Ethanol Molecular Dynamics Measured by Coupled Spin Relaxation Exhibiting Cross Correlation between Dipole-Dipole and Chemical-Shift Anisotropy

Abstract

The motional dynamics of ethanol in chloroform are studied by carbon- 13 multiplet spin-lattice relaxation over a wide range of temperatures. The coupled methylene ( 13CH 2) spin system is perturbed from thermal equilibrium using a variety of nonselective and selective pulse techniques. Simultaneous fitting of the resulting partially relaxed transition intensities with a model derived from Redfield theory allows the extraction of both auto-and cross-correlated dipolar spectral densities, from which the three principal elements of a diffusion tensor may be determined. The cross correlation between the chemical-shift anisotropy, CSA, and the dipolar, D, interactions is found to be appreciable. This cross term couples together magnetization modes with different spin-inversion symmetries. Inclusion of the D-CSA interference terms in the model improves the precision of the measurements of dipolar spectral densities, and the determination of the D-CSA interference term itself provides an estimate of the anisotropy of shielding tensors. It is found that the diffusion of ethanol can be described by an axially symmetric diffusion tensor with its unique axis about 5° from the C-O internuclear vector. Calculation of the D-CSA interference terms for an asymmetric shielding tensor under anisotropic diffusion is reported. The experiments were conducted using CD 3CH 2OD so that interference from intramolecular interactions with the adjacent protons is minimized. When other isotopomers of ethanol, CD 3CH 2OH, CH 3CH 2OD, and CH 3CH 2OH, were used, measurably different relaxation was observed. However, the experimental results in all cases can be analyzed by grouping these other interactions into a random-field interaction term. The determined dipolar spectral densities and D-CSA interference terms for each isotopomer are within experimental error of the values obtained for CD 3CH 2OD. As the adjacent protons only minimally affect the evaluation of anisotropic diffusion, this work supports the extension of this technique to systems where more complex isotopic enrichment schemes are not practical.