Ab initio calculations of tautomer equilibrium and protonation enthalpy of 4- and 5-methylimidazole in the gas phase: Basis set and correlation effects

Abstract

Ab initio calculations are performed to evaluate molecular properties of 4- and 5-methylimidazole (MeIm) and the protonated form (MeImH +) with extended basis sets ranging from 6-31G ∗ to 6-311 + +G ∗∗ at Hartree-Fock, Møller-Plesset perturbation (MP2, MP3, MP4) and density functional (BLYP, B3LYP, B3PW91) levels. Optimized geometry structures, energies and thermochemical properties are carefully analysed. The geometries are not particularly sensitive to the basis set employed, but change appreciably when correlation effects are included. The tautomer equilibrium constant and the protonation energy are, in contrast, much more sensitive to both basis set and correlation effects, although good results are obtained at the simple MP2 level. The computationally less costly density functional (DF) method leads to results similar to those from MP2. The gas phase tautomer equilibrium constant and the protonation enthalpy of 4- and 5-MeIm are calculated at all levels. The results obtained with the 6-31+G ∗, 6-311+G ∗ and 6-311++G ∗∗ basis sets at MP2 and DF levels are in excellent agreement with available experimental values.