13 C and 1 H nuclear magnetic resonance of methyl‐substituted acetophenones and methyl benzoates: steric hindrance and inhibited conjugation

The 1 H and 13 C NMR spectra of 14 methyl‐substituted acetophenones and 14 methyl‐substituted methyl benzoates were assigned and interpreted with respect to the conformation of the C ar —C(O) bond. The substituent effects are proportional in the two series and can be divided into polar and steric: each has different effects on the 13 C SCS of the individual atoms. In the case of C atoms C(O), C(1) and CH 3 (CO), the steric effects were quantitatively separated by comparing SCS in the ortho and para positions. The steric effects are proportional for the individual C atoms and also to steric effects estimated from other physical quantities. However, they do not depend simply on the angle of torsion ϕ of the functional group as anticipated hitherto. A better description distinguishes two classes of compounds: sterically not hindered or slightly hindered planar molecules and strongly sterically hindered, markedly non‐planar. In order to confirm this reasoning without empirical correlations, the J (C,C) coupling constants were measured for three acetophenone derivatives labeled with 13 C in the acetyl methyl group. The constants confirm unambiguously the conformation of 2‐methylacetophenone; their zero values are in accord with the conformation of 2,6‐dimethylacetophenone. The zero values in the unsubstituted acetophenone are at variance with previous erroneous report but all J (C,C) values are in accord with calculations at the B3LYP/6‐311++G(2d,2p)//B3LYP/6–311+G(d,p) level. Copyright © 2004 John Wiley & Sons, Ltd.