The electron spin density at the alkali nucleus in radical ion pairs of nitrogen containing aromatic molecules and sodium

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Abstract
This paper deals with the spin density at the alkali nucleus in ion pairs with aromatic radical ions containing nitrogen. The mechanisms inducing the spin density have been investigated experimentally and theoretically. The E.S.R. spectra of 1,10-phenanthroline and 2,2'-dipyridyl reduced with sodium metal in dimethoxyethane have been measured in the temperature range -85 to +50°C. By comparing the alkali hyperfine splittings in the two compounds the importance of the polarization mechanism for the electron spin density at the counterion is estimated. In order to calculate the value of the spin density at the alkali nucleus as a result of the polarization mechanism only, a quantitative theoretical description of the σ-π interaction is given. A ‘McConnell-type’ relation between the π-density at the nitrogen centre and the sodium hyperfine splitting constant in a planar rigid σ-complex is derived. The proportionality constant is calculated as a function of the interionic distance in the complex. On the basis of the experimental and theoretical data it is suggested that the traditional explanation of the temperature dependence of the alkali hyperfine coupling is in error.