EXCITED-STATE POTENTIAL SURFACE CROSSINGS IN ACROLEIN - A MODEL FOR UNDERSTANDING THE PHOTOCHEMISTRY AND PHOTOPHYSICS OF ALPHA,BETA-ENONES

The interplay between the ground- and the three low-lying singlet/triplet excited-state surfaces (S-1, T-2, and T-1) in s-trans- and s-cis-acrolein has been studied using CASSCF computations at the 6-31G(*) level. The objective is to provide a model for understanding alpha,beta-enone photochemistry and photophysics. Two different photochemically active relaxation paths starting from a planar S-1 (1)(n-pi(*)) excited state minimum have been documented. The first of these pathways involves a radiationless decay via intersystem crossing to the triplet manifold, leading to production of a short-lived T-1 (3)(pi-pi(*)) twisted intermediate. This intermediate then decays, via a second intersystem crossing, to the ground state, leading to isomerization of the acrolein double-bond. The second relaxation path involves the singlet manifold only. In this case relaxation to S-0 occurs via a single decay channel which corresponds to a S-1/S-0 conical intersection. This conical intersection lies 15 and 10 kcal mol(-1) above the 1(n-pi(*)) s-trans- and (1)(n-pi(*)) s-cis-acrolein, respectively. Production of oxetene is found to occur via the singlet path starting exclusively from the S-1 (1)(n-pi(*)) s-cis-acrolein. The computational results agree well with the available experimental data. The existence of a T-1 (3)(pi-pi(*)) intermediate is supported by the observation of a 280-310-nm transient absorption in both acyclic and cyclic alpha,beta-enones. Further, the existence of a barrier to production of oxetene is in agreement with the fact that this product accumulates when acyclic alpha,beta-enones are irradiated with light near 250 nm, but it is not detected when alpha,beta-enones are irradiated near 300 nm.