Structural studies of irradiated solids using electron spin resonance

• Main Author: Begum, Afrozi
• Published: University of Leicester 1971
• Subjects: 540
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.674020

This thesis is a detailed report of the structural investigations of some new radicals formed in 8-irradiated inorganic solids. Electron spin resonance spectroscopy is used for such studies as it is a very sensitive tool for identification of radicals, A great deal of information was obtained from the electron spin resonance spectra about the structure of the radicals, especially with respect to the delocalisation of the unpaired electron within the radicals, the shape of the species, possible internal motions, and the role of electronegativity of the constituent atoms; upon these the effect of environment may be impressed as a perturbation. As a part of a continuing study of the structures of twenty five-valence electron oxy-radicals of general type AOsn-, the radical BrOs in 8-irradiated KBrO3 is discussed in Chapter II, This pyramidal radical is found (from its p:s ratio) to be considerably flatter than its parent ion, BrO3-. In Chapter III, the species PO3F-, PO2F2 and PO2F- and also PO42-, PO32- are discussed, The hyperfine interaction with fluorine substituents in phosphates, shows the mechanism of spin transfer from the central to the ligand atoms. Chapter IV describes the new radicals POCls-, PO2Cl2, POCl2 and also PC1, PCl2. The species are compared with corresponding known fluoro species. Chapter V deals with the species PR3+, AIR3- and also SiR3, where R is H or alkyl. The 3p : 3s ratio shows that these species are pyramidal in character but this ratio increases markedly on going from aluminium to phosphorus. This suggests that these isostructural radicals flatten in this order, thus depicting the role of electro-negativity in governing the shape of radicals- This trend is found more prominent for the alkyls than for the hydrides. Radicals of general type RaC-X, where R is H or alkyl and X is alkyl substituted A1 or P have been studied in different systems and are discussed in Chapter VI. It is concluded that the spin is transmitted to the group X by spin polarisation of the C - X 6 bond and the value of Aiso is negative. A clear trend to lower magnitude is observed in the s character on going across the periodic table from aluminium to phosphorus. Chapter VII describes the formation of radicals CH3 and H2COP(O)(OMe)2 in 8-irradiated frozen trimethyl phosphate. Nothing new could be said about the rotating radical CH3 which is universally accepted as being plannar. The latter radical has an e.s.r. spectrum characteristic of radicals in which the-hydrogen atoms are rapidly reorienting. The small phosphorus splitting is explained in terms of structures where two rival mechanisms, conjugation and hyperconjugation are active. The spectra from trapped N-atoms in irradiated sodium and potassium azide are explained in terms of the zero field splitting from D and E in Chapter VIII. In both cases clear temperature effects were noticed, being more marked in the case of sodium azide where the spectrum shows a reversible change between non-axial and axial symmetry. A possible explanation is given for this, and the zero field splitting is explained in terms of slight admixture of the nitrogen 2s orbitals into the 4S ground state of the atoms.