Energetics and Volume Changes in Electron Attachment to Pyrazine in Supercritical Xenon

Abstract

The attachment of electrons to pyrazine occurs reversibly over a wide range of pressures at and above room temperature in supercritical xenon. The rate constant for attachment increases with pressure at low pressures, passes through a maximum, and levels off at values of 1-3x10(12) m(-1) s(-1) at high pressure. The activation volumes for attachment (DeltaVa*) are quite small but show maxima near the compressibility maxima. In contrast, DeltaVa* is always negative for this reaction in sc-ethane and exhibits minima near the compressibility maxima. The rate constants for electron detachment change little with pressure but increase with temperature. Activation volumes for detachment are small. To explain the small volume change observed for this reaction, it is proposed that at the higher pressures clustering around the neutral pyrazine is comparable to that around the ion; i.e., the partial molar volumes are comparable. The free energy change (DeltaGr) of this reaction decreases between 40 and 60 bar and then is fairly constant at higher pressures. The dependence of DeltaGr on pressure is consistent with clustering around the neutral pyrazine at higher pressure. Also, the electron affinity of the clusters, pyrazineXen, increases with n to a few tenths of an eV.

Behavior of charged species in supercritical heavy rare gas fluids

“…2), and the predicted value indicates that the partial molar volume of neutral pyrazine in sc ethane is nearly zero. In contrast, DV r for the same reaction in SC xenon is small throughout, negative in the lower pressure region and become positive in the vicinity of w T maximum (Holroyd et al, 2007). Above the maximum they are practically zero, which means no further shift in K with pressure.…”

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

“…16 Holroyd studied electron attachment to pyrazine in supercritical xenon. 17 The highly polarizable xenon 18 is a simple model for hydrocarbon solvation, and its spherical symmetry conveniently limits the orientational degrees of freedom with respect to the pyrazine solute. The pyrazine-xenon system is conceptually simplified by the fact that dispersion forces dominate the intermolecular interactions.…”

“…Pyrazine (see Figure ) is well-known to the computational and spectroscopic communities, − and the pyrazine−one xenon cluster produced by supersonic expansion has been examined by two-color double-resonance spectroscopy . Holroyd studied electron attachment to pyrazine in supercritical xenon . The highly polarizable xenon is a simple model for hydrocarbon solvation, and its spherical symmetry conveniently limits the orientational degrees of freedom with respect to the pyrazine solute.…”