Ab initio quantum mechanics; Deshielding; GIAO NMR shielding; Phosphorus NMR shielding; Second order many-body perturbation theory

olecular orbital calculations on PH2 and PF2 and some of their low-lying cationic states, followed by Franck-Condon calculations, have been performed with the objective of simulating HeI photoelectron band of these radicals. The molecular orbital calculations involved MP2 and CCSD(T) geometry optimization and frequency calculations, with basis sets of size up to 6-311G(3df,2p), and as well as G1/G2 calculations. Franck-Condon simulations of photoelectron bands were performed using force constants derived from the ab initio calculations. Based on comparison between simulated and observed spectra, the first adiabatic ionization energy of PH2 has been confirmed as (9.84 ± 0.01) eV and the lowest singlet-triplet separation in PH2+ (1A1 − 3B1 has been deduced as (0.78 ± 0.04) eV. Also, the first adiabatic ionization energy of PF2, corresponding to the ionization PF2+ 1A2 ← PF2 X2B1, has been established as (8.84 ± 0.01) eV. The vibrational structure observed in the first band of PF2 has been assigned to excitation of the symmetric stretching mode (v1) in PH2 in PF2+(X1A1) and the vibrational structure observed in the second band of PH2 has been assigned to excitation of the deformation mode (v2) in PH2+(a3B1