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(H₂O)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 (Formula presented.) 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.