Ab initio structures and quartic force fields of phosphine oxide and phosphine sulphide

Phosphine oxide, OPH3, and phosphine sulphide, SPH3 have been studied ab initio using various combinations of theoretical methods (MP2, CCSD, CCSD(T)) and basis sets (cc-pV(Xþd)Z, cc-pCVXZ, X¼T,Q). For both molecules, high-level ab initio full-quartic force fields have been calculated, and these have been used to predict the most important spectroscopic constants for several isotopologues. For OPH3, the semi-experimental equilibrium geometry and harmonic frequencies have been determined, by combining the theoretically calculated anharmonic corrections to the experimentally determined ground-state rotational constants and vibrational fundamentals. It is proved that, after applying anharmonic corrections, the unusually large disagreement between the experimental geometry and any high-level theoretical approach disappears.