Vibrational spectroscopy of phosphaethyne (HCP). I. Potential energy surface, variational calculations, and comparison with experimental data

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Abstract
A new potential energy surface for the electronic ground state of HCP (phosphaethyne) is presented. The ab initio calculations are based on the internally contracted multireference configuration interaction method using atomic basis functions of quintuple-zeta quality. The ca. 1 000 calculated energy points are fitted to a complex analytical function, which is employed in the subsequent quantum-mechanical variational calculations for total angular momentum J=0–2. The majority of the first 850 vibrational states is assigned in terms of three quantum numbers. The calculated energies are compared to various sets of experimental data—obtained from high-resolution Fourier-transform infrared spectra, dispersed fluorescence spectra, and stimulated-emission pumping spectra. The energy regime, which is covered, extends up to about 25 000 cm−1 above the ground vibrational state. The agreement is excellent; every experimentally assigned level is uniquely related to a calculated vibrational state. Some experimental misassignments at the lower ends of the high-energy polyads are corrected. The progression of “isomerization” (i.e., large-amplitude bending) states, which was experimentally observed by Ishikawa et al. [J. Chem. Phys. 106, 2980 (1997)], is quantitatively confirmed.