEMAN. J . Am. Chem. Soc. 101, 760

(1979). 20. J . A. POPLE, J . W. MCIVER, and N. S. OSTLUND. J . Chem. Phys.

49, 2960 ( 1968). 21. P. A. DOBOSH. Quantum Chemistry Program Exchange, 11, 141

(1979). 22. T. SCHAEFER and R. LAATIKAINEN. Can. J. Chem. 61, 224

(1983).

23. J. A. POPLE and M. GORDON. J. Am. Chem. Soc. 89, 4253 (1967).

24. M. BARFIELD. J. Am. Chem. Soc. 102, 1 (1980). 25. M. W. WIECZOREK. Acta Crystallogr. Part B, 36, 1515 (1980). 26. G. W. BURTON, Y. LEPAGE, E. J. GABE, and K. U. INGOLD. J.

Am. Chem. Soc. 102, 7792 (1980). 27. G. W. BURTON and K. U. INGOLD. J. Am. Chem. Soc. 103,6472

(1981). 28. (a) R. A. HOFFMAN. Mol. Phys. 1, 326 (1958); (b) R.

WASYLISHEN and T . SCHAEFER. Can. J. Chem. 50, 1852 (1972). 29. R. VEREGIN. M.Sc. Thesis, 198 I. University of Manitoba. Win-

nipeg, Canada. 30. D. G. LISTER. J . MoI. Struct. 68, 33 (1980). 31. N. L. ALLINGER, J . J. MAUL, and M. J . HICKEY. J. Org. Chem.

36, 2747 (1971). 32. R. J . ABRAHAM and E. BRETSCHNEIDER. In Internal rotation in

molecules. Edited by W. J. Orville-Thomas. Wiley and Sons, London. 1974. Chapt. 13.

33. J . W. EMSLEY, J . C. LINDON, and D. W. STEPHENSON. J . Chem. Soc. Perkin Trans. 2, 1794 (1975).

34. J . HILTON and L. H. SUTCLIFFE. Prog. Nucl. Magn. Reson. 10, 27 (1925).

35. A. D. BAKER, D. P. MAY, and D. W. TRUNER. J. Chem. Soc. B, 22 (1968).

36. J . W. EMSLEY. J. Chem. Soc. A, 1459 (1968). 37. L. LUNAZZI, C. MAGAGNOLI, and D. MACCIENTELLI. J . Chem.

Soc. Perkin Trans. 2, 1704 (1980). 38. T. SCHAEFER, R. P. VEREGIN, R. LAATIKAINEN, R. SEBASTIAN,

K. MARAT, and J . L. CHARLTON. Can. J. Chem. 60,261 1 (1982). 39. R. JOST, J. SOMMER, C. ENGDAHL, and P. AHLBERG. J. Am.

Chem. Soc. 102, 7063 (1980). 40. W. KITCHING, I. DEJONGE, W. ADCOCK, and A. N. ABEY-

WICKREMA. Org. Magn. Reson. 14, 502 (1980). 41. C. DEVERELL, R. E. MORGAN, and J. H. STRANGE. Mol. Phys.

18, 553 (1970). 42. P. STILBS and M. M. MOSELEY. J. Magn. Reson. 31, 55 (1978). 43. L. B. ALEMANY, D. M. GRANT, R. J. PUGMIRE, T. D. ALGER,

and K . W. ZILM. J. Am. Chem. Soc. 105, 2133 (1983).

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