Gas-expanded liquids (GXLs) form a unique class of environmentally benign solvents that offer many of the benefits of both organic liquids and supercritical fluids. A more complete understanding of the interactions between the gas, the organic liquid, and solutes at the molecular level will enable the full exploitation of GXLs. In this work, we have combined kinetic and solvatochromic studies using in situ high-pressure spectroscopy to develop a clearer picture of the local solvent structure of GXLs. We have used two reactions as kinetic probes of local solvent structure: a Menschutkin reaction and a Diels-Alder reaction.

We have studied the Menschutkin reaction of tributylamine with methyl p-nitrobenzenesulfonate as a probe of solvent polarity using in situ high-pressure UV/Vis spectroscopy. We have compared these kinetic data with other measures of solvent polarity including the Kamlet-Taft p* parameter, Kosower's Z, and dielectric constant, to describe polarity effects in the cybotactic region of GXLs.

We have also developed a comprehensive multiparameter approach to add insight into the molecular interactions related to a Diels-Alder reaction in CO₂-expanded acetonitrile. We have studied the kinetics of the Diels-Alder reaction of anthracene and 4-phenyl-1,2,4-triazoline-3,5-dione as a function of solvent composition using in situ high-pressure fluorescence spectroscopy. We have measured the values of the Kamlet-Taft solvatochromic parameters p* (dipolarity/polarizibility), a (hydrogen bonding acidity), and b (hydrogen bonding basicity) in CO₂-expanded acetonitrile using in situ high-pressure UV/Vis spectroscopy. A linear solvation energy relationship (LSER) describes the reaction rate in terms of the measured solvatochromic parameters. We correlate the reaction rates with the solvatochromic parameters based on the LSER and yield satisfactory consistency with the experimental data.