Photodissociation dynamics in the first absorption band of pyrrole. I. Molecular Hamiltonian and the Herzberg-Teller absorption spectrum for the A21(πσ*)←X̃1 A1(ππ) transition

Abstract

This paper opens a series in which the photochemistry of the two lowest πσ* states of pyrrole and their interaction with each other and with the ground electronic state X̃ are studied using ab initio quantum mechanics. New 24-dimensional potential energy surfaces for the photodissociation of the N-H bond and the formation of the pyrrolyl radical are calculated using the multiconfigurational perturbation theory (CASPT2) for the electronic states X̃(ππ), 1A(πσ*), and 1B(πσ*) and locally diabatized. In this paper, the ab initio calculations are described and the photodissociation in the state 1A(πσ*) is analyzed. The excitation 1 A←X̃ is mediated by the coordinate dependent transition dipole moment functions constructed using the Herzberg-Teller expansion. Nuclear dynamics, including 6, 11, and 15 active degrees of freedom, are studied using the multi-configurational time-dependent Hartree method. The focus is on the frequency resolved absorption spectrum as well as on the dissociation time scales and the resonance lifetimes. Calculations are compared with available experimental data. An approximate convolution method is developed and validated, with which absorption spectra can be calculated and assigned in terms of vibrational quantum numbers. The method represents the total absorption spectrum as a convolution of the diffuse spectrum of the detaching H-atom and the Franck-Condon spectrum of the heteroaromatic ring. Convolution calculation requires a minimal quantum chemical input and is a promising tool for studying the πσ* photodissociation in model biochromophores.

Nonadiabatic quantum dynamics of the coherent excited state intramolecular proton transfer of 10-hydroxybenzo[h]quinoline

“…The diabatic PESs are related to the adiabatic ones as and are parametrized according to a predefined ansatz inspired by the reaction path formalism [41][42][43].…”

Cation, Anion, and Radical Isomers of C4H4N: Computational Characterization and Implications for Astrophysical and Planetary Environments

“…With these observations in mind, the presence of pyrrole (C 4 H 5 N) in Titan's haze (or elsewhere in the interstellar medium) cannot be discounted, and we thus consider the direct formation of anionic (A -) from this hydrogenated five member ring to be a viable possibility. The photofragmentation and dynamics of excited state pyrrole have been extensively studied [93][94][95][96][97][98][99][100][101][102][103][104] (and references therin). Irradiation of ground state pyrrole (S 0 ( 1 A 1 )) at wavelengths from 238-250 nm (corresponding to photons of energy >4.8 eV) excites the molecule into the 1 πσ * ( 1 A 2 ) excited state from which hydrogen atoms are ejected quickly with high kinetic energy, yielding the ground state radical (A ) 93,100,101 ; though this is formally electricdipole forbidden, this mode intensifies by borrowing vibrational energy from higher energy states 100,103 .…”

Coupled Cluster Characterization of 1-, 2-, and 3-Pyrrolyl: Parameters for Vibrational and Rotational Spectroscopy

“…), in km/mol. A comparison between the fc-CCSD(T)/ANO0 harmonic vibrational frequencies calculated in this work and those calculated previously for 1-pyrrolyl 22,31,39,62 is shown in Table 4.…”