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Pyrazine in Supercritical Xenon: Local Number Density Defined by Experiment and Calculation

Summary: Pyrazine in Supercritical Xenon: Local Number Density Defined by Experiment and
Bruce J. Hrnjez,*, Abdo Kabarriti, Benjamin I. Dach, Sergey V. Buldyrev, Neer Asherie,,
Georgiy R. Natanov, and Joshua Balderman
Departments of Chemistry, Physics, and Biology, YeshiVa UniVersity, New York, New York 10033
ReceiVed: August 25, 2008; ReVised Manuscript ReceiVed: September 22, 2008
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-* electronic transition was
measured at high dilution in supercritical xenon as a function of pressure from 0 to 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-* (1
B3u) 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,


Source: Asherie, Neer - Departments of Physics & Biology, Yeshiva University


Collections: Biology and Medicine; Physics