Protonation enthalpies in fluorosulfonic acid using ab initio self-consistent reaction field theory

Electrostatic solvation free energies were computed for several small neutral bases and their conjugate acids using a continuum solvation model called the self-consistent isodensity polarizable continuum model (SCIPCM). The solvation energies were computed at the restricted Hartree-Fock (RHF) and second-order Moller-Plesset (MP2) levels of theory, as well as with the Becke3-Lee-Yang-Parr (B3LYP) density functional theory, using the standard 6-31G(**) Gaussian basis set. The RHF solvation energies are similar to those computed at the correlated MP2 and B3LYP theoretical levels. A model for computing protonation enthalpies for neutral bases in fluorosulfonic acid solvent leads to the equation Delta H-prot,H-HSO3F(B) = -PA(B) + Delta E-t(BH+) - Delta E-t(B) + beta, where PA(B) is the gas phase proton affinity for base B, Delta E-t(BH+) is the SCIPCM solvation energy for the conjugate acid, and Delta E-t(B) is the solvation energy for the base. A fit to experimental values of Delta H-prot,H- HSO3F(B) for 10 neutral bases (H2O, MeOH, Me2O, H2S, MeSH, Me2S, NH3, MeNH2, Me2NH, and PH3) gives beta = 238.4 +/- 2.9 kcal/mol when Delta Delta E-t is computed using the 0.0004 e.bohr(-3) isodensity surface for defining the solute cavity at the RHF/6-31G(**) level. The model predicts that for carbon monoxide Delta H-prot,H-HSO3F(CO) = 10 kcal/mol. Thus, protonation of CO is endothermic, and the conjugate acid HCO+ (formyl cation) behaves as a strong acid in fluorosulfonic acid. (C) 1998 John Wiley & Sons, Inc.