Electron spin resonance characterization of novel radicals produced by high energy radiation

Using the technique of electron spin resonance, the structure and bonding of several radicals formed by γ-irradiation have been examined. The general types of radicals examined were either fluorine containing radical anions, organometallic radical cations, or carboncentered neutral radicals.

By combining the technique of partial orientation with ¹³C enrichment, both the ¹³C ¹⁹F and axially symmetric hyperfine tensors have been determined for ¹³CF₃Br^- and ¹³C₂F₄Br^-. This has provided further insight into the structure of these radical anions. While neutral octafluorocyclooctatetraene (OFCOT) is known to be tub-shaped, the radical anion of OFCOT was found to be planar. In the radical anion of perfluoromethylcyclohexane, the unpaired electron was found to be primarily localized along the axial C-F bond which is adjacent to the CF₃ group. This represents the first example of a radical anion where the σ* orbital is along a single C-F bond. The detection ³³S of satellite lines in natural abundances allowed the positive identification of two new members of the 35 and 41 electron family, SF₄^- and SF₄O^-, respectively. The characterization of the SF₄^- is of particular interest, since its detection is only the second example of a 35-electron radical.

The radical cations of the entire family of tetramethyl Group IVB metals (Si, Ge, Sn and Pb) were examined. While the radical cations of (CH₃)₄Si and (CH₃)₄Ge adopt a C_2vstructure, the radical cations of (CH₃)₄Sn and (CH₃)₄Pb were found to adopt a C_3v trigonalpyramidal structure. An extensive study of some isotopically enriched tetramethyltin derivatives ((CD₃)₄Sn, (¹³CH₃)Sn, and (¹³CD₃)₄Sn) and some Q-band investigations helped confirm this analysis.

The radical cations of a series of alkyltrimethyl tin derivatives were examined. The magnitude and the temperature dependence of the hyperfine coupling constants were, in each case, consistent with a simple model based upon the results found for the tertbutyltrimethyltin radical cations. The σ orbital description given for the hexamethyl homonuclear dimetallic species (Me₃MMMe₃; M=Si, Ge and Sn), was found appropriate to explain the ESR results for two heterobimetallic analogues (Me₃SiGeMe₃⁺) and (Me₃SiSnMe₃⁺).

The family of Me₃MCH₂ neutral radicals where M=Si, Ge, Sn, and Pb, were examined. These results indicated the relative unimportance of d(π) - p(π) bonding in α-metal π radicals. The ¹³C satellite lines which were detected in natural abundance for CF₃CCl₂ strongly support the argument that π-conjugative destabilization of the α-substituent is more significant than increasing the electro negativity in inducing pyramidality of the alkyl radical.