Crystallography and magnetism of radicals with hindered hydroxyl groups: 2-(3,5-di-tert-butyl-4-hydroxyphenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl and 2-(3,5-di-tert-butyl-4-hydroxyphenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl.
Radicals form networks of OH... ON and OH...Me(nitroxide) interactions. In 2, a frustrated network forms with insufficient N-O units to form extended chain interactions. The magnetism of 1 fits a 1-D Heisenberg model with J/k=-25 J mol(-1), while 2 shows more complex exchange behaviour consistent with its disordered crystal lattice.
Hybrid density functional theory studies on the magnetic interactions and the weak covalent bonding for the phenalenyl radical dimeric pair.
The phenalenyl radical (1) is a prototype of the hydrocarbon radical. Recently, the single crystal of 2,5,8-tri-tert-butylphenalenyl (2) was isolated and showed that the two phenalenyl radicals form a staggered dimeric pair, giving rise to strong antiferromagnetic interactions. The origin of the antiferromagnetic interactions and the nature of the chemical bond for the dimeric pair are challenging issues for chemists. First, spin-polarized hybrid DFT (Becke's half and half LYP (UB2LYP)) and CASSCF calculations were performed for 2 and its simplified model, the staggered-stacking phenalenyl radical dimeric pair (3a), to elucidate the origin of the strong antiferromagnetic coupling and the characteristics of the chemical bond. The calculated results showed that a SOMO-SOMO overlap effect was responsible for the strong antiferromagnetic interactions and weak or intermediate covalent bonding between phenalenyl radicals. The tert-butyl groups introduced at three beta-positions hardly affected the magnetic coupling, mainly causing steric hindrances in the crystalline state. Next, to obtain insight into ferromagnetic stacking, we investigated the stacking effect of staggered (3a)- and eclipsed (3b)-stacking phenalenyl radical dimeric pairs with a change of the SOMO-SOMO overlap on the basis of the extended McConnell model. We found that the stacking mode of the dimeric pair with both a small SOMO-SOMO overlap and a ferromagnetic spin polarization effect provided a ferromagnetic coupling.
A comparative electron spin resonance study of the selenothiadiazolyl radicals cyclo-Se3–nSnN2+ (n = 0–3)
Electron spin resonance spectra observed in dilute solutions of the hexafluoroarsenate salts of selenothiadiazolylium cations in polar solvents are ascribed to the parent cation radicals , , , and. The g- and nitrogen hyperfine tensors for the three selenium-containing species are entirely in line with the planar π-radical description deduced in earlier studies of trithiadiazolylium. Substantial increases in the g-shifts on substitution of sulphur by selenium result in broadening of the solution spectra and concomitant loss of nitrogen hyperfine structure. From measurements made in solid SO2, the following principal tensor components were deduced for the new, Se-containing radicals:: g = (1.9930, 2.0108, 2.1379), aN(2) = (7.4, ≈0, ≈0) gauss, aSe(2) = (180, 70, 43) gauss;: g = (1.9941, 2.0108, 2.1355), aN(2) = (6.9, ≈0, ≈0) gauss, aSe(2) = (180, 70, 45) gauss;: g = (1.9957, 2.0086, 2.0736), aN(2) = (8.3, ≈0, ≈0) gauss, aSe = (198, 78, 75) gauss, where the values are given in the order x, y, z, x being the ...