Electron Nuclear Double Resonance Spectra Research Articles

Dynamics and spin relaxation of tempone in a host crystal. An ENDOR, high field EPR and electron spin echo study

We studied in the temperature range 4–300 K the dynamics and the electron spin echo decay of the radical 4-oxo-2,2,6,6,-tetramethyl-1-piperidinyloxyl (tempone) doping substitutionally a single crystal of the diketone 2,2,4,4-tetramethyl-cyclobutan-1,3-dione. Electron nuclear double resonance (ENDOR) and high field electron paramagnetic resonance (EPR) spectra (110 GHz) displayed three types of motion: interconversion between twisted-crossover conformations, rotation of the methyl groups and libration of the radical in the crystal matrix. At room temperature all the motions are fast, and they reach the slow motion re′gime each at a different temperature, being all slow below 80 K. An approximate value of 9 kJ mol-1 for the energy barrier hindering the rotation of the tempone methyl groups is estimated. At low temperature their motion is too slow to give any contribution to the echo decay. The echo decays as a linear exponential in the range 100–300 K, due to the radical motions. At T<100 K the interaction with the methyl protons of the matrix is the dominant dephasing mechanism, and the echo decays as a gaussian. At T<20 K it depends on the square root of the time. Echo decays and electron spin echo envelope modulations (ESEEM) are simulated by using the available theoretical models. A damping of ESEEM superimposed to the echo decay is discussed.

EPR and ENDOR studies of NOx and Cu2+ in zeolites: bonding and diffusion

The diffusion and bonding of NOx (x=1, 2) and Cu2+ species in zeolites are reviewed, based mainly on our own research. The molecular motion of adsorbed NO2 has been examined with EPR in several zeolite samples and analyzed using the slow-motional EPR theory. In X- and Y-type zeolites the broadening of the spectra with temperature could be analyzed by simulations using a rotational diffusion model in agreement with earlier results in Vycor glass and Cu-metal. For the diffusion of NO2 in Na-mordenite and Na-ZSM-5 the broadening of the spectra with increased temperature could be better simulated with the Heisenberg type of spin exchange model. The exchange was attributed to the interaction between NO2 molecules diffusing along the zeolite channels. In Na-ZSM-5 the spin exchange rate increased rapidly with an increasing Si/Al ratio of the zeolite. The effect was attributed to the hindrance against diffusion by Na+, the amount of which increases with a decreased Si/Al ratio. The rates increased with increasing number of H2O molecules adsorbed on the zeolite because of weaker interaction between NO2 and the surface. A more detailed slow-motional analysis indicated that at each temperature a distribution of diffusion rates occurs. NO adsorbed on Na-A zeolite was found to be present as monomer at low pressure ( 100 mbar). The triplet was absent in calcium ion-exchanged A-type zeolite indicating that the NO–NO species depends on Na+ for its stability. The detailed structure of a Cu2+ complex with water molecules in Cu-ZSM-5 zeolite was characterised with ENDOR at 4 K. The complex has an axial distorted octahedral structure with two water molecules at a longer distance in axial position and four molecules in the equatorial positions. The Cu2+ complexes with ammonia and deuterated ammonia have been investigated with electron nuclear double resonance (ENDOR). Simulations of the ENDOR spectra with 1H, 2H and 14N hyperfine interactions have been undertaken. The analysis indicates that the complex has a square planar structure with four ammonia ligands.

Electron paramagnetic resonance investigations of nickel defects in natural diamonds

Electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectra of natural blue diamonds from the Argyle mine in Western Australia are reported for the first time. These diamonds are shown to contain the NE2 centre which has been observed primarily in synthetic diamonds and has been proposed to be with three neighbouring nitrogens. A previously unreported EPR centre is observed and is found to have S = 3/2, g = 2.02 and superhyperfine interaction from one nitrogen. The results are found to be consistent with an on a substitutional site with an on a fourth nearest neighbour site. This defect is found exclusively in Argyle blue diamonds.

Matrix Line in Pulsed Electron-Nuclear Double Resonance Spectra

The intense line in Mims and Davies electron-nuclear double resonance (ENDOR) spectra due to the hyperfine interactions of an unpaired electron with distant matrix nuclei is shown to originate from a simultaneous inversion of a large number of nuclear spins by a radiofrequency pulse. Theoretical expressions describing the matrix ENDOR effect are derived and verified experimentally.

