Simulation of the photodetachment spectrum of the pyrrolide anion

The photodetachment spectrum of the pyrrolide anion, C4H4N−, has been measured recently [A.J. Gianola, T. Ichino, R.L. Hoenigman, S. Kato, V.M. Bierbaum, W.C. Lineberger, J. Phys. Chem. A 108 (2004), 10326]. The band associated with the 12A2 ground state of the pyrrolyl radical, C4H4N, can be identified in the spectrum by its resolved vibrational progression. In contrast, the second band, belonging to the 12B1 first excited state of pyrrolyl, is very weak, broad, and unresolved, which suggests the presence of strong vibronic interaction effects. We have performed a theoretical study of the spectrum in the framework of the linear vibronic coupling model, using ab initio calculated parameters. It is shown that a 12B1-12A2 conical intersection is responsible for the unresolved part of the spectrum. The potential-energy surfaces of the 12A2 and 12B1 states of pyrrolyl as a function of the a1 and b2 ground state normal coordinates of pyrrolide have been computed with the MRCI/aug-cc-pVDZ method. Only the b2 modes can couple the involved electronic states in first order. Five totally symmetric modes (a1 symmetry) and four modes of b2 symmetry have been identified as the vibronically most active tuning and coupling vibrations, respectively. Model Hamiltonians for the description of the dynamics in the coupled vibronic manifold of the 12A2 and 12B1 states, including different subsets of these nine modes, have been constructed. The simulated spectra predict a 12A2 band with sharp peaks and a very diffuse 12B1 band stretching from 2.6 to 3.3 eV with a maximum close to 3.0 eV. The calculated spectra are in good agreement with experiment. Reasons for the unexpectedly low intensity of the 12B1 band such as an extremely short lifetime of 12B1 vibronic levels or very different photodetachment cross sections for the 12A2 and 12B1 states are discussed.