The inversional dependence of hyperfine quadrupole coupling in 14NH3

A simple theoretical model explaining the inversional dependence of hyperfine quadrupole coupling in ¹⁴NH₃ is presented. The model, when related to available experimental hyperfine splittings, can be used to predict the effective hyperfine quadrupole coupling constants for all (low-lying) ro-inversional levels. The actual predictions reveal that the inversional dependence of the coupling constants is not only rather strong but also non-monotonic.     

EPR studies of X-ray irradiated dithiophosphate metal complexes

Different metal O,O’-disubstituted dithiophosphates have been studied by Electron Paramagnetic Resonance (EPR) spectroscopy after X-ray irradiation at 77K. Formation of different radicals has been recorded depending on the central metal atom and on the substituents in the ligands. Phosphorus-centered radicals with hyperfine constants of about 800 G were recorded after radiolysis of zinc and cadmium complexes. Their shape and EPR parameters lead to their identification as phosphoranyl radicals (RO)₂P (S)S^?. In these compounds two different types of coordination of the ligands have been found by X-ray analysis. One dithiophosphate group is coordinated to the metal atom forming a chelate four-membered ring, and another one is a bridging group. The formation of phosphorus-centered radicals is related to the presence of a bridging, or interchelating group. In other metal dithiophosphates, with only chelating groups present, such radicals were not found. In platinum, palladium and nickel dithiophosphates similar EPR signals with small hyperfine coupling were observed. They have been identified as carbon-centered radicals with one phosphorus in γ-position.     

The hyperfine structure and hyperfine anomaly of 161Dy and 163Dy

The hyperfine structure of ¹⁶¹Dy and ¹⁶³Dy for five transitions hasbeen measured with a tunable dye laser and a thermal velocity atomic beam. Values for the magnetic dipole and electric quadrupole coupling constants have been determined for the relevant excited states. The hyperfine anomaly parameters, Δ, which are extracted are interpreted in terms of available atomic wave functions.     

Hyperfine anisotropy and crystal field effects in Mu adducts to naphthalene

Muonated cyclohexadienyl type radicals have been studied in a naphthalene single crystal using the standard transverse-field μSR technique. Muonium addition occurs at both the α and the β positions. Because of the multiplicity and weakness of the lines in the Fourier spectrum, special techniques were developed to allow the signals to be found and identified. The isotropic hyperfine coupling constants show large shifts from the solution values, with significant anisotropy in the hyperfine tensors. The results for the α radicals are similar to those observed for protonated α-hydronaphthyl radicals, but isotope effects are evident. Based on the hyperfine tensor directions and geometrical considerations, each radical has been assigned to a specific muon site within the crystal.     

The calculation of fluorine hyperfine interactions in π-electron radicals

Q factors have been calculated for fluorine hyperfine interaction in π-electron radicals using an independent-electron model for the σ orbitals. It is shown that the off-diagonal elements give an important contribution to fluorine coupling constants. Combining these Q factors with calculated π-electron spin densities of fluoro-nitrobenzenes gives good agreement with experimental coupling constants.     

EPR spectra of γ-irradiated hydrated barium dithionate single crystals

EPR spectra of barium dithionate hydrate single crystals γ-irradiated at low (80 Gy) and high (10 kGy) doses are studied. Four lines, the strongest of which is due to the SO₃⁻ radical, are observed in the EPR spectrum of the low-irradiated samples. Another line seems to belong to SO₂⁻. The strong line and weak lines with hyperfine structure and lines for pairs of closely spaced SO₃⁻ centers with a strong angular dependence are observed at high irradiation doses. The main values of the SO₃⁻ hyperfine coupling tensor and the dipole-dipole coupling constants of the SO₃⁻ pairs are determined. A quantum-chemical calculation of the electronic structure of isolated SO₃⁻ and SO₂⁻ radicals is performed. Values of the g tensors and hyperfine couplings are calculated. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 1, pp. 18–22, January–February, 2008.     

Vibrationally averaged spin densities on muons and protons in the hydroxyl and ethyl radicals

The observations of the residual isotope effect for muonated radicals are discussed using calculations on the hydroxyl and ethyl radicals. The origins are partly due to the non-linear increase of the hyperfine coupling constants with increase in bond length, although a significant contribution comes from the anharmonicity of the molecular vibrations.     

