Photochemical reactions of charge-transfer complexes. IV. The orientation effects of charge-transfer complexes: theory and experiment

Abstract

In paper number III of this series (2) ab initio and PCILO molecular orbital calculations for the four π–π* charge-transfer (ct) complexes formed between maleonitrile (MN) and fumaronitrile (FN), and cis (CS) and trans-1,2-dimethoxyethylene (TR) were reported. This paper reports the ct transition absorption characteristics and association constants of these complexes as a function of temperature. From this data the heats of formation of the complexes are determined and these experimental results are then compared with the theoretical values; good agreement is obtained. The order of complex stability (−ΔHθ (kcal mol)) measured in chloroform solution is: MN−TR(0.55) < FN−TR(0.65) < MN−CS(0.78) < FN−CS(1.25).We reach the conclusion that, for these complexes, orientation effects of the substituents are quite small in magnitude and that the dominant factor influencing complex stability follows the redox potential difference. The structures of the complexes are discussed in view of these results.