Theoretical study of the interaction of a proton with the O, F and Cl atoms of enflurane (CHFCl-CF2-O-CHF2)

Ab initio MP2 and density functional B3LYP calculations were performed to investigate the interaction of a proton with the O, F and Cl atoms of enflurane (CHFCl-CF2-O-CHF2) in the gas phase. The study included the optimized structures, proton affinities, interactions energies and thermodynamic properties of protonated enflurane. The proton affinities (PAs) of the O and Cl atoms are 154.5 and 139.8 kcal mol−1, respectively, whereas PAs of five of the fluorine atoms are between 143.6 and 165.5 kcal mol−1 (MP2 results). In contrast to protonation at the O and Cl atoms, protonation at each of the F atoms of enflurane reveals a striking result, it leads to a cleavage of the C-F bond and formation of an ion-dipole complex between the enfluranyl cation and neutral hydrogen fluoride. The [(enfluranyl)+⋯FH] complexes are weakly bound, the SAPT-calculated interaction energy varies between −12.5 and −11.7 kcal mol−1. The long range attraction in these complexes is dominated by the electrostatic term (70%), whereas the induction and dispersion components contribute by about 15% each. Protonation at the chlorine atom of enflurane does not lead to a cleavage of the C-Cl bond. For the O-protonated enflurane the results from the natural bond orbital analysis (NBO) are discussed in details.