Photochemical reactions of charge-transfer complexes. III. Molecular orbital studies of the charge-transfer complexes between 1,2-dimethoxyethylenes and 1,2-dicyanoethylenes

Abstract

Ab initio and PCILO molecular orbital (MO) calculations for the four π–π* charge-transfer complexes formed between maleonitrile (MN) and fumaronitrile (FN), and cis (CS) and trans-1,2-dimethoxyethyIene (TR) are reported. The donor and the acceptor molecules are stacked in parallel planes. Trans and cis-l,2-dihydroxyethylene are used as the model compounds for TR and CS in the determination of the optimum geometries of the complexes. Both types of MO calculations indicate that the eclipsed geometry (with the two central double bonds parallel and the centres of the double bonds centred over one another) is relatively unstable. With limited geometry searching, we find that the slipped geometry (with the two central double bonds parallel but separated at a projected distance of 2.2 Å and the oxygen atoms above the cyanocarbon atoms) is the preferred structure. According to the ab initio calculations the relative order of the calculated heats of formation (ΔHθ's) for these complexes is: MN−CS > FN−CS > FN−TR > MN−TR.