Theoretical Study on the Excited-state Intramolecular Hydrogen Abstraction Reactions of Butanal

The excited-state intramolecular hydrogen abstraction reactions of butanal have been investigated using the CAS-MP2/6-311+G*//CASSCF/6-31G* methods. Calculated results show that the hydrogen transfer induced fluorescence quenching of the n,π*-excited state of covalent butanal with three paths: (1) The first path corresponds to direct S_0-react reconstitution,which involves the first S_1 decay by partial hydrogen atom transfer.(2) The second stepwise mechanism can be viewed as a full hydrogen atom transfer followed by a partial hydrogen atom back transfer,electron transfer (near S_1/S_0 or S_0-TS) and finally a proton transfer to S_0-react. (3) On the triplet surface,the surface crossing to the singlet state would be clearly much efficient at the T_1/S_0 region due to the large SOC value of 8.3 cm~(-1). The S_0-react decay route from T_1/S_0 was studied with an intrinsic reaction coordinate (IRC) calculation at the CASSCF level,resulting in the S_0-React minimum.