Studies on solute-solvent interactions in gaseous and supercritical carbon dioxide by high-pressure 1H 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 shifts of nonpolar (n-hexane and benzene) and polar (dichloromethane, chloroform, 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 concentration. A comparison of the concentration dependence with relevant data 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 susceptibility 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 interpreted in terms of three distinct density regions, i.e., gaslike, intermediate, and liquidlike. In the gaslike and liquidlike states the solvation structure rapidly varies as the bulk density increases, whereas in the intermediate state the solvation structure remains almost unchanged despite the drastic change in the bulk density. It was demonstrated that the H-1 chemical shift is quite a sensitive probe to a variation of surroundings. The solvent-induced H-1 chemical shifts were analyzed on the basis of two different models.