Electron spin resonance studies of structures and reactions of radical cations of a series of cycloalkanes in low-temperature matrices

Abstract

Radical cations of a series of cycloalkanes (C₃–C₈) produced radiolytically at 4.2 K in a variety of halogenated matrices have been characterized by e.s.r. spectroscopy as σ-delocalized radicals, in which the unpaired electron occupies the molecular orbital lying in the molecular (equatorial) plane. Thus the protons in the equatorial C—H bonds participating in the MO give rise to large hyperfine couplings as compared with the axial protons. Among these, radical cations of C₃, C₄, C₆ and C₈ are Jahn–Teller (J.T.) active and exhibit static distortions at 4.2 K, giving non-equivalent proton couplings. Regardless of the matrices used, the cation distorts to the same direction along the J.T.-active ring-deformation mode giving essentially the same geometry and singly occupied molecular orbital (SOMO). The distorted structure is dynamically averaged at elevated temperatures, giving equivalent couplings. The temperture changes of the spectra of c-C₃H⁺ ₆ and c-C₆H⁺ ₁₂ are accounted for by a three-site jumping (or tunneling) model using a modified Bloch treatment. The site-jumping process may be pseudorotation in the J.T. potential trough. However, the onset temperature of the jumping process and thus the activation energy differ from matrix to matrix, so that the observed distortion may be a matrix-assisted static J.T. distortion. The results may be also accounted for in terms of a static distortion due to matrix effects and an averaging of the matrix field by reorientation of the cations around the normal to the molecular plane. However, the latter explanation seems less probable because the onset of the averaging process of c C₆H⁺ ₁₂ at temperatures as low as ≲4.2 K does not appear to be ascribable to molecular reorientation. Most of these cations undergo deprotonation to form cycloalkyl radicals in SF₆ and in CFCl₂CF₂Cl at ≳100 K.