Photochemistry of pyrrole: Time-dependent quantum wave-packet description of the dynamics at the ¹φσ*-S ₀ conical intersections

The photoinduced hydrogen-elimination reaction in pyrrole via the conical intersections of the two π 1 σ* excited states with the electronic ground states [B11 (π σ*) - S0 and A21 (π σ*) - S0] have been investigated by time-dependent quantum wave-packet calculations. Model potential-energy surfaces of reduced dimensionality have been constructed on the basis of accurate multireference ab initio electronic-structure calculations. For the B11 - S0 conical intersection, the model includes the NH stretching coordinate as the tuning mode and the hydrogen out-of-plane bending coordinate as the coupling mode. For the A21 - S0 conical intersection, the NH stretching coordinate and the screwing coordinate of the ring hydrogens are taken into account. The latter is the dominant coupling mode of this conical intersection. The electronic population-transfer processes at the conical intersections, the branching ratio between the dissociation channels, and their dependence on the initial preparation of the system have been investigated for pyrrole and deuterated pyrrole. It is shown that the excitation of the NH stretching mode strongly enhances the reaction rate, while the excitation of the coupling mode influences the branching ratio of different dissociation channels. The results suggest that laser control of the photodissociation of pyrrole via mode-specific vibrational excitation should be possible. The calculations provide insight into the microscopic details of ultrafast internal-conversion processes in pyrrole via hydrogen-detachment processes, which are aborted at the π 1 σ* - S0 conical intersections. These mechanisms are of relevance for the photostability of the building blocks of life (e.g., the DNA bases).