An electron spin resonance study of the structure of CCl+4 radical cation in carbon tetrachloride γ-irradiated at low temperatures by powder and single crystal analyses

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Abstract
The CCl+4 radical cation produced in CCl4 by γ irradiation at low temperatures was studied by electron spin resonance (ESR) spectroscopy to clarify the electronic structure. By combining an analysis of the angular dependence of ESR spectra in a single crystal plane with a spectral simulation of the powder pattern including the isotropic combinations of 35Cl and 37Cl atoms, it was found that two chlorine atoms are concerned in the radical, having different hyperfine coupling tensors of A(35Cl1)=11.2, 0.5, 1.3 mT and A(35Cl2)=6.2, 0.8, 1.8 mT. The g tensor is approximately axially symmetric and shows a large positive g⊥ shift (g=1.999, 2.10, 2.116), different from the rhombic g tensor of σ* dimer cations of alkyl halides and freons (gmax=2.04). The directions of the maximum hyperfine coupling of the two chlorine atoms Cl1 and Cl2 are close to each other and close to the gmin direction, though having a small deviation from the gmin direction by about 3°–10°. From a consideration on the g anisotropy combined with the optical absorption spectrum, the radical was suggested to be a Cl atom‐like species, in which one of the C–Cl bonds in the parent CCl+4 is released and the atom‐like chlorine nuclei formed makes a three electron bond [(σCl⋅⋅⋅Cl)2 (σ*Cl⋅⋅⋅Cl)1] with a Cl atom in the released CCl+3 group. A discussion is given for the difference in the formation of the cationic species in CCl4 and the σ* dimer radical cation in the other alkyl halides.