Proton Hyperfine Interactions Research Articles

Isotropic Proton Hyperfine Interaction and Properties of the CH Bond

The proton hyperfine splitting (hfs) of aromatic hydrocarbon radicals or ions was investigated in connection with the properties of the CH bond. The calculation was performed for the CH fragment with the use of the first-order perturbation. The result is in general agreement with observation only if MO's of minimum energy are used, but it is not valid for the homopolar CH bond. The dependence of the hfs on the bond angle is very small. In most molecular species, the value of the semiempirical constant Q defined by McConnell1 is between —20 and —30 G, unless there is large spin density on the carbon atom adjacent to the proton in question.

Paramagnetic Resonance Measurements of the Phosphorescent State of Quinoxaline

Paramagnetic resonance absorption has been observed in a single-crystal solid solution of quinoxaline in durene when irradiated at 77°K with uv light from an AH-6 mercury-arc source. The fine structure of the resonance spectrum observed at 9.8 Gc/sec may be described by the spin Hamiltonian H=BH·g·S+DSz2+E(Sx2−Sy2),in which S = 1. D/hc=±0.1007±0.0003 cm−1E/hc=∓0.0182±0.0002 cm−1 gxx=2.0047±0.0005 gyy=2.0035±0.0005gzz=2.0019±0.0005.The fine structure is practically identical to that of the naphthalene triplet state observed by Hutchison and Mangum. The observed hyperfine structure is assigned to a coupling with the 5, 8 protons, and the two N14 nuclei. The angular dependence is in qualitative agreement with the tensor for C–H proton hyperfine interaction for doublet-state aromatic radicals measured by Cole, Heller, and McConnell, as well as the tensor for aromatic N14 interaction derived from experiments by Weissman. With the assumption that hyperfine interaction in a triplet state may be described by the same tensors, normalized π-electron spin densities are approximately 0.28 for the 5, 8 carbon atoms, and 0.1 for the N14 atoms. Satellite lines which are split from the primary hyperfine lines by the proton resonance frequency are observed and interpreted as simultaneous electron-spin nuclear-spin transitions. The interactions leading to these transitions are assumed to be primarily due to hyperfine coupling with protons whose unresolved first-order interaction is much smaller than the proton-resonance frequency.

Photoinduced Paramagnetism in Solutions of Nitrobenzene in Tetrahydrofuran

ESR studies are reported of photoinduced paramagnetism in solutions of nitrobenzene in undeuterated and deuterated tetrahydrofuran (THF). The ESR spectrum exhibits a widely spaced set of identical triplets, and the extra proton hyperfine interaction (0.38 plus or minus 0.04 Oe) is due to a single alpha proton of the THF molecule. The experiments of Lagercrantz and Yhland with trinitrobenzene were also repeated, and the following mechanism for the production of the paramagnetic species is proposed: Light irradiation causes the nitro group to abstract an alpha hydrogen atom from the THF molecule, and the resulting THF free radicais rapidly dimerize or disproportionate, leaving a neutral nitrobenzene free radical in solution. The radical has a totai decay time of approximates 2 sec and is suspected to reside on an oxygen atom. (D.L.C.)

ESR Spectrum and Structure of the Formyl Radical

The electron spin resonance (ESR) spectra of the formyl radical (HCO) and the deuterated radical (DCO) have been observed in solid carbon monoxide over the temperature range 4.2° to 30°K. The observed line shapes were temperature dependent, but the cause of the temperature dependence was not definitely determined. The radical was produced by the reaction of CO with hydrogen atoms produced by the photolytic decomposition of HI. The radical was also produced by the photolytic decomposition of formaldehyde in solid argon, and it was found to be one of the products of the photolysis of methyl alcohol in solid argon. The most striking feature of the ESR spectrum of HCO is the very large proton hyperfine splitting (137 oe). Also the lines were broad and unsymmetrical with the linewidth varying from one hyperfine component to the next. It was shown that the line shapes were due to the combination of a rather pronounced anisotropy in the electronic g factor with a smaller anisotropy in the proton hyperfine interaction. These characteristics of the radical gave a good deal of information about its structure which supported the conclusion of other workers that the formyl radical is not a π-electron radical.

Calculation of the Electron Spin Resonance Line Shape for a Polycrystalline Radical with Anisotropic g Tensor and Proton Hyperfine Interactions

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Relation between the Proton Hyperfine Interaction and the C13–H1 Spin-Spin Coupling in Free Radicals

It is shown that the C13–H1 spin-spin coupling constant J(C13–H1) and the proton hyperfine constant Q(H1) in a free radical can be considered as interdependent, and that they both can be estimated by calculating the perturbation produced in the electronic wave functions by the magnetic moment of the proton. For a C–H fragment with an unpaired π electron on the carbon atom an approximate relation is established between Q(H1) and J(C13–H1). Using the experimental value of J(C13–H1) for the benzene molecule, Q(H1) is found to have a negative sign and order of magnitude agreement with experiment.

