Electron spin resonance study of oxygen-17 enriched N,N-dimethyl-4-nitroaniline radical anion

N,N-Dimethyl-d₆-4-nitroaniline enriched to approximately 11 atom. per cent in oxygen-17 has been prepared and the oxygen-17 isotropic hyperfine interaction in the corresponding radical anion has been examined. In dry dimethylformamide solution we find a _O = -8·85 ± 0·02 and a _N = 11·39 ± 0·10 gauss, while in moist acetonitrile solution a _O = -8·82 ± 0·07 and a _N = 12·23 ± 0·11 gauss. These results substantiate the recent suggestion that the isotropic oxygen-17 splittings in π-radicals may be described by a single parameter equation analogous to McConnell's relation for aromatic proton splittings, i.e. a _O = Q _OρO ^π, where Q _O = -41 ± 3 gauss.     

Electron spin resonance spectra of nitrogen heterocyclic radical ions

Abstract

We introduce a representative benchmark database of 20 cycloreversion reaction energies obtained by means of the high-level W1 thermochemical protocol. We use these benchmark values to assess the performance of a variety of contemporary DFT, double-hybrid DFT (DHDFT), standard ab initio, and compound thermochemistry methods. We show that this set of reaction energies provides an extremely challenging test for nearly all of the considered DFT and DHDFT methods. For example, about 80% of the considered functionals result in root-mean-square deviations (RMSDs) above 10 kJ mol^?1. The best DFT and DHDFT procedures are ωB97X and DSD-PBEP86-D3, with RMSDs of 4.7 and 7.9 kJ mol^?1, respectively. Coupled with the fact that the barrier heights for these reactions also pose a significant challenge for many DFT methods, this work shows that only a handful of functionals can quantitatively describe all aspects of the potential energy surface of this important class of reactions. In addition, this work shows that London dispersion effects are particularly large for this class of reactions. For example, empirical D3 dispersion corrections reduce the RMSDs for the DFT and DHDFT procedures by amounts ranging from 3.5 (PBE and B2K-PLYP) to 22.0 (BLYP) kJ mol^?1.     

Electron spin resonance investigation of o-chloranil alkali metal ion pairs

The electron spin resonance spectra of the ion pairs of ortho-tetrachloro benzosemiquinone with the whole series of the alkali metals have been recorded in different solvents in a range of temperatures. ¹³C hyperfine splittings from nuclei in all the positions in natural isotopic abundance were measured; they are discussed together with the g factor and the alkali metal coupling constants. The sign of the coupling constants is obtained from the analysis of the linewidths of the different hyperfine components. The experimental data are consistent with a structure of the ion pairs with the metal ion chelated by the two oxygens of the semiquinone molecule. With Rb and Cs in tetrahydrofuran a different structure is proposed with the metal ion bonded to only one oxygen.     

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 in superconductors

The internal field distribution in type II superconductors substantially alters the electron spin resonance g value and line shape. The resulting changes can be used to determine both the width and approximate shape of the internal field distribution.     

Electron spin resonance in polyphenylacetylene

By polymerizing phenylacetylene in the absence of initiator and in vacuo while observing electron spin resonance, it was established that the paramagnetism in the thermal polymer (PPA-T) cannot be due to interaction with oxygen but must be a property of the chemically uncontaminated macromolecule. The ESR behavior of a concentrated solution of PPA(T) in monomer and of solid (amorphous) PPA(T) displays a Curie dependence, whereas the ESR absorption of crystalline polymer made with coordination catalysts increases by one to two orders of magnitude over the temperature of about 60 to 140°C. The major portion of the XRD crystallinity disappears more sharply, at about 120°C. The paramagnetism in PPA must therefore be associated with “conformational defects” in the individual macromolecules.     

Electron spin resonance shift in spin ladder compounds

We analyze the effects of different coupling anisotropies in a spin-1/2 ladder on the electron spin resonance (ESR) shift. Combining a perturbative expression in the anisotropies with density matrix renormalization group computation of the short range correlations at finite temperature, we provide the full temperature and magnetic field evolution of the ESR paramagnetic shift. We show that for well chosen parameters the ESR shift can be in principle used to extract quantitatively the anisotropies and, as an example, discuss the material BPCB.     

