The effect of site-specific isotopic substitutions on transport coefficients of liquid methanol

A new approach is suggested for studying the nature of molecular transport in simple liquids, which makes use of site-specific isotopic substitutions. Its application represents the first systematic experimental study of a theoretically predicted correlation between transport coefficients in liquids and molecular moments of inertia. For this purpose, we have determined the viscosities and self-diffusion coefficients at 25°C of normal methanol and seven isotopically labeled methanol species: CH3OD, CD3OH, CD3OD, CH2DOH, CHD2OH, CHD2OD, and 13CH3OH. Except for 12C/13C substitution, the observed isotope effects are significantly larger than predicted by a square root of mass dependence, but are well correlated with the square roots of the moments of inertia. The results give strong evidence that translation-rotation coupling influences the transport processes in methanol, thus confirming earlier interpretations of isotope effects upon the transport in water.