Electric properties of hydrated uracil: From micro- to macrohydration
Résumé
The accurate description of solvent effects on electric and optical properties of a solvated molecule is a complex task involving appropriate consideration of short-range as well as long-range intermolecular interactions having direct (local field induced) and indirect (solvent structure induced) impacts on the properties. In this study, we investigate the effects of hydration on dipole moment and dipole polarizability of uracil (U), focusing on the evolution of the properties from micro- to macrohydration regime. The microhydrated structures were generated by Global Search Algorithm of Minima (GSAM). Our results show a general increase in both induced dipole moment and isotropic polarizability with the cluster size, with a sudden decrease of the polarizability when passing from U(H2O)5 to U(H2O)6. To explain the underlying effects, the variational-perturbational energy decomposition scheme (VP-EDS) was used. The interplay of hydrogen bonding between water molecules and the uracil molecule and hydrogen bonding between water molecules themselves is shown to be the driving force behind these trends. To represent the macrohydrated uracil, supermolecular (SM) and rigorous local field (RLF) methods were used, with representative structure generation performed by molecular dynamics (MD). The trends of induced electric properties with the cluster size are shown to be consistent between micro- and macrohydration regimes. While the induced dipole moment increases monotonically to a converged value of 1.305 ± 0.009 au, the induced isotropic polarizability, reaching maximum of 5.94 au for n = 8, slowly decreases again, and converges to the negative value (−1.21 ± 0.12 au), showing the decrease of the total polarizability of uracil in water. This study also clearly demonstrates that it is the electrostatic interaction which governs the significant property changes on going from micro- to macrohydration.