Paramagnetic impurities containing oxygen were produced in the reactions of KBr, KCl, NaCl and SrCl(2) with fluorine. Molecular oxygen, FOO･ radical and C10(2) were identified by ESR as reaction by-products. The spectra assigned to CIO(2) have predominant powder character even in single crystals of halides. The results are correlated with previous work in this laboratory. The origin of oxygen impurity is suggested to be surface hydroxide ions in KBr and KC1, and entrapped water in NaCl and SrCl(2). Nucleation processes and other anomalous features observed in these reactions by previous workers are explained by the presence of impurities. Oxygen was found to be incorporated into the SrCl(2) crystals by recrystallisation from the melt in the presence of oxygen. It exists in the form of superoxide ion O(2)¯ which occupies an interstitial position between two lattice anions and is associated with two anion vacancies. The molecular axis lies in a [00l] direction of the crystal and the degeneracy of the 2pπ(g) -molecular orbital is lifted by the crystal field. The bonding between the O(2)¯ ion and its neighbouring cations and anions is discussed in terms of a Cl(6)Sr･O(2)¯･SrCl(6) "complex". The orbital angular momentum reduction factor for O(2)¯ in SrCl(2) has been calculated from the experimental g-factors and found to be anomalously large. A survey was made on impurities incorporated into strontium compounds through processes of recrystallisation from melts or from aqueous solutions. Strontium carbonate was identified in melt-recrystallised Sr(NO3)(2) by Infrared Spectroscopy and X-ray powder method. It was produced by partial decomposition of the nitrate in the presence of atmospheric CO(2). NO(3)²¯ ion was identified in the recrystallised material by ESR. ESR and infrared studies suggest that the NO(3)²¯ ion substitutes a carbonate ion in the SrCO(3) lattice. Results from a similar study on Ba(NO3)(2) also support this conclusion. / Science, Faculty of / Chemistry, Department of / Graduate

X-irradiated cyanoacetylurea CNCH₂CONHCONH₂ and x-irradiated dicyandiaraide NCNC(NH₂)₂ single crystals were studied,by Electron Spin Resonance. Two radical species were formed when cyanoacetylurea, CNCH₂CONHCONH₂, was irradiated with x-rays. One was a π-electron radical, CNCHCONHCONH₂, and the other a σ-electron radical, CNCH₂CONHCONH . The σ electron radical was found to have a large, isotropic proton coupling tensor and an anisotropic nitrogen coupling tensor very similar to those of H₂NCOCH₂CONH . The π-electron radical had similar proton and nitrogen coupling tensors as CNCHCOOH which was formed in γ-irradiated cyanoacetic acid. In x-irradiated dicyandiamide crystals, the main species formed was shown to be NCNC(NH₂)NH, having also a large, isotropic proton coupling tensor, and hyperfine interactions with two nitrogen coupling nuclei were also observed and measured. ESR studies were carried out both at room and at liquid nitrogen temperatures. The effect of temperature on the spectra is discussed. INDO-SCF-LCAO-MO calculations were carried out for a model compound HCONH , with the amide proton assuming different inplane and out of plane positions. The spin density was found to vary over a wide range and could be used to interpret the large proton coupling of the dicyandiamide radical. MO calculations were also performed on the radical NCCHCOO ̄. The calculated results correlate fairly satisfactorily with the observed ones. Comparison of the direction of the unpaired electron p-orbital symmetry axis with bond directions and with normals to the fragment planes showed that while malonamide radical, H₂NCOCH₂CONH was a genuine σ-electron radical, dicyandiamide radical, NCNC(NH₂)NH was most likely a π-electron radical. / Science, Faculty of / Chemistry, Department of / Graduate

