A QM/MM Monte Carlo simulation study of solvent effects on the decarboxylation reaction of N-carboxy-2-imidazolidinone anion in aqueous solution

The solvent effects on the decarboxylation reaction of N-carboxy-2-imidazolidinone anion in aqueous solution have been investigated by a combined quantum mechanical and molecular mechanical (QM/MM) Monte Carlo simulation method. In the present approach, the gas-phase intrinsic reaction coordinate of the reaction is first obtained by ab initio molecular orbital calculations at the RHF/6-31+G(d) level. Then, the potential of mean force for the decarboxylation reaction is determined via statistical perturbation theory using the combined QM/MM-AM1/TIP3P potential in Monte Carlo simulations. The computed free energy of activation in water is 22.7 +/- 0.2 kcal/mol, in very good agreement with the experimental value of 23.2 kcal/mol. Detailed insights into the structural and energetic nature of the differential solvation of the reactant and the transition state are presented.