Double ENDOR Research Articles

Characterization of Defect Centres in Semiconductors by Advanced ENDOR Techniques

The advanced magnetic resonance techniques and their application to the studies of defects in semiconductors will be reviewed. Transient and stationary ENDOR, optically detected ENDOR and double ENDOR variations will be briefly discussed while special attention will be given to the Field-Stepped-ENDOR technique. The successful application of the advanced ENDOR techniques for the structure determination of complex defects will be illustrated by the examples concerning the boron-vacancy complex and thermal donors in silicon and the gallium vacancy in gallium phosphide.

An electron nuclear double resonance investigation of redox-induced electronic structural change at CuA2+ in cytochrome c oxidase.

We measured an electronic change at cysteine ligand(s) of the CuA2+ center brought on by reduction of other metal centers within cytochrome c oxidase, notably cytochrome a. This change specifically manifested itself as a modification in magnetic hyperfine coupling to the beta-protons of the beta-carbons adjacent to the cysteine sulfur in the CuA2+ coordination sphere. The electron nuclear double resonance ENDOR signals of these beta-protons had previously been assigned through study of selectively deuterated yeast oxidase. In the present study the ENDOR signals of the CuA2+ center were compared from the following forms of oxidase: resting (a3+.CuA2+.a3+3.CuB2+); mixed valence, 2-electron-reduced CO-ligated oxidase (a3+.CuA2+.a2+3CO.CuB+), and a more completely reduced mixed-valence CO-ligated oxidase. In agreement with previous studies on 3-electron-reduced oxidase, the latter more completely reduced oxidase showed cytochrome a preferentially reduced with respect to CuA, implying that the majority of paramagnetic CuA2+ centers had reduced cytochrome a partners. The ENDOR-resolved splitting of the beta-proton hyperfine features substantially decreased in going from the first two more oxidized forms to the more fully reduced latter form. Thus, the electronic structure of the CuA2+ center specifically monitored by hyperfine couplings to cysteine protons changed in response to a reductive event elsewhere in the protein. This structural change may correlate with the anticooperative redox interaction recently reported between cytochrome a and CuA.

Two types of F centres in alkaline earth fluoro‐halides II. Double ENDOR experiments

AbstractIn crystals containing simultaneously F(Cl−) and F(F−) centres in certain frequency regions the ENDOR spectra are superimposed to such a degree that an analysis is not possible. With double ENDOR experiments the ENDOR spectra of both centres can be separated. The ratio of the ENDOR signal heights compared to the double ENDOR signal heights is found to be characteristic for the shell of the nuclei involved. It can be used to discriminate between ENDOR lines of different shells. The effect cannot be understood on the basis of rate equations with reasonable assumptions for the spin—lattice relaxation times. It is probably due to spin diffusion.

A novel application of double endor spectroscopy: improved resolution in powder samples

A novel application of Double Endor is described which improves the orientation selectivity of paramagnetic species diluted into powders or frozen liquid crystals. The technique is illustrated on the planar metal complex CuTPP oriented in a nematic glass. The magnetic parameters obtained with this method are compared with data from a single-crystal ENDOR study.

Triple resonance investigation of F centres in BaFCl

Abstract In BaFCl two types of F centres can be produced, where the electron occupies a Cl− vacancy [F(Cl−)-centres] or an F− vacancy [F(F−)-centres]. When producing F(F−)-centres the simultaneous production of F(Cl−)-centres cannot be avoided. Both ESR spectra overlap strongly. The ENDOR spectra contain very many lines of both centres and have a very complicated angular dependence. It is shown that the angular dependence of the ENDOR spectra of both F centres can be measured separately by applying a double ENDOR technique (called also Triple Resonance). The ratio of the signal heights of the ENDOR lines compared to the double ENDOR lines is characteristic for the neighbour shell to which the nuclei involved belong. The use of this effect for the analysis of the spectra and its explanation are discussed.

DOUBLE ENDOR with a linearly and a circularly polarized radiofrequency field

The combination of the two spectroscopical techniques, DOUBLE ENDOR and ENDOR with a circularly polarized radiofrequency field (CP-ENDOR), is described. with this new method, termed by the acronym CP-DOUBLE ENDOR, the selective induction of transitions of different types of nuclei and of different paramagnetic species allows a drastic reduction of the number of observed ENDOR lines. With this technique, analysis of hitherto not interpretable ENDOR spectra is often made possible. The experimental setup of the CP-DOUBLE ENDOR spectrometer is described. The advantage of using circularly polarized rf fields in DOUBLE ENDOR spectroscopy is illustrated by two applications on transition metal complexes in single crystals.

