Simulation of Emission Spectra of the PF2 (2B2 → X2B1) Transition by ab Initio Calculation and Franck−Condon Analysis

Abstract

Geometry optimization and harmonic vibrational frequency calculations were carried out on the X2B1 and the lowest 2B2 states of PF2 at the MP2/6-31G* and MP2/6-311G(2df) levels. CCSD(T)/6-311G(2df)//MP2/6-31lG(2df) calculations were also performed to obtain improved adiabatic and vertical transition energies for the two combining electronic states. In addition, Franck−Condon analyses were carried out employing the ab initio data obtained to simulate emission spectra of the PF2 (2B2 → X2B1) transition. Both the computed relative energies and the theoretical spectra confirm that the observed emission spectra by Zhao and Setser were due to the 2B2 → X2B1 transition. Furthermore, the geometry of PF2 in the lowest 2B2 state was also varied in the vibrational intensity calculations to give the best agreement between the theoretical and observed spectra. The bond length and bond angle thus deduced for PF2 in the 2B2 state are 1.628 ± 0.008 Å and 84.9 ± 0.2°, respectively. Spectra involving excited vibrational levels of the 2B2 state were also generated by assuming the Boltzmann distribution at selected temperatures. Comparison of these spectra with the observed one suggested that the vibrational population distribution in the upper state does not follow the Boltzmann rule. The potential surface of the upper 2B2 state under study may be perturbed vibronically by that of the lower, near-linear, 2A1 state via the asymmetric stretching mode. This may explain why the theoretical spectra as obtained using the harmonic oscillator model are different from the experimental one.