Quelques aspects actuels de la radiolyse du méthanol liquide: Une revue

Abstract

In this work, we have revisited, on the basis of existing data, some fundamental aspects of the radiolysis of liquid methanol while trying to establish useful comparisons with the radiolysis of water. In irradiated methanol, the free radicals formed at early time are principally the H · atom, CH 3O ·, and the solvated electron (e s −). The free radical CH 3O · is produced either from the positive ion by a proton transfer 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 radiolysis of water. However, an important difference appears, namely, the time scale involved in electron solvation for both solvents at stake differs by a factor of ten, the phenomenon beingg slower in methanol (about 6–10 ps at 20°C). Besides, the ultimate fate of the preceding products differs from that of their homologues formed in irradiated water. In fact, contrary to the case of water, the three free radicals H ·, CH 3O ·, and e s − have the possibility to react rapidly with the solvent. The H · atom and the solvated electron finally form H 2, while CH 3O · preferentially yields the thermodynamically more stable isomer radical ·CH 2OH. For these reactions, the heterogeneity in the initial radical distribution is without any influence. The solvated electron can also react either with itself to give H 2, or with CH 3 OH 2 + to yield H · and subsequently H 2. The whole mechanism would imply a total inhibition of molecular hydrogen on addition of H · or e s − scavengers. 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 (that is, ∼ 32% of the total H 2 yield has been measured. For liquid water, an “inscavengeable” H 2 yield of 0.15 molecules/100 eV (that is, ∼ 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 dissociation of electronically excited molecules, and the second one is based on the dissociative capture of slow electrons by the solvent molecules. 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 comparison, 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 disappearance yield of irradiated methanol is, on the other hand, higher than its corresponding value for water, as a result of the fact that, for methanol, 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 process 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 water, the electron hydration process also involves, at least for the largest part of the relaxation, two distinct electronic states. Nevertheless, recent results have shown that, in this medium also, the fundamental state involved in such a process relaxes in a continuous way towards the hydrated electron. On the basis of these considerations, it can be seen that the two electron solvation schemes in liquid methanol and water show important resemblances. Additional experiments currently appear to be required to better understand and to quantify the various overall intervening processes. In particular, a systematic analysis of the radiolysis of short-cahin alcoholos in comparison with that of water could bring useful information about the concept of universality of electron solvation in polar liquids.