Reorientational motions in compressed viscous fluids: Selectively deuterated glycerol

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Abstract
The pressure and temperature dependence of the deuteron NMR spin–lattice relaxation time T1 have been measured in liquid glycerol‐d4, (D2COH)2CHOH, and glycerol‐d3, C3H5(OD)3, within the pressure range 1 bar to 5 kbar from −10° to 125 °C. The experimental deuteron relaxation data which reflect intramolecular reorientational motions are interpreted in terms of a Cole–Davidson distribution of correlation times. Within experimental error the distribution width β=0.44 is found to be independent of temperature and pressure. A single correlation time model and a model allowing for internal and overall reorientation fail to explain the experimental data without introducing an arbitrary adjustable parameter. The reorientational motions of the carbon backbone and of the hydroxyl groups are strongly correlated reflecting the effect of hydrogen bonds. By comparing the magnitudes of the T1 values at the minimum of the T1 vs T plots in glycerol‐d4 and glycerol‐d3 the deuteron quadrupole coupling constant is found to be 202 kHz for the hydroxyl deuteron. The dependence of the average correlation time ?2 upon viscosity and the ?2 variation with temperature and density is explained on the basis of a Debye type two term phenomenological equation. A discussion of the experimental data in terms of this equation suggests that the extent of the hydrogen bond network in liquid glycerol is relatively insensitive to density changes but very much reflects the temperature effects. The temperature and pressure dependence of the average correlation time ?2 suggests that the reorientation of the glycerol molecule proceeds via a large‐angle jump diffusion mechanism. A detailed comparison of the present results with the conclusions of other NMR studies is presented.

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