ELECTRON-SPIN-RESONANCE AND OPTICAL STUDIES ON THE RADIOLYSIS OF CARBON-TETRACHLORIDE .2. STRUCTURE AND REACTION OF (CCL4)-CL-(.-) RADICAL-ANION IN TETRAMETHYLSILANE LOW-TEMPERATURE SOLIDS

An electron spin resonance (ESR) and optical study of carbon tetrachloride radical anion has been made to provide for a better understanding of the radiolysis of CCl4, following CCl4.+ cation previously studied. It was found that the anion was metastably trapped in tetramethylsilane (TMS) matrices gamma irradiated at 4 or 77 K. The g tensor and the hyperfine coupling tensors of all atoms of the radical were determined from ESR spectral simulation by using (CCl4)-C-12 and the C-13 enriched compound: g parallel-to = 2.004-5, g1 = 2.015, (A parallel-to, A perpendicular-to) = (24.3,18.3) mT for C-13, (0.9, 0.2) mT for one Cl-35 atom, and (A1,A2 = A3) = (1.98,0.45) mT for the other three equivalent Cl-35 atoms. From these parameters and a consideration on the g anisotropy combined with the optical data, the anion was found to have a predissociating molecular structure (CCl3...Cl) - with C3v symmetry, where the unpaired electron occupies A 1-gamma* antibonding orbital. The carbon atom has a large spin density and near sp3 hybridization: rho-p = 0.62, rho-s = 0.18, rho-p/rho-s = 3.4, and three Cl atoms and the other Cl atom have the spin densities rho-p = 0.10 and rho-p = 0.05, respectively. The species had two optical absorptions at lambda-max = 265 and 370 nm which were assigned to the E-gamma-A 1-gamma* and A 1-gamma-A 1-gamma* electronic transitions, respectively. The anion converted to CCl3. radical by warming to approximately 150 K in the TMS matrix. The present results have given unequivocal ESR and optical spectroscopic evidence and support for the assignment of the 370 nm band reported in the radiolyses of organic solutions containing CCl4.