Electron spin resonance studies of irradiated alkyl halides.
2015-11-19T08:46:27Z (GMT) by
The Electron Spin Resonance (E.S.R.) technique has been widely used to detect the reactive intermediates during the radiolysis of condensed systems which provide valuable information about the overall reaction mechanism. The present work is an attempt to identify the varieties of free radical species which are produced in ?-radiolysis of various organic halides and learn more about the radiation damage mechanisms. As the field of Electron Spin Resonance is now well established, no attempt has been made to review the theory and practice of the technique. The introductory chapter embodies some brief historical developments in radiation chemistry, interaction of radiation with matter, and relevant descriptions of the radiolysis of alkyl halides. Chapter II describes the findings of the novel species (?---Hal-) in which the halogen ion weakly interacts with the parent alkyl radical. The spin density on halogens in these alkyl radical-halide ion complexes is of smaller order compared to ?* radicals of N-halogenoamides. The detailed aspects of ?-mono- and di-chloro radicals as revealed by their powder and single crystal spectra is presented in chapter III. The assignments of the e.s.r. parameters include low but significant 'quadrupole' effects which provide authentic spectral analysis. There has been a lack of an authentic solid-state-spectrum of ?-bromo radicals due to spectral complexities caused by multiple features of two magnetic bromine isotopes (79Br and 81Br) and large 'quadrupole moments'. A detailed study of the detection of these radicals in ?-irradiated alkyl bromides was taken and the apparent difficulties were resolved by working on the single crystal spectra in some cases. Details of the findings are given in Chapter IV. An interesting problem to the organic chemists has been the electronic structure and geometry of the intermediate R2?-CR2Hal, which has often been formulated with a cyclic structure. Production and identification of such radical intermediates in the radiolysis of alkyl halides was undertaken and the first authentic parameter-isation of ?-bromo radicals (R2?-CR2Br) is embodied in Chapter V. The large isotopic coupling due to bromine atoms is explained in the light of a 'hyperconjugative' delocalisation mechanism. ?-Radiolysis of a series of trichloromethyl compounds was carried in pure materials and in solvents at 77 K and ?Cl3 radicals along with other species were identified by their characteristic e.s.r. spectra. It is of interest to find that ?Cl3 radicals can be reasonably detected in the presence of other chloro radicals produced during irradiation. These findings are reported in Chapter VI. Several dihalide anions and related species (Hal2-, R2CHal+, RHal+, RHal-HaR+, RHa1-Hal., ROHal-) have been recognised as products of radiolysis of organic halides. These species were identified by their characteristic e.s.r. spectra and a full description is presented in Chapter VII. The general experimental aspects of Chapters II-VII are presented in Chapter VIII.