Photochemistry of butyrophenone: Combined complete-active-space self-consistent field and density functional theory study of norrish type I and II reactions

Abstract

The complete-active-space self-consistent field (CASSCF) and density functional theory (DFT) approaches V have been used to study the mechanistic details of Norrish type I and II reactions of aromatic carbonyl compounds, with butyrophenone (PhCOCH 2CH 2CH 3) as a representative. A minimum energy crossing point was found to exist among three potential energy surfaces (S 1, T 1, and T 2), and the three-surface crossing allows the T 2 state to act as a relay that enables the intersystem crossing (ISC) from S 1 to T 1 to occur with a high efficiency for PhCOCH 2CH 2CH 3. Once the molecule is in the T 1 state, the 1,5-H shift reaction is the predominant reaction pathway and yields a triplet 1,4-biradical of PhC(OH)CH 2CH 2CH 2 as an intermediate species. Since the formation of excited triplet products is energetically improbable, the subsequent decomposition, cyclicization, and disproportionation of the 1,4-biradical proceed after intersystem crossing from the triplet to singlet state. The singlet 1,4-biradical was found to have three isomers, which determine to a certain extent the branching ratios of the subsequent reactions. The study given here provides new insights into the Si relaxation dynamics of aromatic carbonyl compounds and their subsequent reaction mechanisms.