A simulation of the photoelectron spectrum of pyrazolide

Building on previous theoretical and spectroscopic studies of the pyrazolyl radical, a new three-state quasidiabatic Hamiltonian is reported which reproduces not only the equilibrium geometries and harmonic frequencies of the nominal X (2)A(2) state and low-lying A B-2(1) excited state, but also the minimum energy points on the lowest two-state (X (2)A(2),A B-2(1)) and three-state (X (2)A(2),A B-2(1),B B-2(2)) seams of conical intersection. The three-state Hamiltonian includes all terms through second order in both the diagonal and off-diagonal blocks. Its construction is accomplished in two steps. First, a nascent Hamiltonian, centered at the lowest energy two-state conical intersection, is determined using ab initio gradients and derivative couplings. Then, the nascent Hamiltonian is improved by optimizing selected contributions to the second-order coefficients to better reproduce relevant minima and harmonic frequencies. This Hamiltonian is then expressed in a basis tailored to describe the neutral states of interest under the multimode vibronic coupling approximation. The vibronic Hamiltonian is diagonalized to obtain negative ion photoelectron spectra for pyrazolide-h(3) and the completely deuterated analog pyrazolide-d(3). The resultant spectra, determined employing vibronic Hamiltonians as large as 500 million terms, compare favorably to recent theoretical and spectroscopic results for pyrazolyl-d(3) and to spectroscopic results for pyrazolyl-h(3), for which no reliable simulations had been available. (C) 2008 American Institute of Physics.