Solvent Effects on Global Reactivity Properties for Neutral and Charged Systems Using the Sequential Monte Carlo Quantum Mechanics Model

The energy of the frontier molecular orbitals and reactivity indices such as chemical potential, hardness, and electrophilicity of neutral and charged molecules have been investigated in aqueous solution using explicit model for the solvent with the sequential Monte Carlo/quantum mechanics methodology. The supermolecular structures of the solute-solvent system were generated by Monte Carlo simulation. Statistically uncorrelated structures have been extracted for quantum mechanical calculations of the solute surrounded by the first solvation shell, using explicit water molecules, and the second and third shells as atomic point charges. The supermolecular calculations treating both the solute and the solvent explicitly were performed within density functional theory. The solvent dependence of the frontier molecular orbital energies was analyzed and used to calculate the reactivity indices in solution. The dependence of the results with respect to the number of explicit solvent molecules is also analyzed. It is seen that for the systems considered here, the energies of the highest occupied molecular orbital and the lowest unoccupied molecular orbital show a strong dependence with the number of solvent molecules. However, the properties derived from these are relatively stable. In particular, the results reported here for the reactivity indices obtained using the first solvation shell are similar to those obtained for the limit bulk value. For comparison, the reactivity indices were also calculated in the gas phase and using the polarizable continuum model (PCM). As frequently reported in the literature, neutral molecules do not show significant changes in the reactivity indices between gas phase and the PCM model. However, with the explicit solvent model some important changes were observed: a larger negative chemical potential, a smaller hardness, and a larger electrophilicity. The stabilization of an anion corresponding to a negative chemical potential is obtained only by using explicit solvent molecules. In general, the solvent effect was well treated by using the first solvation shell, and the calculated results are in agreement with the experimental trends. The delocalization of the solute orbitals over the solvent region is analyzed, and its consequence to the reactivity indices are discussed. The importance of including explicit solvent molecules and adopting a statistical approach for the liquid is emphasized.