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)
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 gk= 2.0031 and gRb= 2.0023, while the spin–spin interaction tensors are rhombic, with Dk=(∓305.5, ±221.5, ±88.7 MHz) and DRb=(∓295.1, ±214.9, ±84.7 MHz). The zero-field splitting parameters, in MHz, are thus |Dk|= 458.3, |Ek|= 66.4, and |DRb|= 442.7, |ERb|= 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–.