Nuclear relaxation effects in Davies ENDOR variants

A recent article by Yang and Hoffman [T.-C. Yang, B.M. Hoffman, J. Magn. Reson., 181 (2006) 280] presents a ‘Davies/Hahn ENDOR multi-sequence’ in which the α and β peaks of an electron-nuclear double resonance (ENDOR) spectrum can be distinguished. This represents one instance of a family of ENDOR sequences which have no initial microwave inversion pulse, and which can reveal information about nuclear relaxation rates and the signs of hyperfine coupling constants. Here we discuss the more general set of such sequences, which we refer to as Saturated Pulsed ENDOR, and show how signal sensitivity can be optimised within the context of this new technique. Through simulations, we compare its performance to other techniques based on Davies ENDOR, and experimentally illustrate its properties using the non-heme Fe enzyme anthranilate dioxygenase AntDO. Finally, we suggest a protocol for extracting both the magnitude and sign of the hyperfine tensor using a combination of ENDOR techniques.     

ENDOR spectroscopic and molecular orbital study of the dynamical properties of the side chain in radical anions of ubiquinones Q-1, Q-2, Q-6, and Q-10

The dynamics of the side chain of the radical anions of ubiquinones Q-1 (2,3-dimethoxy-5-methyl-6-[3-methyl-2-butenyl]-1,4-benzoquinone), Q-2, Q-6, and Q-10 have been investigated using electron nuclear double-resonance (ENDOR) spectroscopy. When radicals are produced in the liquid phase, secondary radicals are also formed. The EPR spectra of these additional radicals overlap with the radical of interest. ENDOR spectroscopy was found to be capable for studying the dynamical properties of such conditions. The temperature dependence of the isotropic hyperfine coupling constants of the beta- and gamma-protons of the side chain was measured. The activation energy of the rotation and other dynamical properties of the side chain were calculated assuming that rotation can be modeled by the classical two-jump model. The rotation energy barrier for Q-1 was also determined by the hybrid Hartree-Fock/density functional method UB3LYP with the 6-31G(d) basis set. Calculated results were in good agreement with the experimental results. Despite the numerous parameters affecting the ENDOR linewidth ENDOR spectroscopy was shown to be a potential method for studying the dynamical properties of the mixtures of the radicals. Prominent forbidden transitions appear in the ENDOR spectra when alkali ions are present in the sample. From these transitions measured ENDOR-induced EPR spectra showed an additional doublet and phase transition in electron Zeeman frequency.     

Schonland ambiguity in the electron nuclear double resonance analysis of hyperfine interactions: principles and practice

For the analysis of the angular dependence of electron paramagnetic resonance (EPR) spectra of low-symmetry centres with S=1/2 in three independent planes, it is well-established-but often overlooked-that an ambiguity may arise in the best-fit g<--> tensor result. We investigate here whether a corresponding ambiguity also arises when determining the hyperfine coupling (HFC) A<--> tensor for nuclei with I=1/2 from angular dependent electron nuclear double resonance (ENDOR) measurements. It is shown via a perturbation treatment that for each set of M(S) ENDOR branches two best-fit A<--> tensors can be derived, but in general only one unique solution simultaneously fits both. The ambiguity thus only arises when experimental data of only one M(S) multiplet are used in analysis or in certain limiting cases. It is important to realise that the ambiguity occurs in the ENDOR frequencies and therefore the other best-fit result for an ENDOR determined A<--> tensor depends on various details of the ENDOR experiment: the M(S) state of the fitted transitions, the microwave frequency (or static magnetic field) in the ENDOR measurements and the rotation planes in which data have been collected. The results are of particular importance in the identification of radicals based on comparison of theoretical predictions of HFCs with published literature data. A procedure for obtaining the other best-fit result for an ENDOR determined A<--> tensor is outlined.     

