Electron Spin Resonance of Parabenzosemiquinone-17O

Abstract
The electron spin resonance spectrum of 17O labeled parabenzosemiquinone in aqueous ethanol solutions has been examined. The hyperfine splitting constant due to 17O, a0, was 9.02±0.02 G. The splitting constant has the same sign as Δg, the anisotropy of the g tensor, and since Δg is negative, the sign of a0 is also negative. Taking into account the fact that 17O has a negative gyromagnetic ratio, the sign found for a0 implies that the net spin polarization at the 17O nucleus is the same as that in the radical. Marked variations in linewidth occur between the different components of the 17O hyperfine structure due to modulation through molecular tumbling of the anisotropic dipolar and g‐tensor interactions. These variations are quantitatively accounted for by the general theory of ESR linewidths due to Freed and Fraenkel. Analysis of the spectrum gives | Δg | = 0.0029, and a correlation time for the tumbling of the molecule of 9.4×10−11 sec.