Ab initio MO study on the protonated species of fluoro-methanes and fluoro-silanes: Structures and energetics

Abstract

Geometrical surveys of fluoro-methanes, fluoro-silanes, and their protonated species were performed using Hartree—Fock (HF) MO and second-order Møller—Plesset perturbation (MP2) methods employing a 6-31G* basis set. The HF method was found to be inaccurate for the protonated difluoro-methane (H 2FCFH +) species for which an excessively long CF bond length (2.048 Å) was predicted; subsequent MP2 optimizations shortened the Hartree—Fock bond length to 1.768 Å. Relative proton affinities computed using the MP2/6-31++G**(2 d) method are in good agreement with experimental values, while the calculated absolute proton affinities are lower by 5–6 kcal mol −1. The introduction of further F atoms into methyl fluoride (CH 3F) and fluoro-silane (SiH 3F) is the cause of the smaller computed absolute proton affinity values. Dissociation energies of the protonated species were computed using the MP2/6-31++G**(2 d) method; that of the protonated difluoro-methane species, H 2FCFH +, is found to be 17 kcal mol −1, which is only about one half of that of other protonated species (such as H 3CFH +, H 3SiFH +, and H 2FSiFH +). This lower value can be clearly interpreted using the perturbed MO approaches that are based on the fragment orbital analysis from Fock matrix partitioning.