The conformational equilibria of 3-methoxybenzaldehyde in polar and nonpolar solutions. 'H NMR and MO calculations

TED SCHAEFER, RUDY SEBASTIAN, HUA YAN LI,' AND TINH QUACHI Department of Chemistry, University of Manitoba, Winnipeg, Man., Canada R3T 2N2

Received August 3 1, 1989

This paper is dedicated to Professor Arthur S. Perlin on the occasion of his 67th birthday

TED SCHAEFER, RUDY SEBASTIAN, HUA YAN LI, and TINH QUACH. Can. J . Chem. 68, 996 (1990)

Analyses of the 'H nuclear magnetic resonance spectra of 3-methoxybenzaldehyde in CS2/C6DI2 and in acetone-d6 solutions at 300 K yield the values of stereospecific long-range spin-spin coupling constants for the aldehydic and methyl protons. These coupling constants are used to deduce the conformational bias for both substituents. The conformers in which the carbonyl group lies trans to the methoxy group have a population PI , which is 0.63(2)Pc for the CS2/C6D12 solution, PC being the population of the conformers in which the carbonyl group lies cis to the methoxy substituent; in acetone-d6 solution, P, = 0.80(2)PC. Both of these numbers are substantially larger than that deduced from nuclear Overhauser enhancements for a CDC13 solution. If PI is the fractional population of the conformers in which the methoxy is oriented trans to the aldehyde substituent, then PI = 0.53(1)PC in CS2/C6D12 and PI = 0.67(1)Pc in acetone-d6, both numbers again being rather larger than that deduced from polarization experiments. STO-3G and 4-21G MO computations of the conformer stabilities agree qualitatively with the relative stabilities of the four planar conformers. Extrapolation of the populational data in solution yields vapor phase estimates midway between the two theoretical estimates. Limits can be placed on the fractional population of each of the conformers in the vapor and in the solutions. The relative stabilities of the free conformers can be rationalized in terms of electrostatics and of preferences of the carbonyl and methoxy groups for certain orientations with respect to the ring C-C bonds of highest double bond character.

Key words: 3-methoxybenzaldehyde, NMR; 3-methoxybenzaldehyde, conformations for solutions and vapor; 3-methoxy- benzaldehyde, MO calculations.

TED SCHAEFER, RUDY SEBASTIAN, HUA YAN LI et TINH QUACH. Can. J . Chem. 68, 996 (1990). Une analyse des spectres RMN du 'H du 3-mtthoxybenzaldthyde mesurCs a 300 K, en solutions dans le CS2/C6D12 ainsi que

dans I'acCtone-d6, foumit les valeurs des constantes de couplage spin-spin a longue distance qui sont sptcifiques aux protons de I'aldthyde et des mtthyles. On a utilist ces constantes de couplage pour dtduire les tendances conformationnelles des deux substituants. Pour la solution dans le CS2/c6Dl2, la population P, des conformkres dans lesquels le groupement carbonyle se trouve en position trans par rapport au groupement mCthoxy est Cgale i 0,63(2)P, et PC correspond 5 la population des conformkres dans lesquels le groupement carbonyle est cis par rapport au groupement mtthoxy; pour la solution dans I'acCt0ne-d6, P, = 0,80(2)P,. Ces deux valeurs sont beaucoup plus Clevtes que celles obtenues en faisant appel a des mesures de rehaussement de I'effet Overhauser pour des solutions dans le CDC13. Si la valeur P, est Cgale a la fraction de la population des conformkres dans lesquels le groupement mtthoxy est orient6 trans par rapport au substituant aldkhydique, alors PI = 0,53(1)P, pour la solution dans le CS2/C6DI2 et P, = 0,67(1)P, pour la solution dans I'acktone-d6 et ces deux valeurs sont encore une fois plus ClevCes que celles obtenues i partir d'exptriences de polarisation. Des calculs d'OM aux niveaux STO-3G et 4-21G conduisent a des stabilitCs des conformkres qui correspondent qualitativement avec les stabilitts relatives des quatre conformkres plans. Une extrapolation des donntes relatives aux populations en solution foumit une tvaluation des valeurs en phase gazeuse qui est intermtdiaire entre les deux &valuations thtoriques. On peut placer des lirnites sur les fractions de population de chacun des conformkres en phase vapeur et en solutions. On peut rationaliser les stabilitCs relatives des conformkres libres en fonction de facteurs Clectrostatiques et des prCfCrences des groupements carbonyle et mtthoxy pour certaines orientations par rapport aux liaisons C-C du cycle dont le caractkre de double liaison est le plus ClevC.

