Solvated Electron in Acetonitrile: Radiation Yield, Absorption Spectrum, and Equilibrium between Cavity- and Solvent-Localized States

The equilibrium between a solvent cavity-localized electron, ecav-, and a dimeric solvent anion, (CH3CN)(2)(center dot-), which are the two lowest energy states of the solvated electron in acetonitrile, has been investigated by pulse radiolysis at 233-353 K. The enthalpy and entropy for the e(cav)(-) to (CH3CN)(2)(center dot-) conversion amount to - 11.2 +/- 0.3 kcal/mol and -39.3 +/- 1.2 cal/(mol K), corresponding to a 0.44 +/- 0.35 equilibrium constant at 25 degrees C. The radiation yield of the solvated electron has been quantified using a Co(II) macrocycle that scavenges electrons with a 1.55 x 10(11) M-1 s(-1) rate constant. The apparent yield increases without saturation over the attainable scavenger concentration range, reaching 2.8 per 100 eV; this value represents the lower limit for the acetonitrile ionization yield in pulse radiolysis. The apparent molar absorption coefficient of (20.8 +/- 1.5) x 10(3) M-1 cm(-1) at 1450 nm and 20 degrees C for the solvated electron and individual vis-nearinfrared (NIR) absorption spectra of e(cav)(-) and (CH3CN)(2)(center dot-) are derived from the data. Variances with previous reports are thoroughly discussed. Collectively, these results resolve several controversies concerning the solvated electron properties in acetonitrile and furnish requisite data for quantitative pulse radiolysis investigations in this commonly used solvent.