Peculiarities of Spin Exchange in Nitroxide Biradicals Containing Two para-Phenylene Groups in the Bridge: EPR Investigation and DFT Calculations

Abstract

Two nitroxide biradicals of similar composition: R₅-C≡C-Ph-Ph-C≡C-R₅ (B3a) and R₆-C≡C-Ph-Ph-C≡C-R₆ (B3b), where Ph = p-C₆H₄, and R₆ is 2,6,6-tetramethyl-5,6-dihydropyridin-1(2H)-yloxyl, and R₅ is 2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrol-1-yloxyl nitroxide rings, have been studied by electron paramagnetic resonance (EPR) spectroscopy. Variations of the intramolecular electron spin exchange in the biradicals, dissolved in toluene, as a function of temperature were characterized by changes in the isotropic ¹⁴N hyperfine splitting (hfs) constant a, values of the exchange integral |J|, and compared with the data obtained by density functional theory calculations (DFT). Thermodynamic parameters of the conformational rearrangements were calculated. Geometries of nitroxide biradicals in PES local minima and transition states in the triplet state were calculated at UDFT/B3LYP level with split-valence basis set cc-PVTZ. Probable differences in biradicals behavior are discussed.