Electron paramagnetic resonance of a ¹⁰B-containing heterocyclic radical

Electron paramagnetic resonance measurements for a ¹⁰B-containing heterocyclic phenanthrenedione radical, (C₆F₅)₂B(O₂C₁₄H₈), were made at X-band in 9:1 toluene:dichloromethane from 10 to 293 K and in toluene from 180 to 293 K. In well-deoxygenated 0.1 mM toluene solution at room temperature hyperfine couplings to ¹⁰B, four pairs of protons and five pairs of fluorines contribute to a continuous wave spectrum with many resolved lines. Hyperfine couplings were adjusted to provide the best fit for spectra of the radical enriched in ¹⁰B and the analogous radical synthesized with ^10,11B in natural abundance, resulting in small refinements of the hyperfine coupling constants previously reported for the natural abundance sample. Electron spin relaxation rates at temperatures between 15 and 293 K were similar for samples containing ¹⁰B and natural isotope abundance. Analysis of electron spin echo envelope modulation and hyperfine correlation spectroscopy data at 80 K found A_xx = -7.5 ± 0.3, A_yy = -8.5 ± 0.3, and A_zz = -10.8 ± 0.3 MHz for ¹¹B, which indicates small spin density on the boron. The spin echo and hyperfine spectroscopy data for the ¹⁰B -containing radical are consistent with the factor of 2.99 smaller hyperfine values for ¹⁰B than for ¹¹B.