Electron Spin Resonance Studies of the Potassium Salt of m-Dinitrobenzene Negative Ion

A detailed analysis of the electron spin resonance spectrum for the potassium salt of m-dinitrobenzene negative ion in DME is presented. Isotopic substitution has been performed in various positions in the molecule. m-Dinitrobenzene negative ion exhibits only one nitrogen hyperfine interaction of 9.0 gauss, and possesses proton hyperfine interactions of 4.6 gauss for the number 2, 4, and 6 protons and 1.2 gauss for the number 5 proton. The hyperfine spectrum is different for each of the alkali metals, Li, Na, K, Rb, and Cs, but still retains only one nitrogen hyperfine interaction. An enriched Li6 metal salt of m-dinitrobenzene indicates no alkali metal hyperfine interaction.

Isotropic Proton Hyperfine Interaction in the Methyl Radical

The isotropic hyperfine interactions of radicals or ions have been discussed from the viewpoint of the self-consistent field (SCF) molecular orbital theory. In these phenomena, the most important electron configurations concerned are those arising due to the one electron excitations from the double occupied orbital to the vacant orbital together with the lowest energy configuration. The proton hyperfine splitting of the methyl radical was calculated nonempirically by using the SCF-LCAO-MOs in adopting the first order perturbation. The agreement with the observed value is quite good when one uses MOs that include the 3s AO of carbon atom, while it is not so satisfactory in the case of the MOs without the 3s AO, because of the improvement of the form of the totally symmetric antibonding vacant orbital which gives the main contribution to the phenomena.

σ-π Interaction and Proton Hyperfine Interaction

After the equation (42) on page 786 the following should be inserted.

σ-π Interaction and Proton Hyperfine Interaction

σ-π Interaction and Proton Hyperfine Interaction

Theory of anisotropic hyperfine interactions inπ-electron radicals

Anisotropic proton hyperfine interactions in π-electron radicals are approximated by a magnetic dipole interaction between each proton and an electron spin magnetization that is distributed in 2s and 2p Slater atomic orbitals centred on carbon atoms.

Theory of Isotropic Hyperfine Interactions in π-Electron Radicals

Indirect proton hyperfine interactions in π-electron radicals are first discussed in terms of a hypothetical CH fragment which holds one unpaired π electron and two σ-CH bonding electrons. Molecular orbital theory and valence bond theory yield almost identical results for the unpaired electron density at the proton due to exchange coupling between the π electron and the σ electrons. The unrestricted Hartree-Fock approximation leads to qualitatively similar results. The unpaired electron spin density at the proton tends to be antiparallel to the average spin of the π electron, and this leads to a negative proton hyperfine coupling constant. The theory of indirect proton hyperfine interaction in the CH fragment is generalized to the case of polyatomic π-electron radical systems; e.g., large planar aromatic radicals. In making this generalization there is introduced an unpaired π-electron spin density operator, ρN, where N refers to carbon atom N. Expectation values of the spin density operator ρN are called ``spin densities,'' ρN, and can be positive or negative. In the simple one-electron molecular orbital approximation a π-electron radical always has a positive or zero spin density at carbon atom N, 0≤ρN≤1. In certain π-electron radical systems; e.g., odd-alternate hydrocarbon radicals, the spin densities at certain (unstarred) carbon atoms are negative when the effects of π—π configuration interaction are included in the π-electron wave function. The previously proposed linear relation between the hyperfine splitting due to proton N, aN, and the unpaired spin density on carbon atom N, ρN,aN=QρNis derived under very general conditions. Two basic approximations are necessary in the derivation of this linear relation. First, it is necessary that σ—π exchange interaction can be treated as a first-order perturbation in π-electron systems. Second, it is necessary that the energy of the triplet antibonding state of the C–H bond be much larger than the excitation energies of certain doublet and quartet states of the π electrons. This derivation of the above linear relation makes no restrictive assumptions regarding the degree of π—π or σ—σ configuration interaction. The validity of the above approximations is discussed and illustrated by highly simplified calculations of the proton hyperfine splittings in the allyl radical, assuming the π—π configuration interaction—and hence the negative spin density on the central carbon atom—to be small. Isotropic hyperfine interactions in molecules in liquid solution can also arise from spin-orbital interaction effects, and it is shown that these effects are negligible for proton hyperfine interactions in aromatic radicals.

VECTOR MODEL FOR INDIRECT PROTON HYPERFINE INTERACTIONS IN π-ELECTRON RADICALS

Significant information on unpaired electron distributions in molecules, liquids, and solids can sometimes be obtained from magnetic resonance studies of electron-nucleus hyperfine interactions. A particularly important problem in this field involves the proton hyperfine interactions that have been observed in the electron magnetic resonance spectra of a large number of aromatic radicals in solution [1-3]. The problem concerns the theoretical basis for a proposal that the observed hyperfine splitting aN due to aromatic proton N in an aromatic radical can be used to estimate the unpaired spin density ρN at carbon atom N to which proton N is attached, according to the equation [2,4-6] aN = QρN. (1) A semiempirical value of Q derived from observed splittings aN and theoretically calculated spin densities ρN is -30 ± 5 gauss, or -85 ± 15 Mc [2]. Equation (1) has been derived recently using molecular orbital theory with arbitrary π-π configuration interaction [6]. We show here how the simple proportionality in equation (1) may be derived using the Dirac vector model.

Proton Hyperfine Interactions in Paramagnetic Resonance of Semiquinones1

Proton Hyperfine Interactions in Paramagnetic Resonance of Semiquinones¹