Restricted open-shell Kohn-Sham theory: Simulation of the pyrrole photodissociation

Abstract

The authors study the photodissociation reactions of pyrrole and N-methylpyrrole using first-principles molecular dynamics. The first excited state is described with restricted open-shell Kohn-Sham theory. They find a small barrier in the excited state potential energy surface. The possibility of energy redistribution near the Franck-Condon region leads to two different reaction channels in on-the-fly simulations on a single diabatic potential energy surface. The results are discussed in comparison with previous ab initio calculations and with experiments.

Cluster‐Controlled Photofragmentation: The Case of the Xe–Pyrrole Cluster

“…[14] Although for the slow component the agreement between our experiment and the calculations of Frank et al [14] is very good, there is a discrepancy for the fast component.…”

“…We believe that the dissociation of Py in this red wing of the absorption spectrum takes place entirely on the S 1 ( 1 ps*, 1 1 A 2 ) excited electronic state, as was proposed by Frank et al [14] The fast H component appears due to rapid dissociation (< 0.1 ps) on this electronic state caused by the interaction of the antibonding s* orbital localized on the NÀH bond. The slow H component appears due to excited-state dissociation following the relaxation in the local minimum near the FranckCondon region.…”

“…[12] Others have proposed dissociation on the 1 ps* state following relaxation within this excited state. [14] For excitation at short wavelengths, dissociation on the S 0 (X 1 A 1 ) ground state after internal conversion from the initially excited S 2 ( 1 pp*, 1 B 2 ) state has been documented. [3] The excited molecules fall through the S 2 /S 1 conical intersection promoted by the out-of-plane deformation of the Py ring.…”

“…In the wavelength region between 193 and 254 nm, the velocity distribution of the H-atom photofragment is bimodal and consists of a fast and a slow component. [3][4][5][6][7][8][9][10][11][12][13][14] At long wavelengths the fast H-atom component dominates the slow one, while at low wavelengths this trend is reversed. [6] It is a general agreement amongst theorists and experimentalists that the origin of the fast H atoms is the direct dissociation of the NÀH bond following excitation of Py to the 1 ps* state.…”

Non-adiabatic dynamics of pyrrole: Dependence of deactivation mechanisms on the excitation energy

“…1b). The NH-stretching mechanism, which has been explored by quantum [19][20][21] and trajectory-based [22,23] dynamics simulations, should be the main source of fast H atoms. Time-resolved femtoseconds investigations [24] have revealed that when pyrrole is pumped at 250 nm, this process takes place in approximately 0.1 ps, which has been also confirmed by dynamics simulations [23].…”