Magnetic Resonance Studies of Hyperfine Interactions in the Anion Radicals of Stilbene and Related Molecules

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Abstract
Electron spin resonance spectra are reported for the anions of tolan, stilbene, 1,2-bis(4-pyridyl)-ethylene, and azobenzene. In each experiment only one radical species was observed. The spectra were analyzed with the aid of third-derivative presentations and a nuclear magnetic resonance line-broadening experiment. All of the anions except tolan exhibit hindered rotation which allows the asymmetries of the spin-density distributions in the rings to be observed. Molecular orbital calculations with the McLachlan correction are in satisfactory agreement with the experimental hyperfine splitting constants. Methods for introducing asymmetry into the molecular orbital calculations are critically discussed and it is concluded that the so-called β effect is probably the most desirable of the crude corrections. It is argued on the basis of Hückel molecular orbital theory that moderately rapid isomerization is reasonable for the stilbene anion.

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