Electron spin resonance studies of small free radicals trapped in inert matrices at 4.2 degrees K

Small free radicals trapped in solid argon, krypton and carbon tetrachloride at 4.2°K have been studied using electron spin resonance (ESR). An attempt was made to determine whether the methylene radical, produced by the photolysis of diazomethane and ketene trapped in the solid matrix, has a triplet ground state. No signal definitely attributable to the methylene radical was observed. It is postulated that the zero field splitting due to the spin-spin coupling of the unpaired electrons broadened any ESR signal beyond detectability„ The ESR signal of trapped methyl radicals was observed in some experiments, and it is suggested that they were formed by abstraction of hydrogen atoms from another deposited material by methylene radicals. An experiment in which diazomethane was photolysed in the presence of D₂O in an argon matrix at 4.2°K yielded an ESR signal which may possibly have been due to the CH₂D radical. An investigation has been carried out of the populations of the rotational levels of methyl radicals produced by the photolysis of trapped methyl iodide and dimethyl mercury at 4.2°K. For thermal equilibrium freely rotating radicals should populate only the ground state at this temperature, but it was found that the lowest two levels were both populated. It is suggested that either there was not thermal equilibrium, or, more likely, the methyl radicals were undergoing hindered rotation. Room temperature equilibrium mixtures of N₂F₄ - NF₂ were trapped in the three matrices at 4.2°K, and ESR absorption due to the trapped NF₂ radicals was observed. Three lines were observed at this temperature, with the centre one of greater amplitude and smaller line width than the outer two. During warmup the amplitudes and widths of these lines became approximately equal and two further triplets appeared, symmetrically distributed about the centre line. From the warmup spectra the isotropic hyperfine splitting constants for fluorine and nitrogen have been deduced to be 168 and 48 mc./sec. respectively. It is suggested that the radicals underwent slow isotropic rotation at 4.2°K. The degree of s-character of the molecular orbital containing the unpaired electron is discussed in the light of the isotropic hyperfine splitting constants. An unsuccessful attempt to find hyperfine and rotational structure in the ESR signal of the NF₂ radical in the gas phase was carried out. The photolysis of CF₃I in krypton and carbon tetrachloride matrices at 4.2°K yielded a very complicated ESR spectrum. A phase reversal of some of the lines was observed. A broad single line was observed when CF₃I in carbon tetrachloride was irradiated at 77°K. At the time of writing no definite interpretation of the spectra can be suggested.

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