The Photoelectron Spectrum of Pyrrolide: Nonadiabatic Effects due to Conical Intersections

Abstract

The negative ion photoelectron spectrum of the pyrrolide-h4 anion and the completely deuterated analogue, pyrrolide-d4, are computed employing the multimode vibronic coupling approach, based on a two-state, fully quadratic, quasi-diabatic Hamiltonian, Hd, which accurately represents the vicinity of the ab initio determined equilibrium geometry of the ground 2A2 state as well as the minimum energy crossing point (MECP) on the symmetry-allowed 2A2−2B1 accidental seam of conical intersection. The ab initio data are obtained from multireference configuration interaction wave functions based on triple-ζ quality atomic orbital bases. The determined photoelectron spectra, which are converged with respect to the vibronic basis, compare favorably with previous spectroscopic results, for pyrrolide-h4. The principal impact of the seam of conical intersection is a significant perturbation of the portion of the spectrum attributable to the 2B1 state, in agreement with previous analyses. However, despite the fact that the MECP is approximately 0.5 eV above the 2A2 minimum, its manifestations are shown to be evident in the photoelectron spectrum near threshold. By comparing the simulated and measured photoelectron spectrum, it is deduced that the electronic transition moment for the production of the 2A2 state of pyrrolyl from pyrrolide is approximately twice that for the production of the 2B1 state.