Beryllium-9 NMR Spectroscopy and Molecular Orbital Studies of Hydrogen-Bonding Interaction of Tetraaquaberyllium(II) Ion with Water in the Second Solvation Sphere in Propiononitrile

摘要

The equilibrium constants of the hydrogen-bonding interactions between the tetraaqua-beryllium(II) ion ([Be(H2O)4]2+) and water in the second solvation sphere were determined from the dependence of the 9Be NMR chemical shift (δBe) on the water concentration (C W) in propiononitrile (PN). The 9Be NMR line assigned to [Be(H2O)4]2+ is downfield shifted with the increasing C W, and the change in δBe as a function of C W is explained in terms of a two-step hydrogen-bonding interaction to form [Be(H2O)4](H2O) n 2+ (n = 1, 2). Molecular orbital calculations for [Be(H2O)4](H2O) n 2+ (n = 0−2) revealed that the electron density on the Be(II) ion increases with an increase in n due to an enhanced polarization of the bound water molecules accompanied by the hydrogen bonding in the second solvation sphere. The observed downfield shift of δBe is interpreted as a significant compensation of the diamagnetic upfield shift by the paramagnetic contribution due to the mixing of the vacant s and p orbitals of Be with the σ-type lone-pair orbitals of the coordinating water molecules.