Trapped electrons and hydrogen formation mechanism in λ-irradiated glassy alcohols and 2-methyltetrahydrofuran

Abstract

The electrons trapped in glassy alcohols at 77 K are characterized by an optical absorption spectrum which is composed of partial individual spectra differing only very little from each other. The kinetics of photobleaching under these circumstances is analysed. The quantum yield is low (⩽10 −1) and increases with the photon energy when it surpasses the value of about 2·3 eV corresponding to the absorption maximum. The kinetics of photobleaching is not significantly affected by the presence of various solutes while the kinetics of the thermal decay of trapped electrons is. The trapped electrons excited by light are converted into alkoxy anions and H atoms which further abstract mainly from alkyl groups. The trapped electrons migrating at higher temperature react similarly to solvated electrons. Their reaction with the protonated alcohols produces hydrogen atoms which do not abstract; this results in lower hydrogen yields than those obtained after photobleaching. Besides the reaction of trapped electrons the reaction of hydrogen atoms formed independently of trapped electrons and the formation of unscavengeable hydrogen participate in the total hydrogen yield. A model of the trapped electron and of its reactions is suggested. In methyltetrahydrofuran the electrons are detrapped by light, their kinetics of photobleaching being significantly affected by electron scavengers. Either the detrapped electrons are retrapped or they react with the radicals or scavengers present. The trapped electrons are responsible for hydrogen formation only to a very small extent. The exact hydrogen formation mechanism is still unknown.