Calculation of the vertical ionization potentials of HCNO,HNCO, HOCN,and HN3 by perturbation corrections to Koopmans'Theorem

Vertical ionization potentials (VIP's) have been calculated for HCNO, HNCO, HOCN, and HN₃, using Rayleigh—Schrödinger perturbation theory (RSPT) to determine corrections to Koopmans' Theorem. The calculated VIP's are used to resolve discordances between previous assignments of the PE spectra of HCNO, HNCO, and HN₃. Very little is known about HOCN. The present work contains predicted VIP's for this molecule, which it is hoped will aid in its identification in the laboratory. A diagram is presented correlating the VIP's of the isoelectronic series N₂O, HN₃, HOCN, HNCO, HCNO, and CO₂. It is shown that the variation in the magnitudes of the VIP's associated with the ionization of π electrons, and in the splittings induced in these VIP's by deviations from a linear geometry, can be accounted for qualitatively using a simple Hückel model.