Mots clis : 3-mCthoxybenzaldChyde, RMN; 3-mCthoxybenzaldthyde, conformations en phase vapeur et en solutions; 3-mCthoxybenzaldthyde, calculs d'OM.

[Traduit par la revue]

Introduction H\c//O H\c/P O \ \ C / ~

In the vapor phase, low-resolution microwave spectroscopy provides evidence for the existence of three of the four possible planar conformers of 3-methoxybenzaldehyde (1). The spectrum of 1 is not seen but, noting the broadness and weakness of the C H 0

\ 0 & 0 spectra of 2-4, the probable low dipole moment of 1 may account for its apparent absence. The relative populations of the

I CH3

I conformers are difficult to extract from these spectra (1).

CH3 1

In solution, the barriers to rotation about the C,Z-Csp2 and 2 3

C,2-0 bonds for the aldehyde (2) and methoxy (3) groups are large enough to allow a discussion of the conformational behaviour of 3-methoxybenzaldehyde at ambient temperatures

O\c/H

in terms of the planar conformers, 1-4. In acetone-d6 solution (4), 5 ~ ( H - 5 , C H O ) is 0.375(13) Hz; as a stereospecific spin- spin coupling constant, it indicates the population of 1 + 2 & /CH3

0

'Undergraduate research participant, 1987-1988.

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!R ET AL. 997

relative to that of 3 + 4, although this population was not deduced. Decoupling experiments show that the methoxy protons are coupled to H-2 and H-4 but the coupling constants are not resolved (4). On the basis of PCILO computations (4), it is suggested that electrostatic interactions between meta substituents make an important contribution to the relative stability of the conformers of 2,5-dimethoxybenzaldehyde and, by inference, of 3-methoxybenzaldehyde. In other words, the roughly antiparallel electric dipoles in 1 presumably favour this conformation. No populations are deduced (4).

An alternative interpretation of the conformational prefer- ences of the aldehyde and methoxy groups in aromatic systems (5-7) is that the former is preferentially arranged so that the carbonyl lies trans to the ortho position of highest double bond character, as in 5 (valence-bond picture), or of highest

T-electron density (molecular orbital picture); the latter, how- ever, prefers to lie cis to the bond of the highest double character, as in vinyl ether. It is not clear to us whether it is the methoxy group or the aldehyde group which causes the lower energy of the resonance form indicated in 5. In any event, the nuclear Overhauser effect for the protons ortho to the aldehyde functionality (irradiation of the aldehyde proton peaks) is interpreted to mean that K = (P1 + P2)/(P3 + P4) is roughly 1.8 for CDC13 or benzene-d6 solutions (7). There exist (6, 8) various complications in the interpretation of these polarization experiments. Nevertheless, from measurements of the polariza- tions (enhancements) and of spin-lattice relaxation times of 13C and 'H nuclei of 3-methoxyacetophenone in CDC13 solutions (6), it is concluded that the acetyl group displays the same conformational behaviour as the aldehyde group in 3-methoxy- benzaldehyde and, furthermore, that the methoxy group has K = (P I + P4)/(P2 + P4) as 2.2. In other words, the methoxy group is strongly oriented towards the aldehyde site.

