Microsolvation versus Encapsulation in Mono, Di, and Trivalent Cations

The effects of the formal charge in the stability and bonding of water cavities when solvating a cation are studied here using [X(H₂ O)₂₀ ]^q+ clusters starting with the well known 5¹² isomer of (water)₂₀ , placing a single mono, di, or trivalent X^q+ cation at the interior, and then optimizing and characterizing the resulting clusters. Highly correlated interaction and deformation energies are calculated using the CCSD(T)-DLPNO formalism. Bonding interactions are characterized using the tools provided by the quantum theory of atoms in molecules, natural bond orbitals, and non-covalent surfaces. Our results indicate that water to water hydrogen bonds are sensibly strengthened resulting in strong cooperative effects, which amount to $$  kcal/mol per hydrogen bond in the bare cavity and to larger values for the systems including the cations. Approximate encapsulation, that is, surrounding the cation by a network of hydrogen bonds akin to the well known methane clathrate seems to be preferred by cations with smaller charge densities while microsolvation, that is, cluster structures having explicit X⋯O contacts seem to be preferred by cations with larger charge densities which severely deform the cavity.