CURRENT ASPECTS OF RADIOLYSIS OF LIQUID M ETHANOL - A REVIEW

In this work, we have revisited, on the basis of existing data, some f undamental aspects of the radiolysis of liquid methanol while trying t o establish useful comparisons with the radiolysis of water. In irradi ated methanol, the free radicals formed at early time are principally the H-. atom, CH3O., and the solvated electron (e(s)(-)). The free rad ical CH3O. is produced either from the positive ion by a proton transf er or from excited molecules of the solvent; in this latter case, H-. atoms are also formed simultaneously. It can be seen that there exists an analogy of these mechanisms with those intervening in the radiolys is of water. However, an important difference appears, namely, the tim e scale involved in electron solvation for both solvents at stake diff ers by a factor of ten, the phenomenon being slower in methanol (about 6-10 ps at 20 degrees C). Besides, the ultimate fate of the preceding products differs from that of their homologues formed in irradiated w ater. In fact, contrary to the case of water, the three free radicals H-., CH3O., and e(s)(-) have the possibility to react rapidly with the solvent. The H-. atom and the solvated electron finally form H-2, whi le CH3O. preferentially yields the thermodynamically more stable isome r radical (CH2OH)-C-.. For these reactions, the heterogeneity in the i nitial radical distribution is without any influence. The solvated ele ctron can also react either with itself to give H-2, or with CH3OH2+ t o yield H-. and subsequently H-2. The whole mechanism would imply a to tal inhibition of molecular hydrogen on addition of H-. or e(s)(-) sca vengers. This is not the case, however, since H-2 is very difficult to eliminate totally. A residual H-2 yield of 1.75 molecules/100 eV (tha t is, similar to 32% of the total H-2 yield) has been measured. For li quid water, an ''unscavengeable'' H-2 yield of 0.15 molecules/100 eV ( that is, similar to 33% of the corresponding total H-2 yield) is also found. In order to interpret the origin of such yields, two hypotheses are put forward: the first one corresponds to the intramolecular diss ociation of electronically excited molecules, and the second one is ba sed on the dissociative capture of slow electrons by the solvent molec ules. By gathering the experimental data available from the literature , we have also compared the curves of the e(s)(-) yields as a function of time for methanol and water at room temperature. From this compari son, we note that homogenization of the radiolytic species present in the bulk of these media seems to be faster in methanol than in water, while the yield of e(s)(-) is always higher in water. The disappearanc e yield of irradiated methanol is, on the other hand, higher than its corresponding value for water, as a result of the fact that, for metha nol, the recombination reverse reactions due to the heterogeneity are disfavored with respect to the reactions of H-. atoms with the solvent . Finally, the most recent femtosecond laser spectroscopy experiments have revealed that, in pure liquid methanol, the electron solvation pr ocess can be described according to a hybrid model. This model implies two electronic states, both relaxing via a continuous blue shift and between which there is a stepwise transfer mechanism. In the case of w ater, the electron hydration process also involves, at least for the l argest part of the relaxation, two distinct electronic states. Neverth eless, recent results have shown that, in this medium also, the fundam ental state involved in such a process relaxes in a continuous way tow ards the hydrated electron. On the basis of these considerations, it c an be seen that the two electron solvation schemes in liquid methanol and water show important resemblances. Additional experiments currentl y appear to be required to better understand and to quantify the vario us overall intervening processes. In particular, a systematic analysis of the radiolysis of short-chain alcohols in comparison with that of water could bring useful information about the concept of universality of electron solvation in polar liquids. Copyright (C) 1996 Elsevier S cience Ltd