Photochemistry of Butyrophenone:  Combined Complete-Active-Space Self-Consistent Field and Density Functional Theory Study of Norrish Type I and II Reactions

The complete-active-space self-consistent field (CASSCF) and density functional theory (DFT) approaches have been used to study the mechanistic details of Norrish type I and II reactions of aromatic carbonyl compounds, with butyrophenone (PhCOCH2CH2CH3) as a representative. A minimum energy crossing point was found to exist among three potential energy surfaces (S1, T1, and T2), and the three-surface crossing allows the T2 state to act as a relay that enables the intersystem crossing (ISC) from S1 to T1 to occur with a high efficiency for PhCOCH2CH2CH3. Once the molecule is in the T1 state, the 1,5-H shift reaction is the predominant reaction pathway and yields a triplet 1,4-biradical of PhC(OH)CH2CH2CH2 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 S1 relaxation dynamics of aromatic carbonyl compounds and their subsequent reaction mechanisms.