OD ESR line inversion in viscous solution. Indication of ST_ mechanism of radical ion pair recombination

The optically detected ESR spectra (OD ESR) of naphthalene radical ion pairs in squalane show inversion near the glass formation point of the solvent. A similar inversion of OD ESR spectra is also observed for some other additives, such as biphenyl and tetrafluorobenzene in glassy nonpolar solvents (squalane, 3-methylpentane). A theory of radical pairs can be used to account for the observed inversion of signal amplitude as a result of adiabatic passage of the anticrossing point of radical pair S and T-levels upon counter ion recombination. The comparison between the shapes of experimentally observed and theoretically calculated spectra qualitatively confirm this hypothesis.