Pulsed ENDOR at 95 GHz on the Primary Acceptor Ubisemiquinone in Photosynthetic Bacterial Reaction Centers and Related Model Systems

Davies-type pulsed H electron nuclear double resonance (ENDOR) measurements were performed at a magnetic field of 3.4 T and a microwave (MW) frequency of 95 GHz (W-band). By taking advantage of the increased electron Zeeman interaction at high field, the small g-anisotropy of the semiquinone anion radicals could be resolved in frozen solutions. Hence, the W-band ENDOR spectra could be taken at the well-separated canonical peaks of the powder electron paramagnetic resonance (EPR) spectra, thereby becoming highly orientation-selective with respect to the relative orientation of the radicals to the external magnetic field. The measurements were performed on various randomly oriented semiquinone anion radicals in frozen alcoholic solution and on the primary ubiquinone anion radical, Q, frozen photosynthetic bacterial reaction centers (RCs) of Rhodobacter sphaeroides in which the Fe was replaced by Zn (ZnRC). A simulation program was used to obtain magnitudes and orientations of the hyperfine tensors. The W-band ENDOR spectra of the immobilized radicals show not only hyperfine couplings (HFC) of local protons of the semiquinones, but also those of protons from the environment. These are, for example, involved in hydrogen bonds between the amino acid surrounding and the quinone carbonyl groups. For Q in ZnRCs, a particularly large H-bond HFC was obtained from which direction and distance of the H-bond could be estimated. These data were compared with those measured for the respective ubisemiquinone radical, UQ-10, in protonated and deuterated 2-propanol, where two H-bonds of comparable strength but different directions could be detected. Comparison of H ENDOR spectra of the 2,3,5,6-tetra-methyl-1,4-benzoquinone anion radical in frozen solution, performed both at W-band and at X-band frequency, demonstrated the limitations of achieving orientation selected ENDOR spectra from X-band experiments on systems with small g-anisotropy.

ENDOR Determined Structure of a Complex of α-Chymotrypsin with a Spin-Labeled Transition-State Inhibitor Analogue

The conformation of the spin-labeled compound N-(2,2,5,5-tetramethyl-1-oxypyrrolinyl-3-carboxyl)-L-phenylalaninal (SLPheal), synthesized as a transition-state inhibitor analogue of α-chymotrypsin (αCT), has been determined by electron nuclear double resonance (ENDOR) and molecular modeling methods for both the free inhibitor in solution and the hemiacetal enzyme:inhibitor adduct. SLPheal exhibited linear, competitive inhibition of αCT with KI = (0.21 ± 0.03) × 10-3 M. By a combination of NMR and ENDOR spectroscopy, we have established that 92% of the aldehyde is found as the hydrated aldehyde species RCH(OH)2 at pH 7.0. From ENDOR spectra, principal hyperfine coupling (hfc) components of specific protons assigned by selective deuteration were determined, and their corresponding dipolar hfc components were computed to estimate electron−proton distances. The principal hfc components of the fluorine substituent of a ζF-phenylalaninal inhibitor analogue were also determined for estimation of electron−fluorine...

A sensitive broadband high-frequency electron spin resonance/electron nuclear double resonance spectrometer operating at 5–7.5 mm wavelength

A novel high-frequency electron spin resonance (ESR) spectrometer is described. The features are high sensitivity, a continuously tunable operating frequency of 40–60 GHz, a cylindrical cavity with a novel coupling scheme, and simultaneous detection of absorption and dispersion. A radio frequency coil can be integrated into the cavity for electron nuclear double resonance (ENDOR) experiments. ESR and ENDOR spectra of a test sample are presented in order to demonstrate in particular the scientific potential of a variable-frequency spectrometer.

Pulsed ENDOR Studies at 95 GHz of the Triplet State of13C60

Fully13C-enriched C60has been investigated by pulsed electron-nuclear double-resonance (ENDOR) spectroscopy at W-band frequency at 1.2 K. For molecular C60the ENDOR spectra reflect the distortion of the molecule upon triplet excitation, and the fine-structure parameterDis found to be negative. For a single crystal of C60the ENDOR spectra characterize the triplet excitation as delocalized over a pair of neighboring C60molecules.

