Saturation recovery electron spin–lattice relaxation studies on benzene anion radical and its derivatives: application of Kivelson–Orbach mechanism of electron spin relaxation

Abstract

The role of low-lying excited states on the spin–lattice relaxation times (T1) of organic radicals has been investigated. To test the applicability of Kivelson's electric field fluctuation model (D. Kivelson, J. Chem. Phys. 45, 1324 (1966)), based on the Orbach mechanism of spin relaxation, the T1s of the anion radicals of benzene, benzene-1-d, toluene, ethyl benzene, isopropyl benzene, t-butyl benzene, p-xylene, 1,2,4-trimethyl benzene and 1,3,5-trimethyl benzene in liquid solutions, with potassium cation as the counter ion, have been measured by the pulse saturation recovery technique. The energy gap between the ground and the first excited electronic states changed with the substitutions to different extent. The spin–lattice relaxation rates showed correlation with this energy gap. Anion radicals of benzene and benzene-1-d showed the shortest T1 among the radicals studied here. A small but measurable energy splitting due to the deuterium substitution in benzene-1-d radical was obtained from the temperature dependence of T1. Spin–lattice relaxation times of benzene anion measured here decreased monotonically in the range of −60 to −125 °C, in contrast to some reported claims of very unusual temperature dependence, based on the continuous wave microwave power saturation studies. Our results also showed that the ion pairing between benzene anion and potassium cation did not significantly influence the spin–lattice relaxation times.