Electron spin resonance spectra of transient tin-centered radicals in solution

ESR data for short-living stannyl radicals in solution are presented. The radicals are generated in the cavity of the ESR spectrometer by UV irradiation of di-t-butyl peroxide with the appropriate tin hydride. The radicals R₃Sn^. (R = Me, n-Pr, n-Bu) show multiplets caused by interaction with the β-protons (a_H = 0.30–0.31 mT, g = 2.0160–2.0163) and line broadenings depending on R₃SnH]. The radicals Ph_nMe_3−nSn^. (n = 1, 2) exhibit only splittings due to the methyl protons (a_H = 0.30 mT). The observed linewidths show that a_o,p 0.05 mT, a_m 0.03 mT. The radicals Ph_nEt_3−nSn^. (n = 0–2) show no splittings caused by the methylene protons because of exchange narrowing effects. The g values decrease with increasing n from 2.0163 and 2.015 (n = 0) to 2.0023 (n = 3) because of increasing deviations from the planar conformation at the radical center. The line broadening and exchange narrowing effects are caused by rapid hydrogen exchange between the radicals and hydride molecules (k 10⁶ M⁻¹ s⁻¹); in Zavitsas' model, the relatively high k values are the consequence of the large Sn---H bond lengths which diminish the repulsive forces between the terminal Sn atoms in the transition state R₃Sn--H--SnR₃]^.. The observation of line broadenings in the NMR spectrum of Me₃SnH during irradiation with di-t-butyl peroxide confirms the ESR results.