EPR, the X-ray Structure and DFT Calculations of the Nitroxide Biradical with One Acetylene Group in the Bridge

Two short nitroxide biradicals of similar composition, R6'aEuro"R6' (B0) and R6'aEuro"(C a parts per thousand C)-R6' (B1), where R6' is 1-oxyl-2,2,6,6-tetramethyl-3,4-ene-nitroxide ring, have been investigated by electron paramagnetic resonance spectroscopy and X-ray structural analysis. Quantum chemical calculations at UDFT/B3LYP/cc-pVDZ level were also performed and compared with the X-ray structural data. Zero-field splitting parameters D for B0 and B1 were found to be equal -0.0048 and -0.0022 cm(-1), respectively, in good agreement with quantum chemical prediction. Potential energy surface scans corresponding to pathways of the conformational rearrangements were calculated; rotation barriers for B0 and B1 were found to be 19.2 and 4.0 kJ/mol, respectively; and structural rigidity and probable differences in biradicals behavior are discussed. Calculations of spin density distribution in biradicals B0 and B1 were also carried out.