Attractive Potential Effect on the Rotational Correlation Times for Benzene-d6 in Organic Solvents

Abstract 2H NMR spin-lattice relaxation times have been measured to obtain the rotational correlation times τ2R for benzene C6D6 (50 mol m−3) in various organic solvents, such as benzene (C6H6), hexane, decane, carbon tetrachloride, chloroform, dichloromethane, methanol, ethanol, 1-propanol, 1-butanol, and acetonitrile over the widest possible range of temperature. Plots of the determined values of τ2R against solvent viscosity η divided by temperature T are linear in the apolar and polar aprotic solvents, as predicted by the Stokes–Einstein–Debye (SED) law. However, the plots of τ2R vs. η/T in the alcohols are concave upward, the slope in the high temperature region being ca. 2 times as large as that in the low temperature region. The nonlinearity and the decrease in the slope reflect the influence of the hydrogen-bonded solvent structure. For the benzene rotation in solutions, fairly good positive linear correlations are found between the slopes and the negative dissolution enthalpies at infinite dilution at room temperature. These results indicate an important role played by attractive solute–solvent interactions in controlling the molecular rotation in solution.