Pseudorotation as a possible origin of fast paramagnetic relaxation in radical ions with a degenerate or quasi-degenerate ground state

Abstract

In early 1960s, aromatic radical ions with a three-fold or higher symmetry axis were shown to haveanomalously high spin–lattice relaxation rates [1, 2].For example, the paramagnetic relaxation rate in ben-zene and coronene radical anions in solution turned outto be one to two orders of magnitude higher than in rad-icals of lower symmetry. An even higher relaxation ratewas later revealed in the fullerene radical anion [3] andin radical cations of some cycloalkanes [4–6].The observed anomalously fast spin–lattice relax-ation in radical ions of high-symmetry molecules insolution cannot be explained by common relaxationmechanisms, such as modulation of anisotropic hyper-fine coupling (HFC) and g tensor by chaotic molecularmotion or modulation of isotropic HFC due to transi-tions between distorted Jahn–Teller structures, as wellas by spin–rotation interaction [2, 7]. At present, thehypothesis that spin–orbit coupling in such radicals isresponsible for the anomalous relaxation is believed tobe the most probable [2–7]. However, despite numer-ous attempts, no concrete theoretical model of thisrelaxation has been suggestedOne of the factors complicating the creation of thetheory of paramagnetic relaxation of high-symmetryradicals is the absence of experimental information onthe nature of the state in which the unpaired electronspin is efficiently coupled to the other degrees of free-dom. In particular, for the best studied benzene radicalanion, the doubly degenerate ground vibronic state [2],in which the average projection of the orbital momentonto the symmetry axis can be nonzero, is suggested assuch a state. These concepts make it possible to realis-tically estimate the relaxation rate of the benzene radi-cal anion [7]; however, such arguments are untenablefor asymmetric particles, lacking symmetry-relateddegeneracy, with fast paramagnetic relaxation [4–6].It is worth noting that studying the paramagneticrelaxation in high-symmetry molecular structures is notonly of theoretical significance. Symmetric structures(fullerenes, fullerites, nanotubes) are candidates fordesign of nanosized devices of molecular electronicsand spintronics [8]. Therefore, it should be possible tocontrol the states of separate spins in them, and, hence,the paramagnetic relaxation of these states should berather slow.Radical ions exhibiting anomalous paramagneticrelaxation are structurally nonrigid [9], and it is quitelikely that there is a correlation between this relaxationand intramolecular dynamics of such particles. Theo-retical analysis of the adiabatic potential energy surface(PES) of the ground state of radical ions is required tostudy this correlation in detail.In this paper, we briefly describe the results of suchanalysis for radical cations (RCs) of a series of alkyl-substituted cyclohexanes ( R–C