Spectroscopy and photochemistry at low temperature. Part 3.—Photochemical production of ions and molecular complexes in dilute solutions of polyhalomethanes

Abstract

Transient products of the thermal decay of the colour centres which develop in frozen solutions of polyhalomethanes at 77 K on absorption of near u.-v. light, have been studied by means of u.-v. and visible spectroscopy at low temperatures. Absorption spectra of solvated trihalide ions have been detected, as well as those of weak complexes of molecular halogens with added olefins or with the polyhalomethane itself. The Aâ†�X absorption system of CF2 has been observed in viscous solutions of CF2Br2 at low temperature for the first time, and shown to be a product of the thermal decay of the colour centre. Halide ions are produced from all the polyhalomethanes, and could be formed following an electron transfer between neighbouring polyhalomethane molecules. Except for CF2Br2, little or no molecular halogen is released into solution unless it contains dissolved olefins. When molecular halogens are produced, their constituent atoms are both drawn from the same polyhalomethane molecule; they are products of a “molecular” reaction rather than association of free halogen atoms.