Conformationally selective photodissociation dynamics of propanal cation.

We have previously reported experimental evidence for conformationally selective dissociation of propanal cation that was interpreted, on the basis of ab initio multiple spawning calculations, as arising from distinct dynamics in the excited state manifold of the cation. Two conical intersections (CIs) are accessible from Franck-Condon points on the dark state; however, different conformers prefer different CIs and quench to different regions on the ground state. In this paper, we extend our initial report to include experimental results for the partially deuterated propanal cation as well as detailed characterization of the ground state potential energy surface and statistical calculations of the ground state dissociation dynamics. The DC slice imaging experiments show a bimodal velocity distribution for H elimination with the observed branching ratio of the two channels different for the cis and gauche conformers. H(D)-elimination experiments from deuterated propanal cation support the dissociation mechanism proposed in the earlier report. We further investigate reaction rates on the ground state using Rice-Ramsperger-Kassel-Marcus theory. We find that the experimental results are consistent with a mechanistic picture where the ground state dissociation is statistical, and conformer specificity of the dissociation products arises because of the different populations in distinct ground state isomers after photoexcitation due to ultrafast quenching to the ground state.