SummaryESR. spectra of the chlorine substituted phenoxy radicals listed in Table 1, which were produced by photo-oxidation of the parent phenols in aprotic solvents, are reported. In most cases the chlorine splittings could be resolved and analysed in detail. Experimental data as well as quantum chemical calculations were used for assignment of all measured chlorine couplings and determination of their relative signs.Spin density distributions and polarization parameters for chlorine are discussed and compared with the results from current semiempirical molecular orbital theories.Chlorine couplings associated with the larger splittings are found positive in sign, whereas signs of small couplings appear difficult to establish. Limitations in the applicability of McConnell or Karplus-Fraenkrl type relations to chlorine substituted aromatic radicals will be discussed and related to peculiar properties of the spin density matrix of such systems.1. Introduction. -The number of reported coupling constants for chlorine substituents in radicals is at present rather limited [I]. In the ca:je of aromatic radicals in particular this has to be traced back to a number of factors, e.g. small hyperfine splittings, overlapping of spectra of isotopic modifications, nuclear quadrupole effects and chemical instability. As a consequence of these difficulties the theory which should explain the chlorine splittings is much less developed than the theory for corresponding fluorinated aromatic radicals [I] [2]. In this work ESR. spectra of a series of chlorine substituted phenoxy radicals were investigated with the aim to obtain sufficient experimental data for an unambiguous assignment of observed hyperfine splittings, in particular for the chlorine substituent. This in turn should provide a basis for testing spin density values as obtained from different MO theories and, eventually, lead to a determination of polarization constants for the chlorine substituent in an aromatic radical.
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SummaryThe temperature and solvent dependence of the ESR. spectra of a number of semiquinone-and semidione-type radicals have been investigated with the aim of obtaining structural and kinetic information about intramolecular hydrogen bonding. Systematic variation of the chemical structure of the radicals indicates that in many cases formation and/or exchange of intramolecular H-bonds is disturbed or even precluded by steric hindrance or concomitant dynamic processes, such as internal rot ation and/or intermolecular proton exchange.
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