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Endor and Epr studies of organic radicals in solution Kennedy, David Edward Bertram

Abstract

The construction and operation of a high power liquid phase Electron Nuclear Double Resonance (ENDOR) spectrometer is described in detail. The several modifications of existing methods that make this spectrometer unique are emphasized, particularly the problem of utilizing very high rf fields without excessive rf interference. Some inexpensive and simple methods that were developed for increasing the strength of the rf field are discussed in detail. The completed spectrometer was used to study very small variations in hyperfine couplings in solutions of the tri-t-butyl phenoxy (TTBP) radical. The theory for the treatment of such ENDOR-measured couplings is discussed, and this means was used to determine the existence of very weak hydrogen bond formation in solutions of TTBP and proton-donating solvents. The formation constants and enthalpy of formation for these bonds can be obtained from this theory. The spectrometer was used for an ENDOR study of some common and similar stable free radicals: α, α¹-Diphenyl-β-Picryl Hydrazyl (DPPH), Picryl-N-Amino Carbazyl (PAC) and α,γ-Bisdiphenylene-β-Phenyl Allyl (BDPA). Although the Electron Paramagnetic Resonance (EPR) of all of these compounds has been known for years, there has never been a complete interpretation of their EPR spectra. The ENDOR-obtained hyperfine coupling constants gave complete interpretations of these complex EPR spectra. Spectra were only obtained for these radicals when stringent ENDOR conditions were satisfied: a better understanding of these conditions will make liquid ENDOR a less specialized tool. Values for the spin-spin relaxation time T₂ could be estimated for solutions of these radicals from the ENDOR spectra. The EPR simulations and the assignment of the ENDOR values together yield the first accurate spin density maps for these three species. The molecular distortion that is inferred from the ENDOR spectra must be taken into account if these accurate values are to be used for detailed molecular spin density calculations. Finally, the optimum conditions mentioned above were used to obtain the ENDOR spectra of some related drug precursors: phenoxazine, phenothiazine and chlorophenothiazine. The ENDOR hyperfine values of generated radicals of these compounds were used to simulate their EPR spectra. A novel chlorine hyperfine coupling constant was measured by a careful study of these simulations. Chlorine couplings have rarely been observed previously in aromatic free radicals in solution. The accurate ENDOR proton couplings and assignment were first required in order to obtain this small and unusual chlorine coupling.

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