Michael H. Palmer, Marcello Coreno, Monica de Simone, Cesare Grazioli, Søren Vrønning Hoffmann, Nykola C. Jones, Kirk A. Peterson, R. Alan Aitken, Cécile Rouxel

New synchrotron based studies of the photoelectron ionization spectra (PES) for the isomeric 1- and 2-methyltetrazoles (1- and 2-MeTet) show markedly higher resolution than previous reports. The unusual spectral profiles suggest that a considerable overlay of the ionic states occurs for both molecules. Under these circumstances of near degeneracy of two or more ionic states, mutual annihilation of vibrational fine structure occurs for all except the strongest vibrational states; the PES just reflects the resultants rather than full spectra. Theoretical determination of the adiabatic ionization energies (AIEs) proved a challenge; the most successful method was second order Møller-Plesset perturbation theory (MP2). These calculations suggest that the lowest PES bands for both isomers contain ionization both from lone pair σ-orbitals (2A') on the N-atoms (LPN) and π-orbitals (2A″). The lowest experimental AIEs are as follows: 1-MeTet is 10.315 eV assigned to 12A', while 2-MeTet is 10.543 eV assigned to 12A″. Franck-Condon analysis shows that the lowest ionization energy regions of both spectra are dominated by IE from the LPN2A' manifold, even though the 2A″ states have a higher absolute intensity. In this example, we have utilized a VUV Rydberg state to assist simplification of the PES; more frequently, the PES assignment is simpler and assists the location of Rydberg states in the VUV. The very slow spectral onset for 2-MeTet demonstrates the importance of vertical ionization energy calculations since maxima are more readily measured than slow onsets. These were performed at the equilibrium structure of the X1A' state, using both multi-reference multi-root configuration interaction and the ionization potential variant of the equations-of-motion coupled cluster method, with single and double excitations (EOMIP-CCSD). This enabled the principal ionization bands to be identified over a wider range of energy. Attempts to study the higher ionic states by EOMIP-CCSD showed that several states of each symmetry are close to degenerate for 1-MeTet, in particular. A multi-configuration self-consistent field study confirmed the small separation of ionic states, but state switching during the optimization process largely disabled this method.