Now, both 'J(H,CHO) and 'J(H,OCH~) are stereospecific (9-17). Thus, 'J(H-5,cHO) in 1 and 2 are expected to vanish (9, 12), whereas they will be as large as 0.9 Hz in 3 and 4. Again, 'J(H-4,0CH3) in 2 and 3 will be near -0.3 Hz but negligible in 1 and 4; corresponding statements (13-17) hold for 'J(H-2,0CH3). 'J (H,OCH~) is a proximate coupling (1 4) and, as such, is sizeable only when the methoxy protons lie very near the ring proton. Therefore, a precise analysis of the 'H NMR spectrum of 3-methoxybenzaldehyde should yield precise values of 'J(H-~,CHO) and 'J(H,0CH3), from which K = (P1 + P2)/(P3 + P4) and K = (P I + P4)/(P2 + P3), respectively, can be deduced for solutions. Molecular orbital calculations may give an indication of the conformational behaviour of the free molecule, including relative populations of each of 1-4. Such measurements and calculations are reported below.

Experimental Solutions of 3-methoxybenzaldehyde in CS2/C6D12 and in acetone-d6

(see Table 1) were transferred to 5 mrn od NMR sample tubes, were degassed by the freeze- ump-thaw procedure, and were then flame- sealed under vacuum. 'H NMR spectra were acquired at a probe temperature of 300 K on a Bruker AM300 spectrometer under

conditions previously described (18). A wide sweepwidth survey spectrum was obtained in order to reference the spectrum with respect to TMS, and to obtain the locations of the aromatic and side-chain regions. Each region was examined in detail, with sweepwidth and data region adjusted to give acquisition times of approximately 40 s. Four to 16 scans were acquired. Zero filling to twice the original data region was done before transforming the FIDs using a small amount of Gaussian multiplication. Linewidths at half height were approximately 0.04 Hz after this resolution enhancement. Because of the many long- range coupling constants between the four ring protons and the four on the side chains, spectra were obtained by decoupling, successively, the aldehydic proton and then the methoxy protons. The ensuing spectra, corresponding to seven and five spin 112 systems, were amenable to reasonably precise analyses.

Molecular orbital computations utilized the computer program GAUSSIAN 86 (19). The geometry optimization procedures contained the constraint that the carbon and hydrogen atoms of the aromatic nucleus remain in a plane; all other bond angles and lengths were allowed to change.

Results and discussion The 'H NMR spectra were analyzed with the computer

program NUMARIT (20), as modified by R. Sebastian in this laboratory. The spectral parameters of 3-methoxybenzaldehyde appear in Table 1. The decoupling procedures during spectral acquisition caused Bloch-Siegert shifts, apparent in Table 1. For the CS2/C6D12 solvent mixture, the existence of large correlations (footnotes to Table 1) between certain coupling constants within the ring, and between them and some chemical shifts, causes a relatively large imprecision in these parameters. Fortunately, those long-range coupling constants of immediate interest to the following discussion do not suffer from such imprecision. Figure 1 shows the 'H NMR spectrum of H-2 in CS2 solution.

The conformational behaviour of the aldehydic group In benzaldehyde, 'J(H,CHO) is 0.43 l(2) Hz for a CS2/C6D12

solution and is 0.430(1) Hz for an acetone-d6 solution (21). The available evidence (9-1 2) on benzaldehyde derivatives implies that 'J(H,CHO) vanishes in 1 and 2, thereby implying that 'J(H,CHO) is 0.861(4) in 3 and 4; the methoxy group is distant from the coupling path for this a electron coupling (21) and will therefore minimally perturb the magnitude of the coupling constant.

Accordingly, because 'J(H,CHO) is 0.332(6) Hz in CS2/C6D12 solution and is 0.382(4) Hz in acetone-d6, K(CH0) = (P3 + P4)/(P1 + P2) = 0.332(6)/(0.861(4) - 0.332(6)) = 0.63(2) for the former and is 0.382(4)/(0.861(4) - 0.382(4)) = 0.80(2) for the latter solution at 300 K. The nuclear Overhauser enhancements measured for CDC13 and (or) C6D6 solutions of 3-methoxybenzaldehyde (7) imply a K(CH0) of 0.56 (tempera- ture not stated). It is unlikely that the meta substituent perturbs 'J(H,CHO) enough to reduce the value of K(CH0) to 0.56. Thus, for 4-fluorobenzaldehyde in acetone-d6 solution, in which the substituent is now rather closer to the coupling path, 'J(H,CHO) is 0.439(1) Hz (unpublished work in this laboratory). If this number were to apply, then K(CH0) would become 0.77(2) for 3-methoxybenzaldehyde in acetone-d6 solution.

