A quantum mechanics/molecular dynamics study of electric field gradient fluctuations in the liquid phase. the case of Na⁺ in aqueous solution

The ²³Na quadrupolar coupling constant of the Na⁺ ion in aqueous solution has been predicted using molecular dynamics simuslations and hybrid quantum mechanics/molecular mechanics methods for the calculation of electric field gradients. The developed computational approach is generally expected to provide reliable estimates of the quadrupolar coupling constants of monoatomic species in condensed phases, and we show here that intermolecular polarization and non-electrostatic interactions are of crucial importance as they result in a 100% increased quadrupolar coupling constant of the ion as compared to a simpler pure electrostatic picture. These findings question the reliability of the commonly applied classical Sternheimer approximation for the calculations of the electric field gradient. As it can be expected from symmetry considerations, the quadrupolar coupling constants of the 5- and 6-coordinated Na⁺ ions in solution are found to differ significantly.