Split-shell molecular orbital calculations on the heteronuclear alkali metal diatomics

One of the radicals formed in irradiated 5-iododeoxyuridine is shown unambiguously to be the α-iodo radical RCH₂-?(I)R′ formed by hydrogen atom addition to C₆. The ¹²⁷I hyperfine tensor components, A_x = + 90 G, A_y = (-) 50 G, A_z = (-)40 G are proposed as being characteristic of the coupling to be expected for α-iodo alkyl radicals. Hence a radical recently detected in irradiated iodoacetamide with a maximum hyperfine coupling of 250 G cannot have this structure. Possible alternative structures are discussed.

The way in which the E.S.R. spectra for the α-iodo radica in 5-iododeoxyuridine are modified by the quadrupole interaction from ¹²⁷I is described and hence an estimate of the quadrupole coupling is obtained.     

Semiempirical molecular orbital calculations of anisotropic 1H, 13C and 19F hyperfine coupling constants in hydrocarbon and fluorocarbon radicals

The anisotropic hyperfine coupling constants (AHCC) from the electron spin resonance (E.S.R.) spectra of a variety of atoms in organic radicals have been calculated by means of semiempirical molecular orbital wavefunctions in the INDO approximation. Hyperfine tensors involving ¹H, ¹³C and ¹⁹F nuclei are obtained for the ?H, ?H₃, CH₃?H₂, (CH₃)₃? hydrocarbon radicals, malonic acid radical, ?H₂F, ?F₂H, ?F₃ and CF₃?H₂ radicals. The calculated values are compared with available experimental, non-empirical and semiempirical values for these radicals. All integrals of the operator entering the electronic contributions have been evaluated over Slater type orbitals. The introduction of deorthogonalized wavefunctions gives generally better calculated results. In particular, the tensor components of the ¹⁹F AHCC are in good agreement with the experimental results without the necessity of readjusting the effective nuclear charges.     

Ortho-m meta-, para-directional effects in muonium addition to benzoic acid in water

Isomeric free radicals formed by Mu adding to o/m/p positions of benzoic acid in water were detected by LCR and the corresponding resonance positions and proton hyperfine coupling constants were obtained. There was an isotope effect of the ‘second kind’ for these Mu-radicals in the range of 1.26–1.32. The ‘fractional’ formation rate constants per site are 2.7×10⁹, 0.45×10⁹ and 0.85×10⁹ M^?1s^?1 respectively. Ortho-addition dominates by a large factor, which is in accord with the electron-withdrawing character of the ?COOH group.     

Fine structure and hyperfine structure of some excited states of helium

The fine structure and the hyperfine structure for some singly-excited and doubly-excited states of helium atom are calculated using Rayleigh-Ritz variational method with multi-configuration-interaction wave functions. The calculated results of the fine structure for the Rydberg series are in good agreement with other theoretical and experimental data. The hyperfine parameters and the hyperfine coupling constants of ³He are also obtained for this system.Received: 17 January 2003, Published online: 29 July 2003PACS: 32.10.Fn Fine and hyperfine structure - 31.30.Gs Hyperfine interactions and isotope effects, Jahn-Teller effect     

An ab initio study of the hyperfine structure in the X2 Π electronic state of HCCS–calculation of vibronically averaged components of the anizotropic hyperfine tensor

The results of ab initio calculations of the vibronically averaged components of the anisotropic magnetic hyperfine tensor in the low-lying vibronic species of the X²Π electronic state of the HCCS radical are reported. The electronically averaged hyperfine coupling constants for hydrogen, deuterium, ¹³C and ³³S are obtained as functions of two bending vibrational modes by the density functional theory method. The vibronic wave functions are calculated with the help of a variational approach which takes into account the Renner–Teller effect and spin-orbit coupling. The results of ab initio calculations are compared to the corresponding experimental findings.     

ESR and ENDOR investigations of the degenerate electron exchange reactions of various viologens in solution. Solvent dynamical effects

The temperature dependences of the rates of the degenerate electron transfer of various viologens (1,1′-di(hydrocarbyl)-4,4′-bipyridinium salts) are measured in seven different solvents by means of electron spin resonance (ESR) line broadening. Rates vary between 1.7·10⁸ and 1.1·10⁹ M⁻¹s⁻¹ at room temperature and clearly show a solvent dynamical effect, which is inferred from the dependence of the rate constants on the longitudinal relaxation time of the solvent. Activation energies ranging from 5.3 to 24.4 kJ mol⁻¹ are found. For the first time, hyperfine coupling constants are reported for the radical cations of the hydroxyethyl viologen and the amino viologen based on both continuous-wave ESR and electron-nuclear double resonance spectroscopy. Furthermore, the temperature and the solvent dependence of the hyperfine coupling constants of the methyl viologen radical cation are reported.     

