The vibrational spectroscopy of some highly conjugated systems. Part 7. Conformation and electron distribution in 1,1-diacylethylenes. The phenomenon of vanishing raman intensity for the carbon double bond

Previous studies on the i.r. and Raman spectroscopy of ethylenes oppositely substituted with one electron acceptor group and one or two donors are here extended to molecules containing two acceptors. Established criteria are used to assign conformation and, in particular, to decide the issue of coplanarity. In terms of this criterion, three such classes of molecule are identified : (1) both acyl groups coplanar with the carbon double bond ; (2) cis-planar with trans-carbonyl out of plane, confined to molecules containing NH; and (3) the reverse trans-planar circumstance. Compounds of class (1) invariably possess an extremely low Raman intensity for the carbon double bond, even though this is usually strong in the i.r. spectrum. This is explained as almost total removal of mobile π-electron density from the carbon double bond, which does not however diminish the overall molecular dipole. It is concluded that conjugation, defined as bond equalisation, and resonance, resulting from charge sharing, are not synonymous for these molecules; the latter probably goes through a maximum as the degree of substitution increases.