Experimental and theoretical conformer distributions of 3-methylbenzaldehyde

Abstract

The long-range coupling constants between the aldehydic and ring protons of 3-methylbenzaldehyde, in CS2/C6D12 and acetone-d6 solutions, imply an equilibrium constant, K(O-cis/O-trans) of 1.07(4) in both solvents at 300 K. The small preference for the O-cis conformer in both solutions suggests that the O-cis conformer is also the more abundant in the vapor, indicating a reassignment of recently reported electronic bands in this state. Geometry-optimized molecular orbital computations, using the AMl, STO-3G, and 6-31G bases, confirm the slightly lower energy of the free O-cis conformer. The 6J(H,CHO) of +0.015(1) Hz for the acetone solution has the correct sign and magnitude implied by a recent simple model for this coupling constant. Other long-range coupling constants are also discussed in terms of their mechanisms. Key words: conformations, 3MB; NMR, 3MB; spin–spin coupling constants, 3MB; MO computations, 3MB.