Competing mechanisms of Norrish and Norrish-like reactions in a wide range of systems – From carbonyl compounds to nitrogen oxide donators

For the first time within the unified quantum chemical scheme of calculations (method CASSCF – complete active space self-consistent field; basis 6-31+G* with diffusion functions on the atoms N, O) the competition of Norrish and Norrish-like reactions of types I and II (for simplicity NRI and NRII) was studied in a wide range of systems. These calculations were carried out for butanal, pentanal and pentan-2-one (methyl propyl ketone), as well as for buten-2-al, butanimine-1 and nitrosopropane. In the case of butanal the adequacy of a basis 6-31+G* for present work was proved by means of comparison of these results with those received by more sophisticated basis 6-311++G**. According to calculations data, the four processes, which are listed below, can possibly be observed in butanal: 1) dissociation of OСН bond in T1 state (NRI), 2) dissociation of bond СαСO in T1 state (NRI), 3) intramolecular hydrogen shift in S1 state (NRII), 4) intramolecular hydrogen shift in T1 state (NRII). methyl propyl ketone NRII process immediately and obviously accelerates. The calculations for homological pentanal confirm the independence of NRI energetic characteristics from the lengthening of carbonic chain, whereas the energy barriers of intramolecular hydrogen shift in NRII are considerably lower as compared to butanal. Preparation of the system with rigidly fixed geometrical configuration of reaction knot might make NRII mechanism preferable, which is observed in the case of reactions in buten-2-al, for which NRII is essentially the process without a barrier. ement of oxygen atom (carbonyl compounds) by isoelectronic NH (butanimine-1) leads to suppression of NRI. However the photochemical cleavage of CN bond in NRI is the only possible process for nitrosopropane, isoelectronic to butanal and modelling the analogous process in the most important precursors of nitrogen oxide.