Tunneling dynamics dictated by the multidimensional conical intersection seam in the pi sigma*-mediated photochemistry of heteroaromatic molecules

Abstract

The pi sigma*-mediated photochemistry of heteroaromatic molecules has provoked the investigation of the conical intersection dynamics. The Born-Oppenheimer approximation fails at the conical intersection where the S-1 (pi pi*) and S-2 (pi sigma*) states cross. The nonadiabatic transitions are much influenced by the nuclear configuration of the reactive flux particularly in the curve-crossing region encountered along the reaction pathway. In this article, we focus on the tunneling dynamics of phenols and thiophenols. The O (S)-H bond cleavage occurs via tunneling through the barrier which is dynamically shaped by the upper-lying S-1/S-2 conical intersection in terms of the couplings at the individual branching planes as well as along the (3N-8) dimensional seam coordinates. State-specific tunneling rates and their interpretation are given for phenol, substituted phenols, thiophenol, ortho-substituted thiophenols, and benzenediols including their 1:1 water clusters. The completely orthogonal modes to the tunneling coordinate are very critical in the dynamic shaping of the reaction barrier.