On the electronic structure ofN,N′-ethylenebis(acetylacetonatiminato)Co(II), Co(II)acacen

A single crystal ligand ENDOR study on N,N′-ethylenebis(acetylacetonatiminato)Co(II), Co(acacen), is reported. The hyperfine tensors of the two nitrogens, of four carbon-13 and of five protons as well as the nitrogen quadrupole tensors have been evaluated. A DOUBLE ENDOR experiment was carried out in order to determine the signs of the carbon-13 hyperfine couplings. The problems of asymmetric contributions to the hyperfine tensors, which are especially important for low symmetry systems with low-lying excited states, are discussed. The ligand hyperfine tensors can be understood assuming π-spin density on the ligand system and hence support the |2 A, yz⟩ groundstate for Co(acacen). They furthermore provide detailed knowledge on the electronic structure of the complex via the spin density distribution. The structural information derived from the proton ENDOR results is in good agreement with the X-ray data.

An examination of the cyanide derivative of bovine superoxide dismutase with electron-nuclear double resonance

The Cu(II) sites of native, azido- and cyano-derivatives of bovine superoxide dismutase (superoxide:superoxide oxidoreductase, EC 1.15.1.1) have been examined by electron-nuclear double resonance ENDOR). The ENDOR spectrum of the native protein taken at the g II extreme shows resolved structure due to the file directly coordinated N-atoms of the histidine ligands. These spectra are too complex for interpretation but suggest inequivalent coupling between the electronic spin and the four ligand N-atoms. By contrast, the azido protein reveals one type of nitrogen with well-resolved hyperfine and quadrupole splittings ( A zz = 37.9± 1 MHz, P zz = 1.54±0.02 MHz), and the cyano form reveals one well-resolved set of nitrogen lines ( A zz =47.8 ±0.4 MHz, P zz =1.62±0.01 MHz) and one type of partially resolved nitrogen ( A zz =37.0±1 MHz). The cyano form also reveals a complex spectrum in the low-frequency domain (1–10 MHz). Through isotopic substitution and computer simulation, the spectrum is shown to be a composite of the ENDOR from the remote imidazole nitrogens and the cyanide nitrogen. The component of the hyperfine constant perpendicular to the C 14N bond axis is A N ⊥ =3.9±0.3 MHz and along the bond axis is A N ⊥ ≊5.7 MHz. The quadrupole interaction appears to be greatest along the CN axis with Q z′ z′ = 1.0±0.1 MHz and Q x′ x′ y′ y′ ≈0. Based on an analysis of the hyperfine and quadrupole interactions seen at two extremes of the electron paramagnetic spectrum, we propose a square-planar arrangement of three imidazole nitrogen and one CN − carbon around the copper. Within this plane two imidazole nitrogens are strongly coupled and magnetically equivalent, the third is inequivalent (slightly weaker hyperfine interaction) and forms a trans relationship with the cyanide. This model is consistent with other observations on the cyano-derivative.

On the electronic structure of N,N′-ethylenebis(acetylacetonatiminato)Co(II), Co(II)acacen

A single crystal ligand ENDOR study on N,N′-ethylenebis(acetylacetonatiminato)Co(II), Co(acacen), is reported. The hyperfine tensors of the two nitrogens, of four carbon-13 and of five protons as well as the nitrogen quadrupole tensors have been evaluated. A DOUBLE ENDOR experiment was carried out in order to determine the signs of the carbon-13 hyperfine couplings. The problems of asymmetric contributions to the hyperfine tensors, which are especially important for low symmetry systems with low-lying excited states, are discussed. The ligand hyperfine tensors can be understood assuming π-spin density on the ligand system and hence support the |2 A, yz⟩ groundstate for Co(acacen). They furthermore provide detailed knowledge on the electronic structure of the complex via the spin density distribution. The structural information derived from the proton ENDOR results is in good agreement with the X-ray data.

