Protonated MF3 (M = N–Bi): Structure, stability, and thermochemistry of the H–MF3+ and HF–MF2+ isomers

The structure, stability, and thermochemistry of the H(MF3)+ isomers (M = N–Bi) have been investigated by MP2 and coupled cluster calculations. All the HF–MF2+ revealed weakly bound ion–dipole complexes between MF2+ and HF. For M = N, As, Sb, and Bi they are more stable than the H–MF3+ covalent structures (free energy differences) by 6.3, 14.3, 32.1, and 73.5 kcal mol−1, respectively. H–PF3+ is instead more stable than HF–PF2+ by 21.8 kcal mol−1. The proton affinities (PAs) of MF3 at the M atom range from 91.9 kcal mol−1 (M = Bi) to 156.5 kcal mol−1 (M = P), and follow the irregular periodic trend BiF3 < SbF3 < AsF3 < NF3 < PF3. The PAs at the F atom range instead from 131.9 kcal mol−1 (M = P) to 164.9 kcal mol−1 (M = Bi), and increase in the more regular order PF3 ≈ NF3 < AsF3 < SbF3 < BiF3. This trend parallels the fluoride-ion affinities of the MF2+ cations. For protonated NF3 and PF3, the calculations are in good agreement with the available experimental results. As for protonated AsF3, they support the formation of HF–AsF2+ rather than the previously proposed H–AsF3+. The calculations indicate also that the still elusive H(SbF3)+ and H(BiF3)+ should be viable species in the gas phase, exothermically obtainable by various protonating agents.