Effects of Deuterium Substitution on the ESR Spectrum of the Benzene Negative Ion

Abstract

An investigation has been made of the effects of deuterium substitution on the electron spin resonance spectrum of the benzene negative ion. Spectra of the benzene-1-d, benzene-1,3-d2, benzene-1,4-d2, benzene-1,3,5-d3, and benzene-d6 anion radicals have been obtained at −100° and −120°C, and a more extensive study of the temperature dependence of the spectrum of the benzene-1,4-d2 anion has also been carried out. The results for the benzene-1,3-d2 and benzene-1,4-d2 anions are in agreement with those reported previously for the benzene-1-d anion: The degeneracy of the benzene negative ion is removed by deuterium substitution. As expected, the degeneracy is not lifted in the benzene-1,3,5-d3 and benzene-d6 anions which have threefold or higher symmetry. The temperature dependence of the hyperfine splittings in those radicals which do not possess threefold or higher symmetry was found to be different from that of the unsubstituted benzene anion, and when compared to the benzene anion, some hyperfine splittings increase with temperature while others decrease. In addition, the proton–deuteron splitting-constant ratio was found to be approximately (aH / aD) = 6.18 in all of the radicals studied instead of the value (aH / aD) = 6.514 expected solely on the basis of the magnetic properties of the two isotopes. The effects of deuterium substitution are analyzed by assuming a rapid thermal equilibration between states corresponding to the symmetric and antisymmetric orbitals of the radicals, and estimates are made from the experimental data of the energy separations and relative populations of the two states. These depend on theoretical values for the spin densities; with a Hückel molecular orbital calculation they are −20, 22, and −38 cm−1, respectively, for the benzene-1-d, benzene-1,3-d2, benzene-1,4-d2 anions. The spin densities and energy separations of the symmetric and antisymmetric forms have been calculated using the Hückel MO method without including vibronic interactions by using both a Coulomb and resonance-integral perturbation for the effects of deuterium substitution.