The effects of modulation frequency and rf ENDOR power on 53Cr3+ ENDOR lines in CsAl sulfate alum

An investigation was made of the effects of modulation frequency and rf ENDOR power on the single and double quantum ENDOR spectral lines of 53Cr3+ ions in single crystals and powders of CsAl sulfate alum. Measurements were made of the linewidths and intensities in the single crystals and of the linewidths in the powders. It was found in the single crystals that a factor of 2/1 exists for the ratio of single/double quantum linewidths at the limit of zero ENDOR power and modulation frequency. Interpretation of the line intensity results with the results of lineshape theory indicates that the ENDOR lines are of Lorentzian form, at least for low values of ENDOR power.

General TRIPLE resonance on free radicals in solution. Determination of relative signs of isotropic hyperfine coupling constants

For the first time a general type of electron–nuclear–nuclear TRIPLE resonance has been performed on radicals in liquid solution. In this method two inequivalent nuclei are irradiated simultaneously at their respective NMR frequencies, the resonance being detected by an enhancement of a saturated ESR line. This general TRIPLE experiment, the analog of which is known already in the solid state (DOUBLE ENDOR), gives direct information about relative signs of hyperfine coupling constants. Several important aspects of the experimental setup are described. The experimental results are discussed on the basis of a simple relaxation model.

ESR, ENDOR, and double ENDOR at 300 °K of DL-serine x irradiated at 340 °K, a new radical species

When DL-serine single crystals are x rayed at 300 °K or below, two stable radicals are observed at room temperature. One of these, IV, is of greater intensity than the other, V, and was previously identified. By irradiating at a slightly elevated temperature, ∼340 °K, the concentration of V becomes more intense than IV. In this work we report on a detailed study of V using ESR, ENDOR, and double ENDOR, all at room temperature. We have determined that V is and have obtained five hyperfine tensors representing one α-H, two β-H’s, the rotating amino H’s, and a fourth H which is probably in a neighboring molecule. We have also observed the ENDOR transition of the nitrogen. The α-H tensor is somewhat unusual in that the isotropic term (−41.3 MHz) is considerably less than for a 2ρπ electron and of the same sign, but the anisotropic term is approximately the same (+31.9, +3.5, −35.4 MHz). This is consistent with our model since the bonding to the α-carbon is strained and would not be expected to be pure sp2.

Double ENDOR measurements in CaF2:Tm2+

The double ENDOR method has been used to measure the relative signs of the hyperfine interactions of the unpaired hole in Tm2+ (4f13) with (a) the 169Tm nucleus, and (b) the 19F ligand nuclei in CaF2. The sign of the interaction (a) is known from theory, hence the measurement determines the sign of the interaction (b); both components A2 and Ap are found to be positive. It has been shown that information about the relative signs of two hyperfine parameters may be obtained in a double ENDOR experiment only if the ENDOR signals are essentially transient, or have a significant transient component. The proportion of transient component is somewhat adjustable by suitable choice of experimental conditions. Rapid spin diffusion tends to decrease the proportion of transient component and hence tends to frustrate measurements of relative signs of hyperfine parameters.

Determination of signs of hyperfine coupling constants for anS= 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.

Electron Nuclear Double Resonance

Feher introduced the electron nuclear double resonance or ENDOR technique in 1 956 (1) . In the ensuing 1 5 years several hundred ENDOR­ related articles have been published. However, no general review of the subject has appeared. This may be partially due to the fact that ENDOR results Can be thought of as an extension of the usual paramagnetic resonance measure­ ments and consequently are subsumed under the more general categories of magnetic resonance. Therefore, I shall not attempt to discuss in detail the significance or ramifications for a given field of the results of ENDOR measure­ ments. Instead, I shall attempt to cover the entire range of ENDOR experi­ ments from those in organic liquids to those on zero-spin deviations in antiferromagnets. My purpose shall be to illustrate with representative ex­ amples the power and limitations of the technique, to summarize briefly but comprehensively the work that has been done, and hopefully to suggest the potential of the technique for problems still to be solved. The term ENDOR shall be restricted to that class of double resonance experiments in which the effect of a resonant change in the nuclear spin quantization direction (NMR) is observed via its effect on the EPR signal intensity. [This is distinct from an interesting group of related double resonance experiments (Overhauser effect) in which EPR transitions are used to enhance the sensitivity of the NMR detection process itself.] Herein lies the essence of

Relative signs of hyperfine coupling constants by a double ENDOR experiment

It is shown that the relative signs of the hyperfine coupling constants of two spin ½ nuclei with isotropic coupling to one unpaired electron can be experimentally determined using the ENDOR technique with two radio-frequency fields in addition to the microwave field.