EPR and ENDOR study of an oxygen-vacancy-associated Ti center in RbTiOPO4 crystals

The dominant Ti³⁺ trapped electron center in flux-grown RbTiOPO₄ (RTP) crystals has been characterized using electron paramagnetic resonance (EPR) and electron–nuclear double resonance (ENDOR). This center is produced during an X-ray irradiation at room temperature when a Ti⁴⁺ ion traps an electron and becomes a Ti³⁺ ion, and is best studied in the 30–40 K range. The EPR spectrum contains a three-line hyperfine pattern from two nearly equivalent neighboring ³¹P nuclei, along with hyperfine lines from the ⁴⁷Ti and ⁴⁹Ti nuclei. The g matrix, determined from the angular dependence of the EPR spectrum, has principal values of 1.819, 1.889, and 1.947. Hyperfine matrices for four ³¹P nuclei are obtained from the angular dependence of the ENDOR spectrum. The proposed model for this defect is a Ti³⁺ ion adjacent to an oxygen vacancy at an OT position. Analogies are made to a similar Ti³⁺ center in KTiOPO₄ (KTP) crystals.     

基于直接数字合成技术的电子自旋-核自旋双共振实验射频信号源设计

为了满足脉冲式电子顺磁共振谱仪中电子自旋-核自旋双共振模块的需要,利用直接数字合成器设计并制作了射频信号源. 该部件产生的射频脉冲具备对频率、幅度和相位快速精确调制的能力,对原子核自旋有较强操控能力.     

A Davies/Hahn multi-sequence for studies of spin relaxation in pulsed ENDOR

We extend earlier studies of the effects of relaxation on the intensities of pulsed ENDOR signals by introducing a Davies/Hahn (D/H) pulsed ENDOR multi-sequence that corresponds to a series of Davies sequences with the preparation pulse 'turned off'. In this pulse train, the Hahn [pi/2, pi] detection pulse pair of sequence n-1 both generates the echo detected for that sequence and acts as the preparation portion of sequence n, in effect replacing the pi preparation pulse of the Davies sequence. We show both theoretically, through a master-equation approach, and with both (1)H(I=1/2) and (14)N(I=1) ENDOR experiments on the non-heme Fe enzymes, superoxide reductase (SOR) (S=1/2) and AntDO (S=3/2), that under conditions of high electron-spin polarization (high microwave frequency/low temperature) the D/H multi-sequence allows simplification of ENDOR spectra by suppression of nuclear transitions associated with the m(S)=+1/2 (alpha) manifold. As such suppression depends on the sign of A, it allows determination of this sign. The suppression as a function of the time between individual sequences is found to exhibit behaviors that can be classified into three regimes of the ratio of cross-relaxation to spin-lattice relaxation rates: strong cross-relaxation (X-case); comparable rates (XL); negligible cross relaxation (L). Interestingly, the ENDOR behavior of the S=1/2 SOR center indicates it is an L case, while the S=3/2 AntDO is an L case. Overall, the D/H protocol appears to be a robust and general tool for using relaxation effects to manipulate ENDOR spectra.     

The effect of spin relaxation on ENDOR spectra recorded at high magnetic fields and low temperatures

A simple theoretical model that describes the pulsed Davies electron-nuclear double resonance (ENDOR) experiment for an electron spin S = (1/2) coupled to a nuclear spin I = (1/2) was developed to account for unusual W-band (95 GHz) ENDOR effects observed at low temperatures. This model takes into account the thermal polarization along with all internal relaxation processes in a four-level system represented by the electron- and nuclear-spin relaxation times T(1e) and T(1n), respectively, and the cross-relaxation time, T(1x). It is shown that under conditions of sufficiently high thermal spin polarization, nuclei can exhibit asymmetric ENDOR spectra in two cases: the first when t(mix) > T(1e) and T(1n), T(1x) > T(1e), where ENDOR signals from the alpha manifold are negative and those of the beta manifold positive, and the second when the cross- and/or nuclear-relaxation times are longer than the repetition time (t(mix) < T(1e) < t(R) and T(1n), T(1x) > t(R)). In that case the polarization of the ENDOR signals becomes opposite to the previous case, the lines in the alpha manifolds are positive, and those of the beta manifold are negative. This case is more likely to be encountered experimentally because it does not require a very long mixing time and is a consequence of the saturation of the nuclear transitions. Using this model the experimental t(mix) and t(R) dependencies of the W-band (1)H ENDOR amplitudes of [Cu(imidazole)(4)]Cl(2) were reproduced and the values of T(1e) and T(1x) > T(1e) were determined. The presence of asymmetry in the ENDOR spectrum is useful as it directly provides the sign of the hyperfine coupling. The presented model allows the experimentalist to adjust experimental parameters, such as t(mix) and t(R), in order to optimize the desired appearance of the spectrum.     

