Ab initio studies toward understanding photoisomerization of acrylic acid in the gas phase

Structures and isomerization reactions of the CH2CHCOOH isomers in S0, T1 and S1 states have been studied with ab initio molecular orbital methods. The obtained results support a mechanistic model for the photochemical rearrangement of acrylic acid based on a non-radiative decay route. The CH2CHCOOH molecules are populated in the S1 state by photo-excitation at 248 nm. Then the T1 intermediate is formed via the S1/T1 intersystem crossing. A significant fraction of the CH2CHCOOH molecules in T1 returns to the ground state by a decay through the T1/S0 crossing point. Vibrationally hot ground-state isomers are generated, which can undergo unimolecular decompositions.