SOLVATION OF NONELECTROLYTES IN WATER PROBED BY 17O NMR RELAXATION OF THE SOLVENT

The concentration dependence of the 17O NMR spin-lattice relaxation rate of the solvent water in nonelectrolyte solutions has been determined for a series of alcohols, amides, ethylene glycol, urea, and dimethyl sulfoxide. The values of T10/ T1 are linearly related to the solute mole fraction or molality with a slope B. This parameter is shown to depend on the number of solvation sites of the solute and on the change in the correlation time of water molecules between bulk and solvation shell. The possible dependence on changes in nuclear quadrupole coupling constant is ruled out by ab initio theoretical calculations, which show that the electric field gradient at the oxygen nucleus of water does not change appreciably between water dimer and methanol- or formamide water complexes. The B parameter is correlated with the enthalpy of hydration of the solute within a given family (alcohols, primary, secondary, or tertiary amides). It is shown that if T10/T1 values are corrected for the solution viscosities, the new Bcv values thus obtained correlate with the enthalpy of hydration regardless of the family.