| Abstract
| - A newly designed high-pressure NMR flow cell has been developed for studies of supercritical fluids. Byusing the high-pressure cell, 1H chemical shifts of nonpolar (n-hexane and benzene) and polar (dichloromethane,chloroform, acetonitrile, water, methanol, and ethanol) solute molecules in gaseous and supercritical carbondioxide were measured in the wide pressure range between 2 and 30 MPa at 313.3 K. The chemical shifts ofhydroxyl protons of water, methanol, and ethanol in carbon dioxide at 20.0 MPa were shifted to higherfrequency due to intermolecular hydrogen bonding with increasing concentration. A comparison of theconcentration dependence with relevant data in carbon tetrachloride indicated a specific interaction betweenalcohol and carbon dioxide molecules. The corrected 1H chemical shifts of nonpolar and polar solute moleculesat infinite dilution, where the bulk magnetic susceptibility contribution was subtracted, were shifted to higherfrequency with increasing density of carbon dioxide. The observed density dependence, represented by apolynomial equation of the third power of density, was interpreted in terms of three distinct density regions,i.e., gaslike, intermediate, and liquidlike. In the gaslike and liquidlike states the solvation structure rapidlyvaries as the bulk density increases, whereas in the intermediate state the solvation structure remains almostunchanged despite the drastic change in the bulk density. It was demonstrated that the 1H chemical shift isquite a sensitive probe to a variation of surroundings. The solvent-induced 1H chemical shifts were analyzedon the basis of two different models.
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