MAS NMR studies of carbon-13 spin exchange in durene

One- (1-D) and two-dimensional (2-D) carbon-13 NMR exchange measurements in powder samples of isotopically normal durene under magic angle spinning (MAS) are reported. The experiments include rotor synchronized 2-D exchange (RS2DE), 1-D magnetization transfer (MT) and time reverse ODESSA (tr-ODESSA). The latter two experiments were performed as a function of several external parameters, including proton decoupling field during mixing time, sample spinning rate and partly, of temperature. The effects of these parameters on the spin exchange induced by spin diffusion and by chemical, or physical exchange, is discussed. Spin exchange between all types of carbons in the durene molecules occurs on the time scale of seconds. From the dependence of the spin exchange rate on the external parameters it is concluded that the process is dominated by spin diffusion. On the basis of these results an upper limit of 10(-16) cm2 s(-1) can be set for the self-diffusion constant in crystalline durene.     

Anisotropic hyperfine interactions and spin relaxation in neptunyl salts

Mössbauer spectra of hexavalent neptunyls X₂(NpO₂)Cl₄ and X₄(NpO₂)(CO₃)₃ with X=Na, K, Cs, (NH₄) and [C₂H₅)₄N] are discussed and compared to EPR data. In cases where reliable values of hyperfine parameters can be extracted, the Mössbauer spectra give fine details of bonding structure. It is found that the O–Np–O bond is not linear and that exchanging the cation results in small but definite changes.Work supported by the Bundesministerium für Forschung und Technologie and the U.S. Department of Energydeceased     

Electron spin resonance study of radiation-produced radical pairs in single crystals of 1-methyluracil

Five different types of radical pairs produced in single crystals of 1-methyl-uracil by irradiation with X-rays were identified. Partial differentiation between pairs was achieved by utilizing different stabilities versus ultraviolet illumination or annealing, and variation of relative yields with temperature. The composition of pairs was derived from hyperfine couplings, the orientation from the maxima of dipolar couplings. These were found to agree with the directional cosines obtained from the crystal structure. Further evidence was provided by half-field spectra (Δm _S=2). The zero-field splitting constant D was determined for five pairs, the value of E for one pair. Second-order effects observed at 9·5 GHz and at 35 GHz have been fully analysed for two pairs by application of the appropriate theory which permitted to determine the absolute sign of D. Two pairs are homogeneously composed of hydrogen abstraction radicals 1-methyleneuracil. These are located two layers apart for one pair with the link normal to the molecular planes (D/gβ = -107·5 G), and within one plane for the other (-224·0 G). The three remaining pairs consist heterogeneously of an abstraction radical and a hydrogen addition radical 1-methyl-5,6-dihydrouracil-6-yl. Two of these are intraplanar (-180·0 G; -129·0 G), and the other one located in neighbouring layers (-183·0 G).     

Electron spin resonance in superconducting materials

The ESR of Gd in the superconducting phase of the type II superconductors CeRu₂ and LaOs₂ shows a shift for the field for resonance and inhomogeneous broadening of the lineshape. Both effects strongly depend on the three different microwave frequencies (resp. magnetic fields). The broadening of max. 800 G is attributed to a non resolved finestructure splitting. The local field distribution in vortex state of these type II superconductors is less than 100 G and is the main contribution for shift of the field for resonance. In addition ESR results of Gd and Eu doped into La are discussed forT >T _c. The comparison of depression of the superconducting transition temperature and exchange spin-flip scattering rate determined from ESR shows a perfect agreement.     

Electron spin resonance in superconducting materials

The ESR of Gd in the superconducting phase of the type II superconductors CeRu₂ and LaOs₂ shows a shift for the field for resonance and inhomogeneous broadening of the lineshape. Both effects strongly depend on the three different microwave frequencies (resp. magnetic fields). The broadening of max. 800 G is attributed to a non resolved finestructure splitting. The local field distribution in vortex state of these type II superconductors is less than 100 G and is the main contribution for shift of the field for resonance. In addition ESR results of Gd and Eu doped into La are discussed forT >T _c. The comparison of depression of the superconducting transition temperature and exchange spin-flip scattering rate determined from ESR shows a perfect agreement.     

Electron spin resonance in pyrolyzed lignin

An investigation has been made into electron spin resonance (ESR) occurring in the ranges of carbons prepared from periodate lignin by vacuum pyrolysis. Line shape, line width, g-factors, saturation effects, and free spin concentration have been measured and the effect of oxygen on these parameters has been examined. A g-shift and lines with inhomogeneous broadening have been observed. In the presence of oxygen the lower-temperature carbons showed an unexpected increase in the detectable free spin concentration. The discussion includes factors affecting the nature of the free spins in the pyrolyzed material.     