The present results show that the conformational bias of the aldehydic group in 3-methoxybenzaldehyde is solvent dependent.

Conformational behaviour of the methoxy group Precise analyses of the 'H NMR spectra of anisole yield

S ~ ( ~ , ~ C H 3 ) as -0.152(0) and -0.155(0) Hz in CS2/C6DI2

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SCHAEFER ET AL

FIG. 1. The 'H NMR spectrum of H-2 in 3-methoxybenzaldehyde, as a dilute solution in CS2/C6DI2/TMS (Table I), is displayed. The linewidth at half-height is about 0.04 Hz and the small splittings (quartets of doublets) are very near 0.07 Hz in magnitude. The theoretical spectrum uses the coupling constants in Table 1, but the chemical shifts lie midway between those given in Table 1. Bloch- Siegert shifts, caused by the decoupling fields, render the true chemical shifts in the fully coupled spectrum somewhat uncertain; this uncertainty probably accounts for the quite small incongruence between the measured and theoretical spectra above.

and acetone-d6 solutions, respectively (16). The evidence is again (13-17) that 5 ~ ( H , ~ ~ ~ 3 ) is vanishingly small if the methoxy group lies trans to the ortho C-H bond containing the coupled proton. If so, then 5 ~ ( ~ , ~ ~ ~ 3 ) is -0.304 and -0.310 Hz for the CS2/C6D12 and acetone-d6 solutions when the methoxy group lies cis to the ortho C-H bond. The sums of 'J(H-~,ocH~) and 5 ~ ( ~ - 4 , ~ ~ ~ 3 ) in Table 1 are -0.298(2) Hz for the CS2 solution and -0.301(2) Hz for the acetone-d6 solution. It appears that the perturbation of this coupling interaction by the aldehyde substituent is rather small.

Based on this appearance, it follows that K(OCH3) =

(P2 + P3)/(P1 + P4) = 0.103(1)/0.195(1) = 0.53(1) for the nonpolar solvent and is 0.121(1)/0.180(1) = 0.67(1) for the acetone-d6 solution. For a 0.1M solution of 3-methoxyaceto- phenone in CDC13 (temperature not given) the nuclear Over- hauser enhancements yield a K(OCH3) of 0.45 if they are corrected for the spin-lattice relaxation times and for the

FIG. 2. The STO-3G MO structure of 3-methoxybenzaldehyde is shown for the planar conformer, 1, which is most stable. Bond angles are in degrees and lengths in angstrom units. The bond lengths within the benzene moiety show some degree of alternation, of some interest to literature interpretations of the conformational bias of the substi- tuents (see text), but appear to be controlled by the orientation of the methoxy group (see F&. 3-5).

correlation times of the protons ortho to the methoxy group. Without these corrections, K(OCH3) would be 0.53. Again, the long-range coupling constants imply a lower conformational bias for the methoxy group than do the combined polarization and relaxation measurements. The spin coupling data also yield a solvent dependence of K(OCH3), the equilibrium constant being nearer unity for the polar solvent.

Of course, both types of measurements give the conforma- tional bias, relative to the other substituent, of each substituent individually. They cannot give the populations of each of 1-4. At this juncture, molecular orbital computations become of some value.

STO-3G MO computations Previous experience with minimal basis set calculations on

benzaldehyde (22, 23) shows that the torsional potential for the exocyclic C-C bond is better reproduced than with split- valence bases, even those containing polarization functions. For anisole, a 6-31G* basis yields a potential for torsion about the C,2-0 bond very similar to that obtained from the minimal basis, STO-3G (24, 25). It appears, therefore, that computations of the conformational properties of 3-methoxy- benzaldehyde with the minimal basis set may well be sensible.

