Conformational distributions hyperfine splitting

The ESR spectra of a large variety of sulfonyl radicals have been obtained photolytically in liquid phase over a wide range of temperature. Some selected data are summarized in Table 2. The magnitudes of hyperfine splittings and the observations of line broadening resulting from restricted rotation about the C—S bond have been used successfully in conjunction with INDO SCF MO calculations to elucidate both structure and conformational properties. Thus the spin distribution in these species is typical of (T-radicals with a pyramidal center at sulfur and in accord with the solid-state ESR data. 

The anisotropy of the spin-label hyperfine splitting is a consequence of the particular charge distribution symmetry within the molecule. The analysis of spectral anisotropy provides the immobilization extent of the spin label and is sensitive to the molecular conformation. Moreover, the spin-label molecules have their charge distribution distorted by the polarity of their environment. The isotropic hyperfine splitting constant (a0) provides a relative polarity indicator (Knowles et al., 1976 Campbell and Dwek, 1984). 

The same group has investigated 10-phenylphenoxazine (263) and -phenothiazine (264) cation-radicals and the influence of substituents in the phenyl group upon the spin distribution. The interest here lay in the effect of the conformation on electronic effects. Earlier studies on similarly shaped radicals had indicated that a twist of about 65° must exist in the V-phenyl bond (cf. phenylxanthenyl and isologs Sections II,B,3,a III,C,1 IV,B,1). This value may be calculated trigonometrically for a model of such radicals which has planar aromatic components, each C—C and C—H bond equal to those in benzene, and in which the phenyl ortho and heterocyclic 1- and 9-protons are at van der Waals separation. A rather similar angle has to be allowed in MO calculations in order correctly to reproduce experimental hyperfine splittings. 

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