Photolysis of ethyl‐iodide in lithium chloride glassy aqueous solutions with light, λ = 254 nm at 77 K

The photolysis of C 2 H 5 I in a glassy salt matrix (5 M , 7.5 M , 10 M ) of aqueous LiCl at 77 K with light of λ = 254 nm has been conducted, product analysis being by ESR and UV spectroscopy. The electrolytic medium causes the ionization of product HI, and I − concentrations can be continuously determined. During photolysis [I − ] is less than the amount of C 2 H 5 I decomposed. But after photolysis thaw‐freeze cycling is accompanied by progressive growth in [I − ] until the yield matches the C 2 H 5 I loss, the quantum yields being 0.26, 0.20, and 0.17 for the three LiCl solutions, respectively. The quantum yield of unionized HI is unchanged, however, at around 0.36, the overall change being due to a fall in the extent to which the HI is ionized in the direct photolysis (ø = 0.22, 0.16, and 0.11). It is proposed that this is a consequence of the density increase of matrix packing as the LiCl concentration is increased so that fewer HI are in contact with the aqueous medium and cage recombination is favored. The results establish that the primary reaction is essentially exclusive:and that substantial aggregates of C 2 H 5 I exist within which HI are caged and cannot be ionized. The direct reaction\documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm C}_{\rm 2}{\rm H}_{\rm 5}{\rm I}+ h\nu \to {\rm C}_{\rm 2}{\rm H}_5^. + {\rm I}^. $$\end{document}occurs only to a trivial extent, ø; = 10 −4 , C 2 H 5 arising virtually totally via\documentclass{article}\pagestyle{empty}\begin{document}$$ {\rm C}_{\rm 2}{\rm H}_{\rm 4}+ {\rm H}^. \to {\rm C}_{\rm 2}{\rm H}_5^. $$\end{document}