Photodissociation and photoisomerization dynamics of CH2=CHCHO in solution

By means of time-resolved Fourier transform infrared absorption spectroscopy, we have investigated the 193 nm photodissociation and photoisomerization dynamics of the prototype molecule of alpha,beta-enones, acrolein (CH2 = CHCHO) in CH3CN solution. The primary photolysis channels and absolute branching ratios are determined. The most probable reaction mechanisms are clarified by control experiments monitoring the product yields varied with the triplet quencher addition. The predominant channel is the 1,3-H migration yielding the rearrangement product CH3CH = C = O with a branching ratio of 0.78 and the less important channel is the alpha cleavage of C - H bond yielding radical fragments CH2 = CHCO+H with a branching ratio of only 0.12. The 1,3-H migration is strongly suggested to correlate with the triplet (3)(pi pi(*)) state rather than the ground S-0 state and the alpha cleavage of C - H bond is more likely to proceed in the singlet S-1 (1)(n pi(*)) state. From the solution experiments we have not only acquired clues clarifying the previous controversial mechanisms, but also explored different photochemistry in solution. Compared to the gas phase photolysis which is dominated by photodissociation channels, the most important channel in solution is the photoisomerization of 1,3-H migration. The reason leading to the different photochemistry in solution is further ascribed to the solvent cage effect.