Azq. Khan et al., TWIST ANGLES AND BOND LENGTHS IN 3 TWISTED PUSH-PULL ETHYLENES - INTERPLAY BETWEEN STERIC AND ELECTRONIC EFFECTS, Perkin transactions. 2, (7), 1994, pp. 1569-1573
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.