Table 2 displays the computed energies of the four planar conformers, as well as their dipole moments. The most stable conformer is computed to be 1, in which the group dipole moments are arranged in such a way as to lead to a small dipole moment and lending some support to the electrostatic arguments (4) discussed above. Figure 2 displays the STO-3G structure of 1 and, judging by the C-C bond lengths within the ring, some apparent support exists for the importance (7) of valence-bond structure 5, as also discussed in the introduction. However, the alternation in bond lengths is not striking and, in Figs. 3-5, showing the computed geometries of the other conformers, it appears that the bond lengths in the benzene nucleus are

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CAN. 1. CHEM. VOL. 68, 1990

H- FIG. 3. Comparison of this STO-3G structure of 3-methoxybenz-

aldehyde with that in Fig. 2 implies that the orientation of the methoxy group determines the bond length variations within the ring.

determined by the conformation of the methoxy group and not by that of the aldehyde substituent. At this level of MO theory, therefore, a combined interpretation of the stability of 1 postu- lates that electrostatic interactions determine the preponderance of 1. This, in turn, leads to the apparent preference of the methoxy group for that ortho ring C-C bond of larger double character and, thereby, also to the apparent preference of the C=O bond to lie trans to a similar C-C bond of the ring. The flaw in this description is that, while the STO-3G dipole moment of anisole is within 5% of the experimental value (26), the computed dipole moment of benzaldehyde, at 1.92 D (22), is much less than the experimental value of 2.96 D (27). Clearly, the charge redistribution2 caused by the aldehydic substituent is incorrectly given by the minimal basis set (it does not follow that conformational energies and internal rotational barriers are badly estimated (28)).

Suppose that 1.0 D is added to each of the computed dipole moments of 1-4; this seems reasonable in that, for example, the dipole moment of 1 becomes 1 .735 D, as compared to 1.74 D assumed on the basis of group moments in a study (27) of the dipole moment of 3-methoxybenzaldehyde. Such an adjustment of the dipole moments in Table 2, combined with the STO-3G MO populations, yields a computed dipole moment of 2.98 D at 300 K. The experimental value, in benzene solution, is 2.94 D at 298 K. This agreement is coincidental, of course.

Thus (Table 2), the populations of 1-4 must alter in passing from CS2 to acetone-d6 solution, according to the long-range coupling constants. No doubt coincidentally, the values of K(CH0) and K(OCH3) obtained from the coupling constants in acetone-d6 solution agree rather closely with those computed for the free molecule from the minimal basis set. In fact,

he computed dipole moment depends on the way in which overlap populations are divided among the atoms in question, the standard Mulliken population analysis being used in this paper.

H FIG. 4. Comparison of this STO-3G structure with that in Fig. 2

implies that the orientation of the aldehyde substituent does not change the computed bond lengths within the benzene moiety.

H FIG. 5. Again, comparison of this STO-3G structure with that in

Fig. 3 bears out the independence of the ring geometry on the aldehyde orientation.

the populations given by the latter at 300 K, combined with the coupling constants expected for each of the conformers (see above) yield 5J(H,CHO) as 0.386 Hz (the measured value in acetone-d6 is 0.382(4) Hz) and 5 ~ ( ~ - 2 , 0 C H 3 ) and 5 ~ ( ~ - 4 , ~ ~ ~ 3 ) as -0.174 and -0.126 Hz, respectively (the measured values are -0.180(1) and -0.121(1) Hz), reflecting the difference of 0.05 between the STO-3G and experimental K(OCH3) values in Table 2. These comparisons, taken with the clear solvent dependence of the populational distribution,

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FIG. 6. The 4-21G structure of the most stable conformer of 3-methoxybenzaldehyde is displayed. The bond lengths are rather smaller than those from the minimal basis (Fig. 2). The carbonyl bond and the methoxy group are situated trans and cis, respectively, to the shorter of the two possible C-C bonds in the ring (see the text for a discussion).