Electron spin resonance spectrum of imidazo p-benzoquinone anion: a possible case of proton jump

The influence of a molecule's dipole moment on its ability to capture an electron into a stable bound state is examined. It is proved that, in the fixednuclei approximation, a value greater than 1·625 Debye for the dipole moment suffices to guarantee the existence of a discrete spectrum of negative ion states. Implications for Hartree-Fock calculations of negative ions are discussed. The interaction of electronic, vibrational and rotational motions in negative dipolar ions is studied, and conclusions are drawn for real molecules.     

Electron spin resonance investigation of ultra-small double walled carbon nanotubes embedded in zeolite nanochannels

We report on the low temperature electron spin resonance (ESR) properties of ultra-small (0.45?nm) double walled carbon nanotubes (DWCNTs) embedded in zeolite nanochannels. An isotropic ESR signal is observed at g(c)?=?2.002?77 with the spin density (S?=?1/2)?～?10(19)?g(-1), which is suggested to originate from the carbon related point defects in the DWCNTs. Measurements of the ESR line width and signal intensity as a function of temperature indicate that the spins are of a localized nature as opposed to the conduction type electrons observed in large diameter CNTs. The results are consistent with the suggestion that electrons are trapped at interstitial defects. The observed linear frequency dependence of the ESR line width of embedded DWCNTs points to 'strain' as the prime source of broadening. By contrast, the study of free standing DWCNTs shows the presence of a distinctly superlinear frequency dependence of the signal width at low temperatures. The possible origin of the frequency dependence is discussed.     

Electron spin resonance of proton-irradiated graphite

In the case of colossal magnetoresistance in the perovskite manganites, "double exchange" mediated by the itinerant spins is believed to play a key role in the ferromagnetism. In contrast, the conventional "Heisenberg" interaction, i.e., direct (unmediated) interaction between the localized spins produced by the proton irradiation, is identified as the origin of proton irradiation-induced ferromagnetism in graphite.     

Electron spin resonance in X-irradiated sodium sulphate

Reliable results are reported for the various triple-dipole dispersion energy coefficients needed to evaluate the (exact) triple-dipole dispersion energy for all three-body interactions involving He, Ne, Ar, Kr, Xe, H₂, N₂, CO, O₂ and NO, and at least one molecule. A total of 645 unique coefficients, corresponding to 185 unique interactions, are considered.     

Electron spin resonance in carbon-doped boron nitride

It is shown that carbon impurity is responsible for the yellow color and paramagnetism in boron nitride which has been heated to 1800–2300°C in graphite furnaces. The yellow BN powders exhibited 10-line ESR spectra with spin concentrations up to 10¹⁹/g. Annealing the yellow BN at 1800–2300°C in a carbon-free environment removed the yellow color and reduced the spin concentration below the observable limit of 10¹⁶/g. Such pure white BN was unaffected by prolonged irradiation with u.v., 50 kV X-rays or 1 MeV γ-rays, whereas other less pure types of BN showed a large enhancement of ESR signal after relatively short exposures to such radiation. The ESR spectra from pure white BN doped with C¹³-enriched carbon were identical to those from BN doped with normal carbon. These results suggest that the yellow color in BN is due to an F-center produced by a carbon impurity ionization mechanism. The anisotropy of the ESR was studied in carbon-doped specimens of highly oriented BN. The room temperature hyperfine splitting at 9·151 GHz was found to vary as (a + b cos²θ), where a = 7·85 ± 0·07 G and b = −1·27 ± 0·07 G. The g-value obeyed a similar relation: g = g_⊥ + Δg cos²θ, where g_⊥ = = 2·00321 ± 0·00002 and Δg = g_∥ − g_⊥ = (−9·5 ± 0·2) × 10⁻⁴.     

Electron spin resonance in nickel near Tc

The temperature dependence of the FMR and EPR linewidth of bulk nickel single crystals was measured from 260 to 410°C at 9.85 GHz. A linewidth falling off rapidly with increasing temperature was found in a region above T_c: 365<T<385°C. An interpretation by the critical behaviour of the spin system is discussed.