Cryogenic 35 GHz pulse ENDOR probehead accommodating large sample sizes: Performance and applications

The construction and performance of a cryogenic 35 GHz pulse electron nuclear double resonance (ENDOR) probehead for large samples is presented. The resonator is based on a rectangular TE₁₀₂ cavity in which the radio frequency (rf) B₂-field is generated by a two turn saddle ENDOR coil crossing the resonator along the sample axis with minimal distance to the sample tube. An rf power efficiency factor is used to define the B₂-field strength per square-root of the transmitted rf power over the frequency range 2–180 MHz. The distributions of the microwave B₁- and E₁-field, and the rf B₂-field are investigated by electromagnetic field calculations. All dielectrics, the sample tube, and coupling elements are included in the calculations. The application range of the probehead and the advantages of using large sample sizes are demonstrated and discussed on a number of paramagnetic samples containing transition metal ions.     

多核芳烃正离子自由基的ENDOR和EPR研究

用EPR和ENDOR方法检测傅氏烷基化反应(Friedel-Crufts reaction)中所生成的多核芳烃正离子自由基:1,4,5,8-四甲基惠、9,10-二氘代-1,4,5,8-四甲基蒽、2,6,9,10-四甲基蒽、2,6-二甲基-9,10-二(对甲苯基)蒽、3,6,11,14-四甲基二苯并(a,c)三亚苯正离子自由基,测定了它们的超精细偶合常数并提出了其生成机理。     

FID detection of EPR and ENDOR spectra at high microwave frequencies

High-frequency pulsed EPR spectroscopy allows FID detection of EPR spectra owing to the short dead time that can be achieved. This FID detection is particularly attractive for EPR and ENDOR spectroscopy of paramagnetic species that exhibit inhomogeneously broadened EPR lines and short dephasing times. Experiments are reported for the metalloprotein azurin at 275 GHz.     

Spin dynamics in continuous wave and pulsed ENDOR spectroscopy of coals

ENDOR experiments on coals recorded using continuous wave (CW) and pulsed techniques appear to give qualitatively different spectra. A matrix proton signal dominates the ENDOR spectrum of coals recorded in the CW ENDOR experiment while both a matrix and local proton ENDOR signals with huperfine couplings of up to 20 MHz are observed in spectra recorded using pulsed excitation techniques. Analysis of these spectra lead to different implications for the structure of the molecules that host the unpaired electron. Using a combination of pulsed EPR (Electron Spin Echo, FID detected hole burning) and pulsed Electron Nuclear Multiple Resonance (Sub-level relaxation, hyperfine selective ENDOR, EPR sub-spectra) experiments, we investigate the electron and nuclear spin dynamics in order to reconcile the different signal amplitudes observed in the CW and pulsed ENDOR spectra. In the CW ENDOR experiment, the results of the FID detected hole burning experiments prove that the low ENDOR signal intensity can not be attributed to spectral diffusion mechanisms competing with ENDOR mechanisms. Instead, we find that an unfavorable ratio of the electron and nuclear spin relaxation rates results in small local ENDOR signals. The matrix line dominates the spectrum because of the large number of matrix protons. In the pulsed ENDOR experiment, the hyperfine contrast selectivity mechanism suppresses the intensity of the matrix ENDOR signal and enhances the amplitudes of the local ENDOR signals. In addition, the ENDOR signal is not a function of the ratio of the electron and nuclear relaxation rates.     