The geometric approximation as an example of Aitken's δ2 transformation

The unrestricted Hartree-Fock method and its spin-projected and spin-extended versions are used for calculating the model correlation energy and isotropic and anisotropic hyperfine splitting constants for LiH⁺, BeH, BH⁺ and OH radicals. Results are compared with those of the method of complete configuration interaction. The gradient method of Murtagh and Sargent is employed for direct minimization of EHF electronic energy.     

Radiation effect studies on some glycine derivatives in solid state

Five glycyl derivatives, glycyl-L-histidine, L-alanyl-glycine, glycine hydrochloride, gly-gly hydrochloride and, gly-gly-glycine in powder form were exposed to 20?kGy doses of ⁶⁰Co gamma radiation to study the effects of ionizing radiation. In these compounds, the paramagnetic centers formed after irradiation were attributed to the R─NHCH─?O^?OH, ?H₂CHNH, H₂NCH₂?O^?OHHCl, NH₂?HCONHCH₂COOHHCl and HNCH₂?O^?OH radicals, respectively. The effect of gamma irradiation to the radical structures and time stability of the radicals were investigated by EPR spectroscopy. The spectra were computer simulated and the hyperfine coupling constants were determined.     

Nitroxide radicals

The proton hyperfine constants have been measured, using N.M.R., for a series of sterically hindered alkyl aryl nitroxides. Computer reconstructions of the electron resonance spectra show that the coupling constants obtained from the pure radical are very similar to those obtained from the electron resonance spectra of dilute solutions. Comparison of methyl and proton splittings in identical positions suggests that the radicals are σ in character although the molecular-orbital calculations are unable to distinguish between σ or π radicals.     

Photooxidation of Histidine by 3,3′,4,4′-Benzophenone Tetracarboxylic Acid in Aqueous Solution: Time-Resolved and Field-Dependent CIDNP Study

The photooxidation reaction between 3,3′,4,4′-benzophenone tetracarboxylic acid (TCBP) and l-histidine (His) has been investigated in neutral aqueous solution using the technique of chemically induced dynamic nuclear polarization (CIDNP). Relative values of ¹³C isotropic hyperfine couplings in the TCBP and His radicals were obtained from the ¹³C-time-resolved CIDNP spectrum, recorded during the photoreaction of TCBP with His at natural abundance of the magnetic isotope ¹³C. Good agreement was found for the hyperfine coupling constants of the TCBP ketyl radical calculated using methods of density functional theory, and those obtained from the ¹³C-time-resolved CIDNP spectrum. The mechanism of the quenching reaction of triplet-excited TCBP by His in neutral aqueous solution was established. ¹H CIDNP field dependencies for the photoreaction of TCBP with His were obtained and the g-factor of the histidyl radical was found.     

Nuclear quadrupole coupling in the microwave spectrum of 1,2,3-triazole

The hyperfine structure in the microwave spectra of 1,2,3-triazole and N-deutero 1,2,3-triazole has been analyzed. The coupling constants derived from the analysis of each isotopic species have been combined to give the principal nuclear quadrupole coupling constants at the sites of the three inequivalent ¹⁴N nuclei.     

Determination of signs of hyperfine coupling constants for an S = ½ system through E.P.R., ENDOR and double ENDOR

A systematic method of obtaining relative signs of hyperfine coupling constants is described. It applies to systems consisting of (a) a set of one or more nuclei coupled fairly strongly to the electron spin, and possessing a two-fold (or higher) axis of symmetry, together with (b) a set of weakly coupled nuclei defining superhyperfine transitions. ENDOR measurements for several E.P.R. hyperfine transitions, with the field oriented along the symmetry axis, give relative signs of hyperfine components for this direction. Signs for the other directions can then be obtained through ENDOR measurements on a single hyperfine transition at various field orientations. Additional double ENDOR measurements may be necessary for very weakly coupled nuclei. This method can complement double ENDOR studies in favourable cases. It is illustrated by the determination of signs of coupling constants of protons and of ⁷⁵As in the AsO₄ ^4- radical in KH₂AsO₄.     

Measurement of the electric hyperfine coupling constant <Emphasis Type="Italic">P/k</Emphasis> in a uranyl rubidium nitrate crystal

New investigations with nuclei oriented in a single-crystal sample of uranyl rubidium nitrate required refining values of the constants that determine the degree of orientation. The results obtained from new measurements of the angular anisotropy of the emission of alpha particles from aligned ²³³U nuclei are presented in this article. A method for experimental-data treatment is described. New values of the hyperfine coupling constants for ²³³U and ²³⁵U are obtained with much smaller uncertainties.