The conformational equilibria of 3-methoxybenzaldehyde in polar and nonpolar solutions. 1H NMR and MO calculations

Abstract

Analyses of the 1H nuclear magnetic resonance spectra of 3-methoxybenzaldehyde in CS2/C6D12 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 Pt, 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, Pt = 0.80(2)Pc. Both of these numbers are substantially larger than that deduced from nuclear Overhauser enhancements for a CDCl3 solution. If Pt is the fractional population of the conformers in which the methoxy is oriented trans to the aldehyde substituent, then Pt = 0.53(1)Pc in CS2/C6D12 and Pt = 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. Keywords: 3-methoxybenzaldehyde, NMR; 3-methoxybenzaldehyde, conformations for solutions and vapor; 3-methoxybenzaldehyde, MO calculations.