Carbon‐13 NMR study of molecular reorientation in glycerol

Molecular dynamics simulation study of glycerol-water liquid mixtures

To study the effects of water on conformational dynamics of polyalcohols, Molecular Dynamics simulations of glycerol-water liquid mixtures have been carried out at different concentrations: 42.9 and 60.0 wt % of glycerol, respectively. On the basis of the analysis of backbone conformer distributions, it is found that the surrounding water molecules have a large impact on the populations of the glycerol conformers. While the local structure of water in the liquid mixture is surprisingly close to that in pure liquid water, the behavior of glycerols can be divided into three different categories where roughly 25% of them occur in a structure similar to that in pure liquid of glycerol, ca. 25% of them exist as monomers, solvated by water, and the remaining 50% of glycerols in the mixture form H-bonded strings as remains of the glycerol H-bond network. The typical glycerol H-bond network still exists even at the lower concentration of 40 wt % of glycerol. The microheterogeneity of water-glycerol mixtures is analyzed using time-averaged distributions of the sizes of the water aggregates. At 40 wt % of glycerol, the cluster sizes from 3 to 10 water molecules are observed. The increase of glycerol content causes a depletion of clusters leading to smaller 3-5 molecule clusters domination. Translational diffusion coefficients have been calculated to study the dynamical behavior of both glycerol and water molecules. Rotational-reorientational motion is studied both in overall and in selected substructures on the basis of time correlation functions. Characteristic time scales for different motional modes are deduced on the basis of the calculated correlation times. The general conclusion is that the presence of water increases the overall mobility of glycerol, while glycerol slows the mobility of water.

Determination of the Dipolar Interaction of 23Na in Solution by Triple-Quantum Relaxation Time Measurements

The measurement of the dipolar interaction of 23Na with hydrogen nuclei in glycerol solution is reported. The method, applied previously to 7Li (U. Eliav and G. Navon, J. Magn. Reson. A 123, 32 (1996)), is based on the measurement of the triple-quantum relaxation time of 23Na. Several models of motion are discussed. The analysis of the results yielded 1.9 MHz and 12.5 kHz for the quadrupolar and the 23Na-1H dipolar interaction, respectively. It is shown that under the conditions of long correlation times the triple-quantum relaxation time can be sensitive to dipolar interactions smaller than the quadrupolar interaction by as much as a factor of 5000. This indicates the possibility of measuring interatomic distances for nuclei with sizable quadrupolar moments. Copyright 1998 Academic Press. Copyright 1998 Academic Press