Bond Angles in Lactones and Lactams

The carbonyl group in lactones and, to a lesser extent, in lactams tends to show a C? C?O angle that is larger than the O? C?O or N? C?O angle, the difference increasing in magnitude as the ring size decreases. The observed trend provides information on ratios of force constants characterising the flexibility of the s-cis-ester group and may be interpreted in terms of incipient chemical reaction to ^?O? (CH₂)_n? C?O⁺. Molecular orbital calculations (MINDO/3, MNDO, EH) for model compounds provide an electronic interpretation of this angle difference in terms of an anomeric interaction between the p-type lone pair on the carbonyl O-atom and the antibonding C-O or C-N orbital.