Several computational methods including the conductor-like polarizable continuum model, CPCM with both UAKS and UAHF cavities, Cramer and Truhlar's generalized Born solvation model, SM5.4(AM1), SM5.4(PM3), and SM5.43R(mPW1PW91/6-31+G(d)), and mixed QM/MM-Ewald simulations were used to calculate the pK(a) values of acetate and bicarbonate anions in aqueous solution. This work provided a critical and comprehensive assessment of the quality of these theoretical models in the calculation of aqueous solvation free energies for the singly charged acetate and bicarbonate ions, as well as the doubly charged acetate dianion and carbonate dianion. It was shown that QM/MM-Ewald simulations could give an accurate and consistent evaluation of the pK(a) values of acetate and bicarbonate based on both the relative and absolute pK(a) formulas, while other methods could yield satisfactory results only for certain calculations. However, this does not mean that the current QM/MM-Ewald protocol is superior to other methods. The useful information obtained in this investigation is that both the absolute and relative pK(a) formulas should better be tested in accurate calculations of pK(a) values based on any methods.

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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.

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