Photodissociation dynamics in the first absorption band of pyrrole: I. Molecular Hamiltonian and the Herzberg-Teller absorption spectrum for the $1^1A_2(\pi\sigma^*) \leftarrow ilde{X}^1A_1(\pi\pi)$ transition

摘要

This paper opens a series in which the photochemistry of the two lowest$\pi\sigma^*$ states of pyrrole and their interaction with each other and withthe ground electronic state $\tilde{X}$ are studied using ab initio quantummechanics. New 24-dimensional potential energy surfaces for thephotodissociation of the N-H bond and the formation of the pyrrolyl radical arecalculated using the CASPT2 method for the electronic states$\tilde{X}(\pi\pi)$, $1^1A_2(\pi\sigma^*)$ and $1^1B_1(\pi\sigma^*)$ andlocally diabatized. In the first paper, the ab initio calculations aredescribed and the photodissociation in the state $1^1A_2(\pi\sigma^*)$ isanalyzed. The excitation $1^1A_2 \leftarrow \tilde{X}$ is mediated by thecoordinate dependent transition dipole moment functions constructed using theHerzberg-Teller expansion. Nuclear dynamics, including 6, 11, and 15 activedegrees of freedom, is studied using the multi-configurational time-dependentHartree method. The focus is on the frequency resolved absorption spectrum, aswell as on the dissociation time scales and the resonance lifetimes.Calculations are compared with available experimental data. An approximateconvolution method is developed and validated, with which absorption spectracan be calculated and assigned in terms of vibrational quantum numbers. Themethod represents the total absorption spectrum as a convolution of the diffusespectrum of the detaching H-atom and the Franck-Condon spectrum of theheteroaromatic ring. Convolution calculation requires a minimal quantumchemical input and is a promising tool for studying the $\pi\sigma^*$photodissociation in model biochromophores.