Combined CASSCF and MR-CI study on photoinduced dissociation and isomerization of acryioyl chloride

The potential energy surfaces of isomerization and dissociation reactions for CH₂CHCOCl in the S₀, T₁, T₂, and S₁ states have been mapped with DFT, CASSCF, MP2, and MR-CI calculations. Rate constants for adiabatic and nonadiabatic processes have been calculated with the RRKM rate theory, in conjugation with the vibronic interaction method. Mechanistic photochemistry of CH₂CHCOC1 at 230-310 nm has been characterized through the computed potential energy surfaces and rate constants. Upon photoexcitation of CH₂CHCOCl at 310 nm, the S₁→T₁ intersystem crossing is the dominant primary process, which is followed by the 1,3-Cl migration along the T₁ pathway. Meanwhile, the S₁→S₀ internal conversion occurs with considerable probability and the subsequent trans-cis isomerization proceeds in the ground state. The C-Cl bond cleavage is an exclusive primary channel upon photoexcitation of gaseous CH₂CHCOCl at 230 nm. The direct C-Cl bond cleavage is partially blocked by effects of the matrix, and the internal conversion from S₁ to S₀ becomes an important process for the excited molecule to deactivate in the condensed phase. The present calculations not only provide a reasonable explanation of the experimental findings, but also give new insight into the mechanistic photochemistry of CH₂CHCOCl.