I demonstrate that the STO-3G MO populations for the free I molecule are probably incorrect. I

4-21 G MO computations From unpublished calculations in this laboratory, using

I various split-valence bases and including polarization func- tions, it appears that the 4-21G basis, with a dipole moment

1 of 3.37D for benzaldehyde, comes closest to experiment. I Accordingly, 4-21G MO computations were performed for

the planar conformers, 1-4. The relative energies and dipole moments of the four planar forms are found in Table 2. comparison of the dipole moments with those from STO-3G computations (note discussion above) and from group moments suggests that the dipole moment of 1 may indeed be very near 1.7 D; the dipole moments of the other conformers are not reproduced by the 4-21G basis, judging by those deduced from group moments in Table 2.

Furthermore, the relative energies of the various conformers, as given by the two bases, agree that P1 >> P3 + P4 + P2 at 300 K (Table 2) but imply strikingly different values for the values of Pl-P4 and, of course, for K(CH0) and K(OCH3). Consequently, a semiempirical approach, outlined in the next section, is adopted.

Prior to that, however, note that the 4-21G structures of 1-4, as given in Figs. 6-9, tend to bear out the relative C-C bond lengths in the ring obtained with the minimal basis set. Again, the relative bond lengths appear to be controlled by the orientation of the methoxy group. The methoxy group lies cis to the shorter C-C bond (in 2 the two C-C bonds are equal in length), but the C=O bond does not always lie trans to the shorter of the two C-C bonds in the ring. In fact, the two structures in which the C=O bond is trans to the longer of the two C-C bonds, 2 and 4, also have the highest computed energies. Furthermore, 2 is of higher energy than 4 and has equal C-C bonds next to the methoxy substituent. In this sense, the relative stabilities of 2-4 can be understood in terms

FIG. 7. The 4-21G structure of conformer 2, the least stable, of 3-methoxybenzaldehyde is shown. The C=O bond is now trans to the longer of the two neighbouring C-C bonds and both C-C bonds next to the methoxy group have the same length. In addition, the group dipoles now repel one another (27). The three unfavorable interactions rationalize the instability of this conformer.

of the valence-bond approach ( 5 , 6), given that 1 is the most stable because of electrostatic attraction between the group dipoles (4).

Extrapolation from solution data An estimate of K(CH0) and K(OCH3) can be had from a plot

of In K versus (E - 1 ) / ( 2 ~ + 1) and extrapolation to a value of unity for the dielectric constant, E, of the solution. This approach is based on the reaction field model (30), noting (3 1, 32) that this model yields free energy differences between conformers (examine eq. 13.18 and p. 562 of ref. 30). Various corrections ignored in such a plot, and the uncertainties in the actual dielectric constants of the CS2 and acetone-d6 solutions in Table 1, at first imply that large errors could be involved even within the assumptions of the model. Yet, because of the logarithm and because of the relatively large difference in the dielectric functions for the two solutions, the error in extrapola- tion is rather small.

The extrapolation yields K(CH0) and K(OCH3) as 0.465 and 0.39(1) in the vapor at 300K, very nearly midway between the two theoretical estimates (Table 2). From the expressions for K(CH0) and K(OCH3), together with the normalization condi- tion for the four fractional populations, sums and differences of populations can be had and are presented in Table 2. It follows that the fractional population of 1, P 1 , is greater than 0.40(2) and less than 0.68(2) in the vapor phase. It also follows that P4 - P2 = 0.035. Suppose that PI = 0.54(4), the mean of the limiting values; then P2 = 0.14(4), P3 = 0.14(4), and P4 = 0.18(4). Qualitatively, but by no means quantitatively, one has agreement with the theoretical estimates of the relative proportions of the conformers in the vapor.

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FIG. 8. The 4-21G structure of conformer 4 is displayed. The C=O bond is again trans to the longer of the ring C-C bonds but the methoxy group lies cis to the shorter of the two C-C bonds. Qualitatively, as discussed in the text, it is therefore sensible that this conformer is somewhat more stable than 2 (Fig. 7).