The e.p.r. spectrum for relatively dilute samples of phosphorus-doped silicon (<5 x 10(16) donors/cm³) has been calculated in detail for an assumed random distribution of impurities. The system of donor electron spins is treated as a collection of nearest neighbor donor pairs. An expression is derived for the donor pair exchange energy using Kohn-Luttinger wavefunctions and a general exchange energy expression. The resultant relationship contains an adjustable parameter a*, the "effective Bohr radius", which is determined from a comparison of the calculated spectrum and the experimental results obtained for the ratio, C, of the "central pair" and "hyperfine" line intensities. The resulting expression J(R), where J represents the exchange energy and R the separation vector connecting the two pair donors, exhibits an oscillatory spatial dependence due to interference from portions of the wavefunction arising from different conduction band valleys. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

A completely general method has been developed for calculating Electron Paramagnetic Resonance (EPR) transition fields and transition probabilities from the spin Hamiltonian [formula omitted] The method is a generalization of a previously existing, limited technique, and can be applied without any restrictions. A general computer program has also been written based on the method. Using the program, the results of an EPR study on a U.V. irradiated single crystal of para-nitrophenyl azide were fitted to the spin Hamiltonian [formula omitted] The observed and calculated fine and hyperfine transition probabilities were found to be in good agreement. The observed hyperfine splittings, due to the nitrene nitrogen, were analyzed in terms of a 6.5 gauss linewidth, combined with the transition probabilities for the nine theoretical components of the hyperfine resonance fields. Evidence of a ¹⁴N quadrupolar splitting was observed, but not completely analyzed. In addition, evidence was found to indicate that the nitrene impurity is twisted somewhat from the position in the crystal lattice held by the parent azide. The EPR spectra obtained on U.V. irradiation of randomly oriented 1,5- and 1,8-diazidonaphthalene were analyzed, and found to originate from ground triplet state species with |D|= 0.8152±0.0010 cm⁻¹ and 0.7599±0.0010 cm⁻¹, respectively. The theoretical spin densities for the triplet and quintet states of 1,5- , 1,8- and 2,7- naphthalene dinitrene were calculated using the method of intermediate neglect of differential overlap (INDO). Attempts to obtain a consistent picture of multiplet spin densities based on the results of the calculations were largely inconclusive. / Science, Faculty of / Chemistry, Department of / Graduate

Electron Nuclear Double Resonance (ENDOR) measurements have been made of hyperfine couplings in x-irradiated single crystals of sodium formate and potassium hydrogen bisphenylacetate (KHBP). In sodium formate ENDOR signals were obtained from both proton and sodium ion neighbours of the CO^" centre formed on irradiation. ENDOR studies 23 of the Na hyperfine interaction together with CNDO calculations indicate that the CO^" forms a tight ion pair with the nearer Na+ cation, thus explaining the four line EPR hfs. observed. Hf. interactions have also been resolved for four pairs of nearest neighbour protons. The tensors are mainly dipolar in character, but contain some isotropic contributions which are interpreted in terms of covalent interactions. 23 Extra so-called 'forbidden' lines are observed in the Na ENDOR and a model of ENDOR enhancements involving cross relaxation with other paramagneti species is suggested. (#2" was also observed in uv-irradiated samples: the threshold energy for radical formation was estimated to be 100+10 kcal mole A previously reported free-radical reaction in sodium formate was found to be reversed by uv irradiation, the reaction obeying second order kinetics. Proton ENDOR studies of x-irradiated KHBP confirmed the presence of the benzyl radical, for which all seven anisotropic proton hyperfine tensors were measured. The isotropic couplings agree with earlier EPR measurements of the benzyl radical undergoing free rotation. The dipolar couplings provide an independent estimate of the spin density distribution in the radical which is not in complete accord with earlier determinations based on the McConnell relation. The dipolar tensors imply a spin distribution close to that predicted by INDO and other calculations, which suggests that these calculations may be qualitatively correct, and the McConnell relation not strictly applicable. Several other radicals are also present in irradiated KHBP. Two are tentatively identified as cyclohexadienyl type radicals formed by hydrogen addition at the phenyl ring ortho and para to the methylene carboxy group. / Science, Faculty of / Chemistry, Department of / Graduate

No description available.

No description available.

No description available.

No description available.

No description available.