The 1,2-hydrogen shift reaction for monohalogenophosphanes PH₂X and HPX (X = F, Cl)

The aim of the present study was to perform a quantum chemical investigation in the 1,2-hydrogen shift reaction for the PH₂X and HPX molecules (X = F,Cl). Several phosphorus–halogen-bearing molecules were studied, including PH₂F, PH₂Cl, HPF, HPCl, HPFH, HPClH, PFH and PClH. The energies of stationary and saddle points on the ground electronic potential energy surface were investigated with post-Hartree–Fock methods [CCSD(T), MP2, QCISD] and different DFT functionals. The PH₂F 1,2-hydrogen shift energy barrier was 75 kcal mol⁻¹ at the CCSD(T) level and only a small increase in this value was observed for the HPF isomerisation. In contrast, the HPCl 1,2-hydrogen shift barrier is higher than the PH₂Cl one, which presented a barrier height of 69 kcal mol⁻¹ among CCSD(T) and composite methods. The rate constants of these unimolecular rearrangements varied from 10⁻⁴⁴ to 10⁻³⁸ s⁻¹, and these isomerisation channels exhibited large half-lives. In addition, the heat of formation of each monohalogenophosphane was also calculated. The Quantum Theory of Atoms in Molecules (QTAIM) and Natural Bond Orbital (NBO) analysis were also employed to characterise the differences between the phosphorous–halogen bonds.