H FIG. 9. The 4-21G structure of conformer 3 has the C-0 bond

trans to the shorter C-C in the ring and the methoxy group cis to such a bond. Presumably this conformer is of higher energy than 1 because in the latter the group dipoles are roughly antiparallel.

In the solutions, P4 - P2 remains at 0.04(1) and P1 apparently decreases, as is sensible for the conformer with the lowest dipole moment. In the CS2 solvent mixture, take PI as 0.44(4), again the means of its limits. Then P2 is 0.18(4), P3 is 0.17(4), and P4 0.21(4). In the acetone-d6 solution, the mean of the limits for P1 is 0.37(4).

Of course, if a stereospecific coupling constant involving nuclei within the two substituents were to exist, then the individual populations could be derived. The existence of such a coupling constant, for instance, that over six bonds between the aldehydic proton and the 13C nucleus in the methoxy group, is rather unlikely. Thus, 'J(H,CHO) in planar benzaldehyde is apparently a pure a electron coupling interaction because in 3,5-dimethylbenzaldehyde (21) there is no evidence for a finite coupling constant 6 ~ ( ~ , ~ ~ 3 ) , involving the aldehydic and methyl protons. It happens (33) that CND0/2 FPT (34) computations of 'J(H,CHO) reproduce its magnitude and stereospecificity. Therefore such computations (35) were done for 1-4 employing the 4-21 G geometries. 6 ~ ( ~ ~ ~ , ~ ~ ~ 3 ) was computed as less than 0.01 Hz for 1, 3, and 4 and as only 0.06 Hz for 2, implying that this parameter cannot be used as a measure of the population of one of these conformers. 'J(cHO,OCH~) was calculated as 0.11 Hz for 2 and 3 and as <0.01 Hz for 1 and 4, again suggesting that an individual population is unavailable from this coupling constant.

Internal rotational barriers The STO-3G energies were also computed for conformations

in which one substituent lay in-plane while the other rotated about the exocyclic bond in steps of 15". These results are not presented here in detail, but are available from the authors.

Rotation of the aldehyde group by 90" for conformer 1 raises the energy by 24.64 kJ/mol, whereas it costs 23.24 kJ/mol to do so for 4. These numbers are very similar to the 24.28 kJ/mol computed with the same basis set for benzaldehyde (23) and that measured as 22.7 * 1.4 kJ/mol from vibronic fluorescence spectra (29). A substantial substituent effect on the CSp2-CSp2 barrier is not apparent.

Again, rotation about the C,2-0 bond by 90" in 1 is calculated to raise the energy by 8.25 kJ/mol but, while the aldehydic rotational potential is predominantly twofold, both for 3-methoxybenzaldehyde and benzaldehyde, that for the methoxy group is now rather complex. In fact, for rotation in conformer 1, the 13 energies in 4 , the angle of twist about the C,2-0 bond, are fit by eq. [I] , the standard error in

the coefficients being 0.10 kJ/mol. Thus, there are significant twofold, in sin2 4 , and fourfold, in sin2 2 4 , components. A very similar equation holds for rotation in 4 for 4. For anisole, the minimal basis set gives the twofold and fourfold components as 5.6(4) and 2.1(4) kJ/mol, respectively (24). It appears that the aldehyde substituent has increased the twofold component by 1.8(5) kJ/mol in 3-methoxybenzaldehyde, whereas in 4- methoxybenzaldehyde this increase is calculated as 1.7(9) W/mol (24). The usual explanation for the latter molecule is that the aldehyde group accepts .sr electron density from the methoxy group, thereby increasing the double-bond character of the C,z-0 bond and hence the twofold component of the 4 barrier. The barrier in the gas phase of anisole itself has not been determined.

Acknowledgments We thank the Natural Sciences and Engineering Research

Council of Canada for financial support.

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SCHAEFER ET AL 1003

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