Definitive ab initio structure for the X̃ 2A′H2PO radical and resolution of the P–O stretching mode assignment

Abstract

Previous ab initio studies of the X̃ 2A′H2PO radical have reported dramatically differing P–O bond distances when using spin-restricted wave functions predicting two artifactual isomers of H2PO: a singly bonded oxygen-centered radical and a doubly bonded phosphorus-centered radical. We show that large basis sets coupled with high levels of dynamical electron correlation are required to correctly describe the P–O bond in H2PO as well as the unpaired electron density as evidenced by the Fermi contact terms and anisotropic components of the 31P, 1H, and 17O hyperfine splitting (hfs) constants. The optimized geometry, harmonic vibrational frequencies, and hfs constants of H2PO were determined at several coupled-cluster levels of theory using both spin-restricted (ROHF) and spin-unrestricted (UHF) Hartree–Fock reference wave functions. The geometrical parameters at the coupled-cluster level with single, double, and perturbatively applied triple substitutions [CCSD(T)] using Dunning’s correlation consistent polarized valence quadruple-ζ basis set (cc-pVQZ) are r(P–O)=1.492 Å; r(P–H)=1.410 Å; ∠(HPH)=102.63°; ∠(HPO)=114.92°. These are in excellent agreement with those derived from recent gas phase microwave data, with the surprising exception of the P–H distance which deviates 0.02 Å from experiment. The value of the P–O harmonic stretching frequency at the CCSD(T) level within the cc-pVQZ basis set is 1190 cm−1, in good agreement with the experimental fundamental frequency of 1147 cm−1 obtained by Withnall and Andrews and in constrast to previous speculation that this experimental band may have been misassigned. Hyperfine splitting constants determined at the TZ2P(f,d)/UHF-CCSD(T) level are in very good agreement with experimental values with an average deviation of 23 MHz.