Electron-spin resonance and theoretical studies of radicals formed from trimethylamine borane : the radicals Me2[Ndot]-BH3 and (Me3N-BH3)-

Exposure of trimethylamine borane to ⁶⁰Co γ-rays at 77 K gave, initially, a four-line E.S.R. spectrum assigned to the radical anion, Me₃[Ndot]-BH₃ ^-, having a large (58 G) hyperfine coupling to boron. Theoretical calculations suggest that the radical resembles a perturbed ^.BH₃ ^- in which the normally planar unit still has considerable pyramidal character.

After annealing above 77 K this species was lost and a multiline, isotropic spectrum grew in which, by comparison with spectra obtained from Me₃N-BD₃, is shown to belong to Me₂[Ndot]-BH₃ radicals. This species is of interest because of the large ¹H coupling to the -BH₃ protons (ca. 45 G). This coupling, qualitatively ascribed to hyperconjugation, is correctly reproduced by theoretical (ab initio UHF) calculations.     

Electron resonance spectra of some radicals derived from acetylenes and nitriles

The electron resonance spectra of a series of radicals derived by hydrogen abstraction from substrates containing carbon-carbon or carbon-nitrogen triple bonds have been studied in fluid solutions. The radicals ·CH₂CN, ·CH₂C ≡ CH and ·CH₂C ≡ C·CH₃ which have a linear heavy-atom framework show interesting temperature dependent line-width variations. These are attributed to spin-rotational interactions arising from anisotropic molecular reorientation. It is suggested that the motion about the long axis of these radicals is inertial in character.

The equilibrium geometries and hyperfine coupling constants of the propargyl and cyanomethyl radicals have been estimated from INDO-LCAO-SCF calculations.     

Electron spin resonance spectra of radicals derived from secondary aromatic amines

The electron spin resonance spectra of diphenyl nitric oxide and the diphenylamino radical are described. Hyperfine structure in both spectra is accounted for in terms of the various interacting magnetic nuclei; the improved resolution obtained allows the coupling parameters to be given with greater precision than previous values. A mechanism is suggested for the formation of these radicals in the systems being investigated and a qualitative explanation is put forward to account for the observed variation in line width across the spectra.     

ENDOR evidence for the quantum exchange of the methylene protons

ENDOR spectroscopy was used for studying methylene proton couplings in the 6-yl (or H-addition) radical in a single crystal of 1-methyluracil at 4.2K. The two methylene protons were magnetically equivalent in all crystal (radical) orientations in the magnetic field, indicating proton quantum exchange. The two protons compose magnetically single entity with spin I=1. The observed small ENDOR line splittings of about 0.60MHz agree well with the theoretical expectations for such a system.     

Electron spin resonance study of neutral radicals derived from cyclic and acyclic carbonate esters

An electron-spin resonance study of radicals formed from cyclic and acyclic carbonate esters by loss of a hydrogen atom is reported. The radicals were generated photolytically near room temperature in solutions containing 1 % H₂O₂. Spectra were analyzed and couplings are discussed for two radicals formed from diethyl carbonate, two from propylene carbonate (4-methyl-1,3-dioxolan-2-one), and one each from dimethyl carbonate and ethylene carbonate (1,3-dioxolan-2-one).     

Electron spin resonance of radicals in X-irradiated single crystals of N-substituted amino acids

Angular distribution measurements of KX reactive scattering of a potassium dimer K₂ beam by allyl and alkyl halides are reported. The centre of mass differential cross sections are uncertainly determined but distinctly different from the rebound cross sections exhibited by K+CH₃I, C₂H₅I atom reactions. This change of dynamics may be rationalized in terms of the transfer of both K₂ valence electrons to the halide molecule. The reaction products seem likely to be a potassium halide and a potassium alkyl or allyl molecule.     

