Equilibrium Structures of the Phosphorus Trihalides PF₃ and PCl₃, and the Phosphoranes PH₃F₂, PF₅, PCl₃F₂, and PCl₅

Among the title species, a reliable and accurate equilibrium geometry ( r_e structure) is available only for PF₃, which has been determined experimentally more than 20 years ago. Here, we report accurate r_e structures for all title molecules, which were obtained using a composite computational approach based on explicitly correlated coupled-cluster theory (CCSD(T)-F12b) in conjunction with a large correlation-consistent basis set (cc-pCVQZ-F12) to take core-valence electron correlation into account. Additional terms were included to correct for the effects of iterative triple excitations (CCSDT), noniterative quadruple excitations (CCSDT(Q)), and scalar relativistic contributions (DKH2-CCSD(T)). The performance of this computational procedure was established through test calculations on selected small molecules (PH, PF, PCl, PH₂, PF₂, and PH₃). For PF₃, PCl₃, PH₃F₂, and PF₅ sufficiently accurate experimental ground-state rotational constants from the literature were used to determine semiexperimental r_e structures, which were found to be in excellent agreement with the corresponding best estimates from the current composite approach. The recommended equilibrium structural parameters are for PCl₃, r_e(PCl) = 203.94 pm and θ_e(ClPCl) = 100.18°; for PH₃F₂, r_e(PH_eq) = 138.38 pm and r_e(PF_ax) = 164.15 pm; for PF₅, r_e(PF_eq) = 153.10 pm and r_e(PF_ax) = 157.14 pm; for PCl₃F₂, r_e(PCl_eq) = 200.21 pm and r_e(PF_ax) = 159.37 pm; and for PCl₅, r_e(PCl_eq) = 201.29 pm and r_e(PCl_ax) = 211.83 pm. The associated uncertainties are estimated to be ±0.10 pm and ±0.10°, respectively.