Photolysis of Low-Temperature Glasses. I. Ethanol and Ether at 77°K

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Abstract
The sensitized photolytic decomposition of ethanol and diethyl ether glasses at 77°K has been investigated utilizing electron paramagnetic resonance (EPR) spectroscopy. Support is given to a proposed mechanism of sensitized solvent decomposition by a two‐step biphotonic absorption by an aromatic impurity molecule followed by a solute—solvent energy‐transfer process. Subsequent dissociation processes from the excited solvent molecule are considered. It is demonstrated that the results obtained upon prolonged photolysis of ethanol or ether can be attributed to a steady‐state competition between the abstraction reactions of the type CH3CH2 (or CH3) +CH3CH2OR→CH3CH3 (or CH4) +CH3CHOR and the photoreaction CH3CHOR+hv→CH3+CH3CH2+ other products. The predicted dependence of the direction of the steady‐state competition with respect to the relative radical concentrations on the incident light intensity is verified experimentally. Extinction of the excitation source results in a cessation of the steady‐state condition, leading to the decay of the ethyl and methyl radical concentrations via the abstraction reactions. The latter abstraction process is discussed, and a significant isotope effect in the decay of the hydrocarbon radicals in CH3CD2OH is indicated. The products of the dark abstraction reactions are the ethanol (CH3CHOH) or etheral (CH3CHOC2H5) radicals. In some cases, an EPR spectrum tentatively assigned to the acetyl radical (CH3CO) is also obtained. The latter species are photolyzable, and the wavelengths responsible for the photolyses are indicated. The EPR and electronic absorption spectral characteristics of the acetyl radical are compared to somewhat similar results obtained in the γ radiolysis of ethanol glasses. It is demonstrated that the similarity of the results is coincidental and that the results in the present study are not attributable to the solvated electron as they are in the case of γ radiolysis. A series of reactions is proposed to qualitatively account for the spectral observations and gaseous product yields. This scheme is in satisfactory agreement with the experimental results.