Pyrazine in supercritical xenon: local number density defined by experiment and calculation

Toward our goal of using supercritical fluids to study solvent effects in physical and chemical phenomena, we develop a method to spatially define the solvent local number density at the solute in the highly compressible regime of a supercritical fluid. Experimentally, the red shift of the pyrazine n-pi* electronic transition was measured at high dilution in supercritical xenon as a function of pressure from 0 to approximately 24 MPa at two temperatures: one (293.2 K) close to the critical temperature and the other (333.2 K) remote. Computationally, several representative stationary points were located on the potential surfaces for pyrazine and 1, 2, 3, and 4 xenons at the MP2/6-311++G(d,p)/aug-cc-pVTZ-PP level. The vertical n-pi* ((1)B(3u)) transition energies were computed for these geometries using a TDDFT/B3LYP/DGDZVP method. The combination of experiment and quantum chemical computation allows prediction of supercritical xenon bulk densities at which the pyrazine primary solvation shell contains an average of 1, 2, 3, and 4 xenon molecules. These density predictions were achieved by graphical superposition of calculated shifts on the experimental shift versus density curves for 293.2 and 333.2 K. Predicted bulk densities are 0.50, 0.91, 1.85, and 2.50 g cm(-3) for average pyrazine primary solvation shell occupancy by 1, 2, 3, and 4 xenons at 293.2 K. Predicted bulk densities are 0.65, 1.20, 1.85, and 2.50 g cm(-3) for average pyrazine primary solvation shell occupancy by 1, 2, 3, and 4 xenons at 333.2 K. These predictions were evaluated with classical Lennard-Jones molecular dynamics simulations designed to replicate experimental conditions at the two temperatures. The average xenon number within 5.0 A of the pyrazine center-of-mass at the predicted densities is 1.3, 2.1, 3.0, and 4.0 at both simulation temperatures. Our three-component method-absorbance measurement, quantum chemical prediction, and evaluation of prediction with classical molecular dynamics simulation-therefore has a high degree of internal consistency for a system in which the intermolecular interactions are dominated by dispersion forces.