Electron paramagnetic resonance and electron nuclear double resonance of the AsO4 4- centre in X-irradiated KH2AsO4

Electron paramagnetic resonance studies of the AsO₄ ^4- centre in X-irradiated crystals of KH₂AsO₄ in the ferroelectric phase at 77 and 4·2 K are reported. The symmetry of the spin-hamiltonian has been found to be orthorhombic, and the g tensor and the A tensor describing the interaction of the unpaired electron with the arsenic nucleus (I = 3/2) have been obtained. Domain splitting has been observed in he spectra recorded in the ab plane of the crystal. By studying these spectra in the presence of an applied electric field, it has been possible to plot the hysteresis curve of ferroelectric KH₂AsO₄. Electron-nuclear double resonance (ENDOR) of protons surrounding the AsO₄ ^4- units has been studied at 4·2 K. Two sets of ENDOR lines have been found arising from the protons in the two equilibrium positions (labelled ‘ close ’ and ‘ far ’) along the hydrogen bonds linking the AsO₄ tetrahedra. The angular variation of the ENDOR lines from both ‘ close ’ and ‘ far ’ protons has been plotted in the crystal symmetry planes. The observed ENDOR frequencies have been fitted to those calculated from the numerical diagonalization of the Hamiltonian. The superhyperfine parameters for the ‘ close ’ and ‘ far ’ protons thus obtained are found to be quite anisotropic. The ENDOR results are shown to explain all details of the partially-resolved proton superhyperfine structure at room temperature as well as at low temperatures. An isotropic contact hyperfine coupling of -2·875 MHz of the unpaired electron to the proton in the ‘ far ’ position of the hydrogen bond has been determined, providing direct evidence for covalency in the hydrogen bonding in KH₂AsO₄ crystals.     

Electron spin resonance of Nd3+ and Gd3+ in AB2 cubic intermetallic compounds

The ESR of Nd³⁺ and Gd³⁺ in the eight intermetallic compounds AB₂ (A = Y, La, Ce, Lu; B = Ir, Rh) is reported. The hyperfine constants and the exchange parameters are extracted.     

Conformational analysis of the chemical shifts for molecules containing diastereotopic methylene protons

Quantum chemistry SCF/GIAO calculations were carried out on a set of compounds containing diastereotopic protons. Five molecules, including recently synthesized 1,3-di(2,3-epoxypropoxy)benzene, containing the chiral or pro-chiral center and the neighboring methylene group, were chosen. The rotational averages (i.e. normalized averages with respect to the rotation about the torsional angle τ with the exponential energy weight at temperature T) calculated individually for each of the methylene protons in 1,3-di(2,3-epoxypropoxy)benzene differ by ca. 0.6 ppm, which is significantly less than the value calculated for the lowest energy conformer. This value turned out to be low enough to guarantee the proper ordering of theoretical chemical shifts, supporting the interpretation of the (1)H NMR spectrum of this important compound. The rotational averages of chemical shifts for methylene protons for a given type of conformer are shown to be essentially equal to the Boltzmann averages (here, the population-weighted averages for the individual conformers representing minima on the E(τ) cross-section). The calculated Boltzmann averages in the representative conformational space may exhibit completely different ordering as compared to the chemical shifts calculated for the lowest-energy conformer. This is especially true in the case of molecules, for which no significant steric effects are present. In this case, only Boltzmann averages account for the experimental pattern of proton signals. In addition, better overall agreement with experiment (lower value of the root-mean-square deviation between calculated and measured chemical shifts) is typically obtained when Boltzmann averages are used.     

Electron spin resonance and magnetocrystalline anisotropy in YCo3

We present the electron spin resonance measurements in powdered samples of the intermetallic compound YCo₃ in Q, K and X microwave bands. The grains of the powdered samples were magnetically aligned in a static magnetic field. The resonant spectra consist of the lines connected with localized and unlocalized magnetic moments. We were able to describe only the part of the spectrum associated with localized moments and to estimate the anisotropy field at the room temperature. Next we present the magnetization data by the pulse magnetometer. The anisotropy field which was determined from these data is much larger than that from ferromagnetic resonance. We suggested a possible reason for such discrepancy.     

Electron spin resonance and structure of the CO- 2 radical ion

Analysis of the electron spin resonance spectrum shows that CO^? ₂ radical ions are trapped in sodium formate at room temperature after irradiation of the crystal with high energy γ-rays. This radical is bent with an angle of about 134° and has a ² A ₁ ground state. From the experimental values of the ¹³C coupling coefficients and the g values, coefficients are derived for the mixing of atomic orbitals in the half-filled 4a ₁ molecular orbital which appears to be 14 per cent carbon 2s, 66 per cent carbon 2p _z and 11 per cent of each oxygen 2p _z . There is also coupling to the sodium nucleus in an adjacent cation.     

