Twist angles and bond lengths in three twisted push–pull ethylenes. Interplay between steric and electronic effects

Abstract

Crystal structures of 1,3-dibenzyl-2-(4,4-dimethyl-2,6-dioxocyclohexylidene)hexahydropyrimidine and two of its sulfur derivatives have been determined by X-ray crystallography. The molecules are formally push–pull ethylenes, but due to strong steric interaction between the donor and acceptor parts they are twisted by 72° to 85° about the formal double bond, and they are best described as zwitterions with an amidinium ion and a carbanion part. The degree of twisting is discussed in terms of steric and electronic effects, and electron distributions from AM1 and CNDO/S calculations are described.