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-I, 3-d 2 , benzene-I, 4-~, 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-I, 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-I, 3, 5-d s and benzene-ds 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 (aR/a D ) =6.18 in all of the radicals studied instead of the value (aR/a D ) =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 Huckel 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 Huckel MO method without including vibronic interactions by using both a Coulomb and resonance-integral perturbation for the effects of deuterium substitution.

Calculations of the Equilibrium Isotope Effects on the Reductions of Benzene-d6 and Cyclooctatetraene-d8

“…The 3-fold axis of symmetry that is present in benzene-1,3,5-d 3 also results in the C 2 v and D 2 geometries of the radical anion both giving essentially the same calculated isotope effect of K = 2.15 at 173 K . As would be expected, the value of K = 2.15 that we calculate for benzene-1,3,5-d 3 is close to the square root of the value of K (1) = 4.63 that we compute for benzene- d 6 .…”

“…Nevertheless, there is reasonably good agreement between the equilibrium isotope effect that we calculate for reduction of benzene- d 6 and that measured by Stevenson and co-workers . The calculated equilibrium constants at 173 K are, respectively, K (1) = 4.63 and 4.67, which are both about 20% larger than the experimental value of K (1) = 3.85.…”

An experimental estimate of the lower bound on the energy separation between the ground and first excited electronic states of the p-xylene anion radical

“…The uncertainties for A represent the rms differences between the experimental points and the points calculated with the linear equation as given. 6 See ref 1. 1.…”

“…See text for details regarding parameter selection. 6 The signs of all the hfs values listed in this table should be negative. Note that in the case of the column headed 868 cm™1 the values go through zero at approximately 245 K. troduced into the benzene ring, one might expect the energy separation for the p-xylenide to be approximately twice the value measured for the toluenide system.5,6 The energy separation reported for toluenide1 is 458 cm™1, thus yielding an estimate of 916 cm™1 for the p-xylenide.…”

Formation of Large Molecular Cluster Anions and Elucidation of Their Electronic Structures

“…71 The formation of the valence anion of a benzene molecule is thus unfavorable in the gas phase. In contrast, a benzene molecular anion can be stably formed in the condensed phases by the solvent-induced stabilization, [72][73][74][75][76] which leads to the following simple question: ''How many benzene molecules are needed to bind an excess electron?'' An answer for this question is given in Fig.…”