The salt effect on the intramolecular proton transfer reaction of glycine in aqueous NaCl solution was considered using the reference interaction-site model self-consistent field theory. The free energy profiles were computed for various salt concentrations. The set of profiles clearly showed that the free energy gap between the zwitterionic form (ZW) and the neutral form (NF) became larger as the NaCl concentration increased. The transition-state structure of the solute glycine approaches the NF structure, and the reaction barrier height is reduced. From the energy decomposition analysis of the free energy of solvation, we found that the changes in the solvation free energy are determined by the balance between the electronic distortion energy, the electrostatic interaction, the solvent reorganization, and the entropic effects caused by the NaCl addition. The salt effect of NaCl makes all the species associated with the reaction unstable; however, the destabilization of NFs caused by NaCl addition is stronger than that of ZWs. In the ZW, the penalty coming from the solvent reorganization and the entropic effects are compensated by the strong solute-solvent electrostatic interaction.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry
- Condensed Matter Physics
- Atomic and Molecular Physics, and Optics
- Electronic, Optical and Magnetic Materials
- Materials Chemistry