Dipolar Hydration Anomaly in the Temperature Dependence: Carbonyl and Nitrile Solutes Studied by 13C NMR Chemical Shifts

Abstract

The 13C chemical shifts δ of carbonyl (13CO) and nitrile (13C⋮N) compounds in aqueous solutions were measured as functions of temperature to study the hydration structure around dipolar solutes. The δ values showed an anomalous maximum in the temperature dependence; at −9 °C for urea, at 23 °C for diethyl ketone, at 24 °C for cyanamide, at 32 °C for acetone, at 63 °C for acetaldehyde, and at 96 °C for acetonitrile. No such anomaly was observed in any other organic solvents. The presence of the maximum is due to the competition of two factors, solute−solvent and solvent−solvent interactions. One is the electrostatic hydrogen bond between the solute and water, and the other is the hydrogen bond network characteristic of solvent water. When the network structure of water is collectively stabilized with decreasing temperature, the effective water−water interaction becomes stronger than the solute−water interaction, giving the maximum in the temperature dependence of δ. The temperature of the maximum chemical shift Tmax δ was found to be sensitive to the solute polarity. The solute dependence of Tmax δ was explained in terms of the solute dipole moment μ obtained from ab initio calculations: the larger the μ, the lower the Tmax δ.