Quantum-chemical study of competitive photochemical reaction pathways in N-methyldiphenylamine derivatives

摘要

The electronic structures of diphenylamine derivatives Ph_2NCH_(3-n)Ph_n, where n = 0-3, in the ground and lowest excited states were studied by semi-empirical PM3 method. A priori these compounds can show two kinds of photochemical reactivity: photocyclization of the diphenylamine group and photodissociation of the N-C bond. To discriminate these possibilities, two cross-sections of the potential energy surface of the triplet excited state for every compound were studied, and the activation energis (E_a) for the two competing reactions were calculated. Comparison of E_a values shows, that activation energy for the cyclization reaction remains nearly constant within 20.5 ± 0.6 kcal/mol for all the compounds studied, whereas E_a value for the dissociation reaction reduces from 27.6 kcal/mol for Ph_2NCH_3 to 12.6 kcal/mol for Ph_2NCPh_3. Thus, PM3 calculations predict predominantly photocyclization of the N-C bond for Ph_2NCPh_3, in full accordance with the experimental data.