INERTIAL AND ATTRACTIVE POTENTIAL EFFECTS ON ROTATION OF SOLITARY WATER-MOLECULES IN APOLAR AND POLAR-SOLVENTS

Deuteron nuclear magnetic resonance spin-lattice relaxation times T1 have been measured for solitary water molecules (D2O) at very low concentrations (8-70 mM) in a series of solvents over a wide range of temperatures; the solvents studied are carbon tetrachloride (10 to 40-degrees-C), benzene (10 to 60-degrees-C), chloroform (-40 to 50-degrees-C), acetonitrile (-40 to 50-degrees-C), and acetone (-40 to 50-degrees-C). The orientational correlation times tau-2R for D2O with extremely small moments of inertia (I) in CCl4, C6H6, CHCl3, CH3CN, and (CH3)2CO at 30-degrees-C are, respectively, 96, 224, 230, 625, and 826 fs, which are all by far smaller than the bulk value (2210 fs). The tau-2R value is proportional to the proton chemical shift of water in each solvent which is taken as a measure of the strength of solute-solvent interactions in the short range. On the other hand, tau-2R is almost inversely proportional to solvent viscosity in disagreement with the simple hydrodynamic friction model. The correlation time (tau*) scaled by the free rotator correlation time square-root I/k(B)T is 1.20, 1.01, and 0.89 in CCl4, respectively, at 10, 30, and 40-degrees-C, suggesting a strong inertial effect on the water rotation in the weakly coupled solvent cage. In other solvents, the quantity tau* increases with an increase in the water proton chemical shift, and the larger the tau* value the stronger the temperature dependence. The usefulness of the solvent cage model is illustrated and limitations of primitive models so far often used are discussed.