TWIST ANGLES AND BOND LENGTHS IN 3 TWISTED PUSH-PULL ETHYLENES - INTERPLAY BETWEEN STERIC AND ELECTRONIC EFFECTS

Crystal structures of methy-2,6-dioxocyclohexylidene)hexahydropyrimidi ne and two of its sulfur derivatives have been determined by X-ray cry stallography. The molecules are formally push-pull ethylenes, but due to strong steric interaction between the donor and acceptor parts they are twisted by 72-degrees to 85-degrees 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 ster ic and electronic effects, and electron distributions from AM1 and CND O/S calculations are described.