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

Area of Science:

• Physical Chemistry
• Computational Chemistry
• Materials Science

Background:

• Studying solvent effects in physical and chemical phenomena requires understanding local solvent density.
• Supercritical fluids offer unique properties for such investigations due to their high compressibility.
• Precisely defining solvent local number density at the solute is challenging in supercritical regimes.

Purpose of the Study:

• To develop and validate a method for spatially defining solvent local number density at a solute.
• To investigate solvent effects in supercritical xenon using pyrazine as a model solute.
• To predict bulk densities corresponding to specific solvation shell occupancies.

Main Methods:

• Experimental measurement of pyrazine's n-pi* electronic transition red shift in supercritical xenon.
• Quantum chemical calculations (MP2, TDDFT) to model pyrazine-xenon interactions and transition energies.
• Classical molecular dynamics simulations (Lennard-Jones) to evaluate predicted densities and solvation structures.

Main Results:

• Successfully predicted supercritical xenon bulk densities for average solvation shell occupancies of 1 to 4 xenon molecules around pyrazine.
• Density predictions at 293.2 K: 0.50, 0.91, 1.85, 2.50 g/cm³; at 333.2 K: 0.65, 1.20, 1.85, 2.50 g/cm³.
• Molecular dynamics simulations confirmed the predicted densities and solvation shell structures with high internal consistency.

Conclusions:

• The combined experimental, computational, and simulation approach accurately determines local solvent density in supercritical fluids.
• This method provides a robust tool for studying solvent effects in systems dominated by dispersion forces.
• The study demonstrates high internal consistency across absorbance measurements, quantum chemical predictions, and molecular dynamics simulations.