The value of the rate constant kl for the reaction e-s,lv + RO-s,lv + H, [I], at 295 K and 1 bar is 51.4 X lo5 s-1 in methanol and 5 8 X 104 s-1 in ethanol. The respective volumes of activation averaged between 1 bar and 2 kbar are AVl* 2 -21 and 5 -22 cm3 mol-1. A high concentration of potassium hydroxide (1 M) or water (5 M) decreases the apparent value of kl somewhat but has little or no effect on the value of AV,". The effect of pressure on the rate constant of e-s,lv + S + product, [2], was also measured for a series of solutes that displays a wide range of reactivity. Experimental values of AV2* depend on the relative contributions of the effects of solvent density on the reactant diffusion rates, the concentrations of the actual reacting species, and the relative energies of the reactant and intermediate states. For reactions whose rates are near the diffusion controlled limit, k2 = 10'0 M-1 s-1 in methanol and ethanol, the values of AV2' are positive and similar to those for the diffusion of simple ions. AV* (e-s,!v diffusion) = 4 cml mol-1 in methanol and 6 cml mol-1 in ethanol. Cadmium chlor~de IS apparently not completely dissociated in alcohols, and k(e-s,lv + CdC12) < k(e-s,lv + CdCl+) < k(e-,,lv + Cd2+). For a series of compounds with lower rate constants there is a correlation between log k2 and AVz*, the latter being negative for very low values of k2. The products of electron capture by benzene, toluene, ethyl acetate, and possibly acetonitrile appear to be stabilized by protonation: e-,,,I,. + S = S-,,lv, 13, -31,; S-s,lv + ROH + SH + ,I,,[4]. The results indicate that the decomposition of e-s,lv in a pure alcohol occurs by protonatlon of the electron site, e-s,lv + ROH -t H + RO-,,,,, 14'1, rather than by electron transfer to an alcohol molecule followed by decomposition of the anion. [Traduit par le journal]
