Richard A. Holroyd, Masaru Nishikawa and Kengo Itoh
J. Phys. Chem. B 103, , 9205-9210 (1999) [Find paper at ACS Publications]
Abstract:
The reaction of excess electrons with pyrimidine and styrene was studied in supercritical ethane. Although their electron affinities are comparable, the equilibrium constant for attachment was large when the solute (S) was pyrimidine and small for styrene. At pressures above 100 bar, the rate constant for electron attachment to pyrimidine (ka) was time resolved using the short pulse of the Laser-Electron Accelerator Facility (LEAF). The rate constant, ka, is large and nearly independent of pressure and temperature; thus the volume of activation is close to zero. At lower pressures the equilibrium constants for this reaction were derived from the changes in the mobility of excess electrons. The free energy is a function of the polarization energy, which was evaluated with a compressible continuum model. The small equilibrium constant in the case of styrene is attributed to a smaller polarization energy. Values of D
Vr, obtained from changes of D
Gr with pressure, range from -9.0 to -0.4 Liters/mol. The observed volume changes agree with electrostriction volumes calculated by the model. Electron attachment occurs with a large decrease in entropy associated with clustering of ethane molecules around the ion formed. Observed values of D
Sr compare well with expected values calculated from D
Sel = (a
/c
T) D
Vr.