Equilibrium Structures of the Phosphorus Trihalides PF and PCl, and the Phosphoranes PHF, PF, PClF, and PCl.

Among the title species, a reliable and accurate equilibrium geometry ( r structure) is available only for PF, which has been determined experimentally more than 20 years ago. Here, we report accurate r 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, PH, PF, and PH). For PF, PCl, PHF, and PF sufficiently accurate experimental ground-state rotational constants from the literature were used to determine semiexperimental r 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 PCl, r(PCl) = 203.94 pm and θ(ClPCl) = 100.18°; for PHF, r(PH) = 138.38 pm and r(PF) = 164.15 pm; for PF, r(PF) = 153.10 pm and r(PF) = 157.14 pm; for PClF, r(PCl) = 200.21 pm and r(PF) = 159.37 pm; and for PCl, r(PCl) = 201.29 pm and r(PCl) = 211.83 pm. The associated uncertainties are estimated to be ±0.10 pm and ±0.10°, respectively.