Proton Hyperfine Structure Research Articles

Electron Spin Resonance Spectrum of the Radical Anion of 1,3,5-Cycloheptatriene and the Spin Polarization Constant QC′CHH

The radical anion of 1,3,5-cycloheptatriene has been prepared by electrolysis in liquid ammonia. The proton hyperfine structure of its electron spin resonance spectrum was assigned with A(1, 6)=7.64, A(2, 5)=0.59, A(3,4)=4.90, and A(7)=2.16 G. Satisfactory agreement between calculated and observed coupling constants is only obtained if the radical anion is assumed to have a nonplanar boat conformation and if the methylene coupling is assumed to arise primarily from spin polarization by the spin density on adjacent carbon atoms. A value for this spin-polarization constant is obtained and we find that either QC′CHH=4.33 or −3.09 G. The recent data of Morton are in good agreement with the positive value, but the theoretical calculation by Colpa and de Boer indicate that it should be probably negative in sign.

Nuclear Hyperfine Interactions in Orbitally Degenerate States of Aromatic Ions

The dynamical Jahn-Teller effect in the benzene negative ion is investigated by estimating the vibronic eigenfunctions and energies for the lowest energy degenerate and lowest energy nondegenerate vibronic states using Hückel molecular orbital theory and the normal coordinates for the 606 cm—1 and 1595 cm—1 E2g vibrations of benzene given by Whiffen. It is concluded that the lowest energy vibronic state is doubly degenerate, and that this degeneracy is removed by interactions with a polar solvent when this ion is in solution. The solvent interaction produces a time-dependent oscillation or switching of the electronic spin distribution relative to the nuclear framework, which in turn leads to an enhanced broadening of the hyperfine structure through a corresponding fluctuation in the isotropic hyperfine interaction. The calculated linewidths are the same as those reported by Townsend and Weissman if the switching spin distribution is described by a correlation time of the order of 10—9 sec. A similar effect is probably responsible for the anomalously broad proton hyperfine structure reported by Weissman and Townsend in the negative ions of coronene and triphenylene. This effect must also contribute to the linewidth of the hyperfine structure of the negative ion of cyclooctatetraene observed by Katz and Strauss. It is suggested that the enhanced spin-lattice relaxation rate reported by Weissman and Townsend for the negative ion of coronene is due to enhanced spin-orbit interaction, also associated with a degeneracy of the ground vibronic state.

Electron Spin Resonance in a Gamma-Irradiated Single Crystal of L-Cystine Dihydrochloride

The electron spin resonance of a gamma-irradiated single crystal of L-cystine dihydrochloride, HOOC–CH–CH2–S–S–CH2–CH–COOH·2HCl, – – NH2 NH2has been measured at 9 kMc/sec and at 24 kMc/sec for various orientations of the crystal in the magnetic field. The resonance pattern was found to be a doublet, the spacing between the components of which is independent of the crystal orientation as well as of the strength of the static magnetic field. The spectroscopic splitting factor was found to be anisotropic with the principal values: gu=2.003, gv=2.025, and gw=2.053. A model of the free radical, HOOC–CH–CH2–S·, – NH2in which the electron spin density is mainly concentrated in a nonbonding 3 p orbital of the sulfur atom, is proposed. The model accounts very well for the principal values of the g tensor as well as their directions relative to the atomic configuration. It also can give rise to the observed proton hyperfine structure.

Proton Hyperfine Interactions in Semiquinone Ions

The proton hyperfine structure in a series of methylsubstituted semiquinone ions has been shown to arise in a general sense from configuration interaction. A crude calculation of the proton hfs to be expected in a hypothetical solution of C2H4+ ions indicates that the σ—π interaction is responsible for the hfs of protons attached directly to an aromatic ring. The molecular orbital theory of hyperconjugation accounts for the detailed hfs associated with the methyl protons.