The effects of double-bond twisting on the photoionization energies of push-pull ethylenes

Abstract

The u.v. photoelectron spectra of a number of push-pull substituted ethylenes with two donor groups and with planar or twisted double bonds have been interpreted by the aid of perturbational arguments, CNDO/2 calculations, and comparison with published spectra of similar compounds. The highest bonding molecular orbital in all cases is of π-type, and its energy is raised with increasing twist about the double bond, whereas the orbital formed by anti-symmetric combination of the donor atom p_z orbitals is unchanged or lowered by increased twist. Through-bond interaction splits the ‘non-bonding’ oxygen orbitals by 0.6–0.7 eV in 1,1-diacetyl-2,2-diaminoethylenes, in which the carbonyl groups are E,Z oriented with respect to the CC double bond. When Z,Z orientation is enforced by ring closure, the splitting is only 0.38 eV, in agreement with calculations. In the 2,2-dithio-analogues, interaction with the C–S bond orbitals leads to a more complex splitting pattern.