ENDOR identification of a Rh+ centre in solution-grown NaCl

X-Irradiation at room temperature produces three stable rhodium-related centres in solution-grown NaCl doped with Rh3+. Two have earlier been identified as Rh2+-centred complexes. The third centre, which could only be observed with electron nuclear double resonance (ENDOR), appeared to have cubic symmetry. In the present work this centre is identified as a (RhCl6)5- complex, substituting for (NaCl6)5-. The central Rh+ ion has a 4 configuration, leading to a centre with effective spin S=1. This is confirmed experimentally by the occurrence of a ΔMS=2 transition in the EPR spectrum. Interactions of the unpaired electrons with the central Rh nucleus, the first-shell CI and the fourth-shell Na nuclei are observed in the ENDOR spectrum. The angular dependence of these interactions shows that the centre has cubic symmetry. The spin Hamiltonian parameters obtained are used to estimate the crystal field splitting and the unpaired spin density over the complex.

Mapping the unpaired electron density in [Pt(bipy)L2]- (bipy=2,2′-bipyridine; L=Cl-, CN-, 13CN-): A combined EPR–ENDOR–theoretical study

The X-band EPR and electron–nuclear double resonance (ENDOR) spectra of the formally , 17-electron radical anions [Pt(bipy)L2]− (bipy = 2,2′-bipyridine; L = Cl−, CN−, 13CN−) are reported. The 195Pt (I = 1/2, 34%) anisotropic hyperfine matrices are analysed in terms of both 5d and 6p contributions to the singly occupied molecular orbitals (SOMOs). For L = Cl− there is 5.0% 5dyz and 7.6% 6pz character, for L = CN−there is <1% 5dyz and 13.1% 6pz. The bulk of the unpaired electron density is delocalised about the bipy π* system and ENDOR spectra reveal the superhyperfine couplings to the 1H, 14N and 13C (L = 13CN−) nuclei. The unpaired electron densities in the 14N 2pz orbitals contributing to the π* systems are calculated to be 12% and 4% for L = Cl− and CN−, respectively. The cyanide π-orbitals make little contribution to the SOMO for [Pt(bipy)(CN)2]−. There is a good agreement with density functional theory (DFT) MO calculations on the radical anions. DFT calculations also show the atomic orbital compositions of the lowest unoccupied (LU) MO of [Pt(bipy)L2] and the SOMO of [Pt(bipy)L2]− to be virtually identical.

An EPR and pulsed ENDOR study of the structure of tyrosine Z ⋅ in Tris-treated photosystem II

Electron paramagnetic resonance (EPR) and pulsed electron-nuclear double resonance (ENDOR) spectra of oxidized tyrosines Z and D (Y Z ⋅ and Y D ⋅, respectively) in Mn-depleted photosystem II have been studied in the pH range from 5.5 to 8.0. Above pH 7.0 the hyperfine couplings of 3,5-ring protons of both tyrosines were similar. The hyperfine constants of the strongly-coupled β-methylene protons were slightly different, 28.4 MHz in Y D ⋅ vs. 33.6 MHz in Y Z ⋅. This difference was explained by different orientations of the β-methylene group with respect to the aromatic ring plane. From the similarity of the hyperfine couplings it was concluded that at high pH Y Z ⋅, same as Y D ⋅, is a neutral radical. The EPR and ENDOR spectra of Y D ⋅ did not change in the studied pH range. However, the light-minus-dark spectra below pH 6.0 were considerably different from those at higher pH. To explain these spectra, two models were discussed, with Y Z supposed to be a cation-radical or a neutral radical with spin densities close to those known in literature. Assuming Y Z to be a cation radical with a spin density distribution close to that known for phenolic cation radicals, we were able to explain the shape of the ENDOR, S-(2GHz) and X-band EPR spectra observed at pH 5.5. The deuteron ENDOR spectra of Y Z ⋅ in D 2O-washed sample were similar both at low and high pH, suggesting the change in hydrogen bondings may be orientational without change in their distance. The change of the Y Z ⋅ structure at low pH was suggested to be caused by the protonation of some amino acid residue (possibly, histidine 190 of the D1 protein) in the vicinity of Y Z.

