A specially synthesized nitroxide biradical R5-C≡13C-(p-C6H4)2-13C≡C-R5 (B4) and two radicals, R5-C≡13CH (RCC) and R5-C≡13C-C6H5 (RCCPh), where R5 is 1-oxyl-2,2,5,5-tetramethyl-pyrroline group, have been studied by X- and W-band electron paramagnetic resonance (EPR) spectroscopy, and by W-band electron-nuclear double resonance (ENDOR). Spin density distribution and hyperfine splitting (hfs) constants on 13C atoms were experimentally determined and also calculated using ORCA 3.0.3 program package. The biradical and radicals geometries were optimized on UKS/B3LYP/cc-pVDZ level. Hfs constants were calculated using density functional theory (DFT) with PBE0 functional and N07D, and were compared with the experimental value of the hfs constant on 13C atoms, measured from ENDOR spectra. It is concluded that at small values of the exchange integral as J ≤ a/2 ≈ 7–8 G, the current quantum chemical approaches do not allow determining precise values of the hfs constants on the 13C atoms in the bridge connecting two paramagnetic nitroxide rings of the biradical.
ASJC Scopus subject areas
- Atomic and Molecular Physics, and Optics