Electron paramagnetic resonance study of isolated free radical pairs in M+ 18-crown-6 TCNQ–. (TCNQ = 7,7′,8,8′-tetracyano-p-quinodimethane; M = K, Rb)

Abstract

Intense EPR spectra observed in M⁺ 18-crown-6 TCNQ^–(M = K, Rb) at ambient temperature and above, in both powder and single-crystal samples, are typical of electronically excited triplet species (S= 1). From measurements of the fine-structure splitting in single-crystal specimens as a function of angle in three orthogonal crystal planes it has been established that the g-tensor is nearly isotropic, with g_k= 2.0031 and g_Rb= 2.0023, while the spin–spin interaction tensors are rhombic, with D_k=(∓305.5, ±221.5, ±88.7 MHz) and D_Rb=(∓295.1, ±214.9, ±84.7 MHz). The zero-field splitting parameters, in MHz, are thus |D_k|= 458.3, |E_k|= 66.4, and |D_Rb|= 442.7, |E_Rb|= 65.1. Comparison of the D eigenvectors with directions determined crystallographically within the crystals shows unequivocally that the triplet spectra are due to the intrinsic localized pairs of TCNQ^– radical anions. The principal values and directions of D in these radical pairs are described very well in terms of classical pairwise dipolar interactions based on literature values of the unpaired spin distribution in TCNQ^–.