Temperature Dependences of the Hyperfine Interactions, Linewidths, and Line Asymmetries in Methyl Radicals Stabilized on Porous Glass Surfaces

Abstract

Methyl radicals were stabilized on the surface of porous Vycor glass at low temperatures by the uv photolysis of methyl iodide, deuterium-enriched methyl iodide, and carbon-13-enriched methyl iodide. The proton, deuterium, and carbon-13 hyperfine splittings were measured as a function of temperature in order to make comparisons with the dependences calculated by Schrader. In addition, the linewidths, line asymmetries, and relative line intensities in the methyl radical ESR spectra were measured as a function of temperature. The absolute values of the temperature coefficients of the proton and deuterium splittings were found to be slightly larger than those predicted by theory. The carbon-13 hyperfine splittings were found to agree with theory above 200°K but a large deviation was observed below 200°K. Below 275°K the proton lines become increasingly asymmetric. The relative line intensities approach but do not attain the binomial values as the temperature is increased. The proton linewidths were found to have different temperature dependences below 200°K, while having the same dependences above 200°K. The observed results may be explained by a physical model based on increased surface interaction at low temperatures.