Studies on solute-solvent interactions in gaseous and supercritical carbondioxide by high-pressure H-1 NMR spectroscopy

A newly designed high-pressure NMR flow cell has been developed for studies of supercritical fluids. By using the high-pressure cell, H-1 chemical shi fts of nonpolar (n-hexane and benzene) and polar (dichloromethane, chlorofo rm, acetonitrile, water, methanol, and ethanol) solute molecules in gaseous and supercritical carbon dioxide were measured in the wide pressure range between 2 and 30 MPa at 313.3 K. The chemical shifts of hydroxyl protons of water, methanol, and ethanol in carbon dioxide at 20.0 MPa were shifted to higher frequency due to intermolecular hydrogen bonding with increasing co ncentration. A comparison of the concentration dependence with relevant dat a in carbon tetrachloride indicated a specific interaction between alcohol and carbon dioxide molecules. The corrected H-1 chemical shifts of nonpolar and polar solute molecules at infinite dilution, where the bulk magnetic s usceptibility contribution was subtracted, were shifted to higher frequency with increasing density of carbon dioxide. The observed density dependence , represented by a polynomial equation of the third power of density, was i nterpreted in terms of three distinct density regions, i.e., gaslike, inter mediate, and liquidlike. In the gaslike and liquidlike states the solvation structure rapidly varies as the bulk density increases, whereas in the int ermediate state the solvation structure remains almost unchanged despite th e drastic change in the bulk density. It was demonstrated that the H-1 chem ical shift is quite a sensitive probe to a variation of surroundings. The s olvent-induced H-1 chemical shifts were analyzed on the basis of two differ ent models.