Electron Spin Resonance of the Monodeuterotropenyl Radical. Effect of the Deuterium Substituent

Publisher Website
Google Scholar
Abstract
Monodeuterotropenyl radical, C7H6D·, is generated by thermal cleavage of liquid x,x′-dideuterobitropenyl (x,x′-dideutero-bi-2,4,6-cycloheptatrien-1-yl) and is investigated by means of electron spin resonance (ESR) spectroscopy. The major goal of this work is an assessment of the effect of the deuterium substituent on the vibronic near degeneracy of this radical. The ESR spectrum of C7H6D· appears to correspond to hyperfine interaction with six equivalent protons and one deuteron. The expected inequivalence of the proton splittings is less than the linewidth. From a detailed analysis of the magnitude of the linewidth, an upper limit of 25 cal/mole is estimated for the energy separation between the nearly degenerate vibronic levels of C7H6D·. This estimate is compared with the energy splitting of C6H5D− and is correlated with the prediction of the resonance integral perturbation model for the effect of a deuterium substituent [J. Am. Chem. Soc. 87 5260 (1965)]. The proton and deuteron hyperfine splittings are measured as a function of temperature (130 ≤ t ≤ 186°C). The temperature dependence of the proton splitting agrees closely with that of C7H7·. The ratio aH / aD decreases with increasing temperature and is most likely greater than 6.514. The g value, 2.00266, is in agreement with that of C7H7·. A small positive temperature coefficient is indicated by the g-value data.