Hyperfine decoupling in electron spin resonance

A new class of experiments is introduced to electron spin resonance (ESR) spectroscopy that utilizes hyperfine decoupling for resolution enhancement and spectrum simplification, and that provides a basis for correlation techniques. A general framework is provided for the discussion of pulse ESR experiments on systems with arbitrary effective electron spin S and an arbitrary number of coupled nuclear spins and is used to describe spin decoupling in pulse ESR and ENDOR spectroscopy. Analytical expressions are given for the hyperfine-decoupled nuclear frequencies and the residual hyperfine splittings of spin-1/2 nuclei during strong decoupling. Pulse sequences are proposed for hyperfine-decoupled electron spin echo envelope modulation (ESEEM) and electron nuclear double resonance (ENDOR) experiments as well as for the correlation of the hyperfine-decoupled ESEEM spectrum with two-pulse and three-pulse ESEEM spectra and of hyperfine-decoupled ENDOR with the hyperfine splittings. It is shown that hyperfine-decoupled ESEEM and ENDOR spectra can reveal information on the magnetic quantum numbers involved in an ESR observer transition, and that choosing a transition mS↔mS+1 with mS≠−1/2 can improve the resolution of a nuclear frequency spectrum. In addition, such experiments can be used to determine the relative signs of hyperfine couplings. The potential of the two-dimensional DECENT (decoupled ESEEM correlated to nuclear transition frequencies) experiment is demonstrated on weakly coupled N14 nuclei in both an ordered and a disordered system and on the hexaquo manganese (II) complex (S=5/2) in a single crystal. It is also shown that for the ESR observer transition mS=(−3/2↔−5/2) the S=5/2 system yields highly resolved hyperfine-decoupled ENDOR spectra which allow for a complete assignment of the ENDOR lines.

EPR AND ENDOR INVESTIGATION OF SINGLE CRYSTALLINE AND POWDERED NaCl:Rh2+ and AgCl:Rh2+

Single crystalline NaCl doped with Rh2+ was investigated with electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR). An orthorhombic Rh2+ centre was found similar to that observed previously in AgCl, where Rh2+ substitutes for Na+ forming an [RhCl6]4− complex elongated along a [001] axis with a charge compensating Na+ vacancy in a second nearest Na+ position in the equatorial plane. In addition, two further axial Rh2+ centres were found, where the Na+ vacancy is not in the nearby equatorial plane. In the powder ENDOR spectra of NaCl:Rh2+ the interactions with axial second nearest 23Na and axial 103Rh nuclei could be resolved. Powder ENDOR spectra of AgCl:Rh2+ precipitated from aqueous solution showed that the Rh2+ defect is different from that generated in Rh2+ -doped AgCl grown from the melt. The limitations and possibilities of powder ENDOR are discussed. © 1997 Elsevier Science Ltd

Right-angle sample spinning electron spin resonance

Right-angle sample spinning electron spin resonance (RAS-ESR) is introduced. In this new pulse ESR technique the sample is rotated along an axis perpendicular to the direction of the static magnetic field B0. RAS-ESR can be applied to improve sensitivity and spectral resolution of ESR, electron spin echo envelope modulation, and electron nuclear double resonance spectra of orientationally disordered systems. A detailed qualitative description of two-dimensional (2D) anisotropy-resolved ESR (AR-ESR), a technique that is based on RAS is given. It is demonstrated that the anisotropy of a spin system can be used to disentangle a field-swept powder ESR spectrum into a second dimension. This new dimension is obtained by Fourier transformation of the stimulated echo modulations caused by incrementing the spinning frequency. Details of the probe assembly for sample spinning at temperatures T⩾10 K with spinning frequencies between 0 and 500 Hz, frequency increments of minimum 0.2 Hz, a long-time frequency stability better than 1 Hz, and the possibility of sample exchange at these temperatures are given. The performance of the RAS-ESR spectrometer is demonstrated by a stimulated echo 2D AR-ESR experiment on a transition metal complex in a polycrystalline host material.

ENDOR spectra of polarons in poly(paraphenylene vinylene)

Electron nuclear double resonance (ENDOR) spectra of π-electron spin defects in undoped poly(paraphenylene vinylene), PPV have been measured using stretch-oriented polymers. Detailed spectral line shape observed with the aid of ENDOR-induced ESR technique reveals that the spin density extends over about 4 phenyl rings in half-width and the maximum and the second largest spin densities reside on the carbons of vinylene sites. These features correspond well to those of the theoretically calculated spin density distribution of polarons in the case of finite electron correlation, supporting that the spin defects are trapped polarons, as we proposed previously. The polaron spin distribution also well reproduces the photoinduced ESR spectra of PPV, showing that polarons are photogenerated in this polymer.