The solution conformation of triarylmethyl radicals

Hyperfine coupling tensors to 1H, 2H, and natural abundance 13C were measured using X-band pulsed electron nuclear double resonance (ENDOR) spectroscopy for two triarylmethyl (trityl) radicals used in electron paramagnetic resonance imaging and oximetry: methyl tris(8-carboxy-2,2,6,6-tetramethyl-benzo[1,2d:4,5-d']bis(1,3)dithiol-4-yl) and methyl tris(8-carboxy-2,2,6,6-tetramethyl(-d3)-benzo[1,2d:4,5-d']bis(1,3)dithiol-4-yl). Quantum chemical calculations using density functional theory predict a structure that reproduces the experimentally determined hyperfine tensors. The radicals are propeller-shaped with the three aryl rings nearly mutually orthogonal. The central carbon atom carrying most of the unpaired electron spin density is surrounded by the sulfur atoms in the radical and is completely shielded from solvent. This structure explains features of the electron spin relaxation of these radicals and suggests ways in which the radicals can be chemically modified to improve their characteristics for imaging and oximetry.     

Matrix ENDOR of ion-exchange systems

Electron nuclear double resonance (ENDOR) has been investigated in sulfocation exchangers, containing free radicals stabilized in polymeric matrix or Cu²⁺ and (VO)²⁺ as counterions. It was shown that the ENDOR signal is mainly due to electron-nuclear dipole-dipole interactions between the unpaired electron and nuclei of polymeric matrix or hydrogen atoms of water molecules which hydrate the charge groups. In order to quantitatively describe the ENDOR line shape and intensity, the theory of matrix ENDOR is developed. The correctness of this theory was tested by comparing the temperature dependence of spin-lattice relaxation times calculated from ENDOR line intensities with the corresponding dependence obtained from stationary saturation electron spin resonance spectra. A good agreement was observed in the temperature range from 200 to 350 K. The structural parameters of surroundings of paramagnetic ions Cu²⁺ and (VO)²⁺, which include four coordinated spheres on the distance from 0.3 to 1.2 nm, were calculated. The motional parameters, correlation time and activation energy of mobile protons were also determined. It is concluded that the activation processes of water self-diffusion and proton exchange take place at high temperature, whereas the proton tunneling transfer is possible at low temperature.     

A Multi-Frequency EPR and ENDOR Study of Rh and Ir Complexes in Alkali and Silver Halides

Aliovalent Rh and Ir cations have been frequently used to influence the photographic properties of silver halide emulsions. The doping introduces several types of related defects with distinct trapping and recombination properties. EPR and ENDOR are, in principle, ideally suited for the determination of the microscopic structure of the individual centres but it will be demonstrated that well-chosen, sometimes sophisticated multi-frequency experiments are necessary in order to (partially) reach this goal. Model studies on single crystals of AgCl and NaCl also appeared indispensable for the unravelling of the spectra.

In the review of Rh-centres in NaCl and AgCl special attention is paid to methods that allow to detect cation vacancies near Rh^2? complexes. An alternative explanation for the high temperature behaviour of the [RhCl₆]^4? complexes in AgCl is presented.     

General formulae for hyperfine structure and endor frequencies

General expressions for hyperfine (superhyperfine) structure and ENDOR frequencies are derived. It is only assumed that the hyperfine (superhyperfine) interaction is much smaller than the electronic spin transition energy but the electronic spin-Hamiltonian may be of arbitrary form. To obtain the ENDOR frequencies one need only know the electron spin matrix elements. These influence the direction and magnitude of the effective magnetic field which act on the nucleus.     

