High-pressure nuclear magnetic resonance study of the dynamical solvent effects on internal rotation of N,N-dimethyltrichloroacetamide

The pressure dependence of the hindered rotation about the amide C–N bond of N,N-dimethyltrichloroacetamide (DMTCA) has been studied at 282.3 K in n-pentane and methylcyclohexane solvents using the high resolution, high pressure nuclear magnetic resonance (NMR) technique. The experimental rotation rate k decreases with increasing pressure in both solvents and the correlation of the rates with solvent viscosity η shows that the rotation falls into the strongly coupled diffusive regime. Interpretation of the experimental k vs η dependence in terms of the Kramers’ model fails to account for the leveling off of the rate constant at high viscosities. The Grote–Hynes theoretical model, which assumes frequency-dependent friction, reproduces well the observed rate behavior with viscosity of the solvent.