Combined multi-level quantum mechanics theories and molecular mechanics study of water-induced transition state of OH~- + CO_2 reaction in aqueous solution

The presence of a solvent interacting with a system brings about qualitative changes from the corresponding gasphase reactions. A solvent can not only change the energetics along the reaction pathway, but also radically alter the reaction mechanism. Here, we investigated the water-induced transition state of the OH~- + CO_2→ HCO_3~- reaction using a multi-level quantum mechanics and molecular mechanics method with an explicit water model. The solvent energy contribution along the reaction pathway has a maximum value which induces the highest energy point on the potential of mean force. The charge transfer from OH~- to CO_2 results in the breaking of the OH~- solvation shell and the forming of the CO_2 solvation shell. The loss of hydrogen bonds in the OH~-solvation shell without being compensated by the formation of hydrogen bonds in the CO_2 solvation shell induces the transition state in the aqueous solution. The calculated free energy reaction barrier at the CCSD(T)/MM level of theory, 11.8 kcal/mol, agrees very well with the experimental value, 12.1 kcal/mol.     

Solvent effects and potential of mean force study of the S_N2 reaction of CH_3F+CN~- in water

We used a combined quantum mechanics and molecular mechanics(QM/MM) method to investigate the solvent effects and potential of mean force of the CH_3F+CN~- reaction in water. Comparing to gas phase, the water solution substantially affects the structures of the stationary points along the reaction path. We quantitatively obtained the solvent effects' contributions to the reaction: 1.7 kcal/mol to the activation barrier and -26.0 kcal/mol to the reaction free energy.The potential mean of force calculated with the density functional theory/MM theory has a barrier height at 19.7 kcal/mol,consistent with the experimental result at 23.0 kcal/mol; the calculated reaction free energy at -43.5 kcal/mol is also consistent with the one estimated based on the gas-phase data at -39.7 kcal/mol.