Protonated MF 3 (M = N–Bi): Structure, stability, and thermochemistry of the H–MF 3+ and HF–MF 2+ isomers

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The structure, stability, and thermochemistry of the H(MF 3) + isomers (M = N–Bi) have been investigated by MP2 and coupled cluster calculations. All the HF–MF 2 + revealed weakly bound ion–dipole complexes between MF 2 + and HF. For M = N, As, Sb, and Bi they are more stable than the H–MF 3 + covalent structures (free energy differences) by 6.3, 14.3, 32.1, and 73.5 kcal mol −1, respectively. H–PF 3 + is instead more stable than HF–PF 2 + by 21.8 kcal mol −1. The proton affinities (PAs) of MF 3 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 BiF 3 < SbF 3 < AsF 3 < NF 3 < PF 3. 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 PF 3 ≈ NF 3 < AsF 3 < SbF 3 < BiF 3. This trend parallels the fluoride-ion affinities of the MF 2 + cations. For protonated NF 3 and PF 3, the calculations are in good agreement with the available experimental results. As for protonated AsF 3, they support the formation of HF–AsF 2 + rather than the previously proposed H–AsF 3 +. The calculations indicate also that the still elusive H(SbF 3) + and H(BiF 3) + should be viable species in the gas phase, exothermically obtainable by various protonating agents.