Investigation of the dynamical behaviour of the FH(CN−) centre in KCl with temperature dependent endor spectroscopy

Abstract

The temperature dependence of the electron nuclear double resonance (ENDOR) spectra of F_H(CN^?) centres in KCl was investigated in the temperature range between 10–220 K. At the lowest temperature of 10 K only one CN^? orientation with respect to the F centre electron is present, in which the nitrogen is thought to be nearer to the F-electron than the carbon. With the very small thermal activation energy of 2.9 meV the opposite orientation is occupied. The superhyperfine interactions of those first shell K nuclei nearest to CN^? and of the ¹³C interaction of the CN^? molecule are strongly temperature dependent between 10 and 60 K, following an exponential law with a thermal activation energy of 4.2 meV. It is assumed that a soft local mode involving those two nearest K nuclei and the CN^? is causing the strong temperature dependence. The shf interactions of ¹⁴N nuclei have not been seen, probably because of the dynamical effects.     

Design of a permanent magnet with a mechanical sweep suitable for variable-temperature continuous-wave and pulsed EPR spectroscopy

A magnetic system is introduced which consists of three nested rings of permanent magnets of a Halbach dipolar layout and is capable for EPR spectroscopy. Two of the rings can be rotated independently to adjust the magnetic flux in the center and even allow for mechanical field sweeps. The presented prototype achieves a magnetic flux range of 0.0282–0.3013 T with a minimal sweep of 0.15 mT and homogeneity of about 10⁻³.First applications with CW and pulsed Mims ENDOR as well as ESEEM experiments on a sample of a glycine single crystal doped with 1% copper nitrate demonstrate that flux range, sweep accuracy and homogeneity of this prototype is sufficient for EPR experiments on most solid samples.Together with a recently improved design magnets can be build which could serve as compact and easily transportable replacement of standard electromagnets with negligible consumption of power or coolants.     

The ups and downs of Feher-style ENDOR

A new transient variation of the “Feher-style” electron-nuclear double resonance (ENDOR) method is examined. In this technique, the passage-mode electron paramagnetic resonance (EPR) signal is monitored following the application of a pulsed radio frequency (RF). Continuous-wave and transient proton ENDOR experiments have been conducted on the nonheme iron center from the protein nitrile hydratase. These experiments show that the transient ENDOR signal response exhibits a complex response with multiple phases in the time evolution of the ENDOR signal. Both increases and decreases in the passage-mode EPR signals are observed at different times following the RF pulse that induces an ENDOR transition. A simple model based on a packet-shifting ENDOR mechanism for a nonadiabatic passage EPR signal is proposed. This model describes many of the features seen in the transient ENDOR experiments and provides new insight into the traditional Feher-style ENDOR measurements. This new model shows that a packet-shifting mechanism can account for many of the “negative ENDOR” effects commonly seen in Feher-style ENDOR, which suggests that more exotic ENDOR mechanisms may not be required to explain these observations.     

Combining steady-state and dynamic methods for determining absolute signs of hyperfine interactions: pulsed ENDOR Saturation and Recovery (PESTRE)

The underlying causes of asymmetric intensities in Davies pulsed ENDOR spectra that are associated with the signs of the hyperfine interaction are reinvestigated. The intensity variations in these asymmetric ENDOR patterns are best described as shifts in an apparent baseline intensity that occurs dynamically following on-resonance ENDOR transitions. We have developed an extremely straightforward multi-sequence protocol that is capable of giving the sign of the hyperfine interaction by probing a single ENDOR transition, without reference to its partner transition. This technique, Pulsed ENDOR Saturation and Recovery (PESTRE) monitors dynamic shifts in the 'baseline' following measurements at a single RF frequency (single ENDOR peak), rather than observing anomalous ENDOR intensity differences between the two branches of an ENDOR response. These baseline shifts, referred to as dynamic reference levels (DRLs), can be directly tied to the electron-spin manifold from which that ENDOR transition arises. The application of this protocol is demonstrated on (57)Fe ENDOR of a 2Fe-2S ferredoxin. We use the (14)N ENDOR transitions of the S = 3/2[Fe(II)NO](2+) center of the non-heme iron enzyme, anthranilate dioxygenase (AntDO) to examine the details of the relaxation model using PESTRE.