AB-INITIO CALCULATIONS OF TAUTOMER EQUILIBRIUM AND PROTONATION ENTHALPY OF 4-METHYLIMIDAZOLE AND 5-METHYLIMIDAZOLE IN THE GAS-PHASE - BASIS-SET AND CORRELATION-EFFECTS

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, Moller-Plesset perturbation (MP2, MP3, MP4) and density functio nal (BLYP, B3LYP, B3PW91) levels. Optimized geometry structures, energ ies and thermochemical properties are carefully analysed. The geometri es are not particularly sensitive to the basis set employed, but chang e appreciably when correlation effects are included. The tautomer equi librium constant and the protonation energy are, in contrast, much mor e sensitive to both basis set and correlation effects, although good r esults are obtained at the simple MP2 level. The computationally less costly density functional (DF) method leads to results similar to thos e from MP2. The gas phase tautomer equilibrium constant and the proton ation enthalpy of 4- and 5-MeIm are calculated at all levels. The resu lts obtained with the 6-31+G, 6-311+G* and 6-311++G** basis sets at M P2 and DF levels are in excellent agreement with available experimenta l values. (C) 1998 Elsevier Science B.V.