C. Rajadurai, A. Ivanova, V. Enkelmann, M. Baumgarten
Study on the heteroatom influence in pyridine-based nitronyl nitroxide biradicals with phenylethynyl spacers on the molecular ground state.
Journal of Organic Chemistry.  68(26), 9907-9915  (2003)
P-03-171

Novel pyridine-based nitronyl nitroxide (NIT) biradicals, 3,5-bis[4-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)phenylethynyl)]pyridine (1) and 2,6-bis[4-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)phenylethynyl)]pyridine (2), and monoradicals, 4-(5-bromopyridine-3-ylethynyl)-1-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)benzene (3), 4-trimethylsilylethynyl-1-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)benzene (4), and 4-trimethylsilylethynyl-1-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)pyridine (5), were synthesized and investigated by ESR and UV-vis spectroscopy. The solution EPR measurements of the biradicals gave well-resolved, nine-line spectra with exact half line spacing as compared to monoradicals (g_iso = 2.0067) with isotropic line spacing \a_N\= 7.36 G. This indicates strong, intramolecular exchange coupling (J >> 7 × 10^-4 cm^-1; J/a_NN >> 1) of the biradicals with in the limit of EPR. The temperature dependence on the ?ms = ±2 signal intensity of biradicals follow Curie behavior down to 4 K ascertaining the triplet ground state or its near-degeneracy with the singlet state. UV-vis studies of 1-5 show characteristic differences in the extinctions of n-?* transitions around 600 nm. Both biradicals 1 and 2 were crystallized in monoclinic space groups C2/c and P2₁/a with the intraradical distances 1.54 and 1.47 nm, respectively. Computational studies of the biradicals 1, 2, and 1,3-bis[4-(1-oxyl-3-oxo-4,4,5,5-tetramethylimidazolin-2-yl)phenylethynyl)]benzene (T) were performed by the AM1/CAS(8,8) method to calculate the singlet-triplet (?E_ST) energy difference and the spin density distribution. Results show that the position of the pyridyl nitrogen in 1 and 2 in comparison with T does not alter the triplet ground-state spin multiplicities supporting the obtained experimental results.