Electron spin resonance analysis of magnetic structures in La2/3Ca1/3MnO3

Measurements of electron spin resonance (ESR) of La_2/3Ca_1/3MnO₃ (LCMO) in the ferromagnetic and paramagnetic phases were carried out. Phase transition and temperature dependence of the peak-to-peak ESR linewidth were determined. The transition temperature between ferromagnetic and paramagnetic phases was observed at 265 K. A prominent increase of the peak-to-peak linewidth with decreasing temperature below T_c was observed. Using the dynamic scale theory and block spin transformation in critical phenomenon, the quantitative calculation of peak-to-peak linewidth at near T_c was made, which was in good agreement with the experimental data. It was believed that the long interactions between the ferromagnetic microregions for LCMO played a key role in determining the ESR linewidth.     

A limit on the site inequivalence in g-As2Se3

X-ray photoelectron spectra of As and Se 3d in g-As₂Se₃ exhibit two distinct As sites, but only an excess Gaussian width of 0.4 eV for the Se sites. The second As site is attributed to the presence of As₄Se₄ clusters in agreement with earlier Raman spectroscopy, the broadening of the Se lines to the site inequivalence of the orpimental rafts.     

Electron spin resonance on enantiomorphic centers in PbZrO3:Gd3+ crystals

The Gd³⁺ ESR spectrum of the antiferroelectric phase of a PbZrO₃ single crystal contains at least one pair of magnetically equivalent components. The angular dependences of the resonance magnetic field that were constructed for them transform into one another by mirror reflection in a plane perpendicular to the antipolarization axis of the crystal. The constants in the spin Hamiltonians for the corresponding pair of paramagnetic centers are determined assuming monoclinic symmetry. Fiz. Tverd. Tela (St. Petersburg) 41, 1279–1281 (July 1999)     

Measurement of one-bond heteronuclear dipolar coupling contributions for amine and diastereotopic methylene protons

One-bond heteronuclear and two-bond homonuclear residual dipolar couplings measured at methylene or amine sites can be utilized as long-range constraints in structure determination of molecules as well as to facilitate characterization of local conformation by stereospecific assignment of diastereotopic protons. We present two J-modulated HMQC type experiments to measure the one-bond heteronuclear dipolar coupling contributions of geminal protons individually. In addition two-bond homonuclear residual dipolar couplings between the diastereotopic protons are also obtained.     

Electron spin resonance in the Heusler alloy YbRh2Pb

An electron spin resonance (ESR) signal was observed in a concentrated Kondo lattice, Heusler alloy YbRh₂Pb. It is attributed to the combined effect of the 4f local magnetic moments of Yb³⁺ and conduction electrons. It is shown that the significant broadening and disappearance of the ESR line at temperatures above 20 K is caused by the processes of the spin-lattice relaxation of the Yb³⁺ ions through the first excited Stark doublet with an activation energy Δ ≈ 73.5 K. A comparison of the ESR data for YbRh₂Pb and some other undoped intermetallic compounds based on ytterbium, cerium, and europium indicates that hybridized electronic states occurring as the result of hybridization between the localized 4f electrons and the collectivized conduction electrons constitute a fundamentally new source of ESR.     

Electron spin resonance in YbRh2Si2: local-moment, unlike-spin and quasiparticle descriptions

Electron spin resonance (ESR) in the Kondo lattice compound YbRh(2)Si(2) has stimulated discussion as to whether the low-field resonance outside the Fermi liquid regime in this material is more appropriately characterized as a local-moment phenomenon or one that requires a Landau quasiparticle interpretation. In earlier work, we outlined a collective mode approach to the ESR that involves only the local 4f moments. In this paper, we extend the collective mode approach to a situation where there are two subsystems of unlike spins: the pseudospins of the ground multiplet of the Yb ions and the spins of the itinerant conduction electrons. We assume a weakly anisotropic exchange interaction between the two subsystems. With suitable approximations our expression for the g-factor also reproduces that found in recent unlike-spin quasiparticle calculations. It is pointed out that the success of the local-moment approach in describing the resonance is due to the fact that the susceptibility of the Yb subsystem dominates that of the conduction electrons with the consequence that the relative shift in the resonance frequency predicted by the unlike-spin models (and absent in the local-moment models) is ? 1. The connection with theoretical studies of a two-component model with like spins is also discussed.