Electronic Structures and Endor Spectra of Polarons in Poly(p-Phenylene Vinylene)

Abstract Polaron states in electroluminescent conjugated polymer, poly(p-phenylene vinylene) (PPV) are studied theoretically. The Pariser-Parr-Pople model which includes a long range electron-electron interaction is applied to a π electron system on a PPV chain. Using the unrestricted Hartree-Fock method, we calculate the properties of polarons, such as the structures of spin and charge densities. Electron-nuclear double-resonance spectra calculated from the theoretical spin density of a polaron well explain corresponding experiments in undoped, oriented PPV.

Electron-nuclear double resonance of deep-boron acceptors in silicon carbide

Electron-nuclear double resonance spectra of deep-boron acceptors in silicon carbide have been observed. The quadrupole and hyperfine interaction constants are determined. A sharp decrease of the quadrupole interaction and suppression of the anisotropic part of the hyperfine interaction, as compared with shallow-boron acceptors, are found. The observed effects are explained by the motion of holes inside the deep-boron acceptor.

Electronic and optical properties of neutral and charged poly ( p-phenylene vinylene )

Electronic and optical properties of a conjugated polymer, poly ( p-phenylene vinylene) (PPV), are theoretically studied. Long-range interactions among π electrons as well as electron-lattice coupling are taken into account in both neutral and charged states of a PPV chain by applying the Pariser-Parr-Pople model with intermediate interaction strength. This model coupled with the single-excitation configuration interaction method gives an excitation energy diagram for neutral PPV, consistent with experiment. The optical absorption spectrum is calculated in a wide frequency range, and its polarization dependence and spectral features are analyzed in detail. A polaron state in doped PPV is investigated, and its spatial pattern and absorption spectrum calculated. Its spin density gives an electron-nuclear double resonance spectrum, which is in good agreement with experiment. The present model provides a unified description of the electronic properties of neutral and doped PPV.

Chirp Hyperfine Spectroscopy

A novel pulse ESR scheme for the direct measurement of hyperfine splittings is proposed. It is shown that this hyperfine spectroscopy reduces the number of the spectral lines considerably, overcomes assignment ambiguities inherent in electron-nuclear double-resonance spectra, and allows one to separate signals that overlap in ENDOR spectra. The resolution enhancement of hyperfine spectroscopy is fully utilized since the hyperfine splittings are measured between nuclear-spin rather than electron-spin transitions. The pulse sequence for the experiment is based on the chirp ENDOR method and can yield one-dimensional as well as two-dimensional spectra that correlate the ENDOR and the hyperfine frequencies. Both experiments are discussed theoretically, and the first experimental spectra of a model system are presented.

The chemistry and the geometric and electronic structures of small naked metal clusters prepared using a rotating cryostat and studied by electron paramagnetic resonance

A rotating cryostat has been used to generate several new naked metal clusters whose structures and reactions have been studied by EPR spectroscopy. Most alkali-metal trimers adopt an obtuse triangular geometry with a 2B2 ground state in C2v symmetry. A new acute Na3 cluster was formed by interaction with water, Na3(H2O)x where x 1. The first EPR spectrum of a mixed alkali-metal trimer, Na2Li, has been recorded. The fluxionality of both Na3 and Li3 in an adamantane matrix is lost and a static form results with the Li located at the apical position. The first ENDOR (electron nuclear double resonance) spectrum of a naked metal cluster, Li3, has been recorded. It gives more accurate paramagnetic parameters and shows conclusively that the cluster is a fluxional, pseudo-rotating Jahn–Teller molecule. The ENDOR spectra of Li3(H2O)x complexes were also observed. A new coinage metal trimer, Au2Ag, has been characterised by EPR spectroscopy and has the silver in the apical position in a static obtuse structure. Reactions of the alkali-metal and coinage-metal trimers with simple molecules such as water and ethene resulted in a change in geometry from an obtuse or fluxional molecule to an acute static entity. The flexible nature of clusters is discussed in the context of modelling the chemical events occurring at the active metallic sites of heterogeneous catalysts.