Empirical relationship between the conformation and the direction of carbonyl group pyramidalization of cyclohexanones

A statistical investigation of precise crystal structures from the Cambridg e Structural Database shows that cyclohexanones have a predictable pyramida lization of the carbonyl group in the chair conformation that depends on th e puckering of the ring. Flattened cyclohexanones are usually "axially pyra midalized" (see Figure 1), whereas more strongly puckered cyclohexanones ar e usually "equatorially pyramidalized". It is conceivable that a cooperatio n between electronic effects (hyperconjugation) and steric effects (relief from torsional strain) leads to this unique correlation. No such conformati on/pyramidalization correlations are observed for other ketones.