Partial Molar Volume of the Hydrated Electron

The partial molar volume of the hydrated electron was investigated with pulse radiolysis and transient absorption by measuring the pressure dependence of the equilibrium constant for e(aq)(-) + NH4+ double left right arrow H+ NH3. At 2 kbar pressure, the equilibrium constant decreases relative to 1 bar by only 6%. Using tabulated molar volumes for ammonia and ammonium, we have the result (V) over bar (e(aq)(-)) - (V) over bar (H) = 11.3 cm(3)/mol at 25 degrees C, confirming that (V) over bar (e(aq)(-)) is positive and even larger than the hydrophobic H atom. Assuming on the basis of recent molecular dynamics simulations that the molar volume of the H atom is somewhat less than that of H-2, we estimate (V) over bar (e(aq)(-)) = 26 +/- 6 cm(3)/mol. The positive molar volume is consistent with an electron that exists largely in a small solvent void (cavity), ruling out a recent model (Larsen, R E.; Glover, W. J.; Schwartz, B. J. Science 2010, 329, 65-69) that suggests a noncavity structure with negative molar volume.