Uniform Field Re-entrant Cylindrical TE$$_{01\text {U}}$$ Cavity for Pulse Electron Paramagnetic Resonance Spectroscopy at Q-band

Uniform field (UF) resonators create a region-of-interest, where the sample volume receives a homogeneous microwave magnetic field (\(B_1\)) excitation. However, as the region-of-interest is increased, resonator efficiency is reduced. In this work, a new class of uniform field resonators is introduced: the uniform field re-entrant cylindrical TE\(_{\text {01U}}\) cavity. Here, a UF cylindrical TE\(_{\text {01U}}\) cavity is designed with re-entrant fins to increase the overall resonator efficiency to match the resonator efficiency maximum of a typical cylindrical TE\(_{011}\) cavity. The new UF re-entrant cylindrical TE\(_{\text {01U}}\) cavity is designed for Q-band (34 GHz) and is calculated to have the same electron paramagnetic resonance (EPR) signal intensity as a TE\(_{011}\) cavity, a 60% increase in average resonator efficiency \(\Lambda _\mathrm{ave}\) over the sample, and has a \(B_1\) profile that is 79.8% uniform over the entire sample volume (98% uniform over the region-of-interest). A new H-type T-junction waveguide coupler with inductive obstacles is introduced that increases the dynamic range of a movable short coupler while reducing the frequency shift by 43% during over-coupling. The resonator assembly is fabricated and tested both on the bench and with EPR experiments. This resonator provides a template to improve EPR spectroscopy for pulse experiments at high frequencies.     

Crucial role of paramagnetic ligands for magnetostructural anomalies in "breathing crystals"

Breathing crystals based on polymer-chain complexes of Cu(hfac)(2) with nitroxides exhibit thermally and light-induced magnetostructural anomalies in many aspects similar to a spin crossover. In the present work, we report the synthesis and investigation of a new family of Cu(hfac)(2) complexes with tert-butylpyrazolylnitroxides and their nonradical structural analogues. The complexes with paramagnetic ligands clearly exhibit structural rearrangements in the copper(II) coordination units and accompanying magnetic phenomena characteristic for breathing crystals. Contrary to that, their structural analogues with diamagnetic ligands do not undergo rearrangements in the copper(II) coordination environments. This confirms experimentally the crucial role of paramagnetic ligands and exchange interactions between them and copper(II) ions for the origin of magnetostructural anomalies in this family of molecular magnets.     

Concepts in high-frequency EPR — Applications to bio-inorganic systems

In this contribution we describe a high-frequency high-field EPR facility which has been developed at the University of Nijmegen. We present the design of a heterodyne quasi-optical bridge based on a millimeter-wave vector network analyzer as source and detection system. The mm-waves are transported in free-space through Gaussian beam optic elements and through a corrugated guide inside the resonator insert. The Fabri-Pérot (TEM_00n) resonator is coupled through a metallic mesh and because of its bimodal property it can be operated using orthogonal detection leading to substantial improvement in sensitivity. In the first stage of the project, a multifrequency CW-facility is realized covering the 100–500 GHz range. In our initial explorative experiments we demonstrate the advantages of HF-EPR of high-spin systems: Due to the large microwave quantum, transitions which would be undetectable at X-band due to the large zerofield splitting can now be observed in good sensitivity. As a model for biological high-spin systems a sample of metmyoglobin was measured at D-band (130 GHz). Theg = 5.9 perpendicular line from theS = 5/2 ferric heme was detected and its line-width was compared to data previously obtained at Q-, X-, S- and L-band. As a model for biological integer spin systems theS = 1 signal of Ni(II) in nickel Tutton salt (Ni(NH₄)₂(SO₄)₂) was studied at 35 and 130 GHz.     

EPR studies of15N- and2H-substituted pH-sensitive spin probes of imidazoline and imidazolidine types

The isotopically substituted analogs of pH-sensitive imidazoline and imidazolidine radicals have been synthesized and investigated with electron paramagnetic resonance (EPR) spectroscopy. The introduction of²H and¹⁵N into the structure of the radical is a useful approach to enhance the information obtained from spin-labeling experiments. The spectra of the radicals have been analyzed with 9.8 (X-band) and 130 GHz (D-band) EPR spectroscopy. The substitution of¹H for²H leads to significant narrowing of Gaussian line width, while the substitution of¹⁴N for¹⁵N in the nitroxyl fragment decreases both the number of spectral lines and Lorentzian line width. These effects result in a significant increase in the peak intensities up to 5–7 times for X-band EPR spectra of one of the imidazoline radicals (R4). The increase in spectral resolution allowed us to reveal the hyperfine interaction splitting with the attached proton (0.36 G) in the protonated form of the radical R4. The influence of proton exchange of the radicals with phosphate and acetate buffers on their EPR spectra has been studied in H₂O and D₂O. The corresponding rate constants of the proton exchange have been calculated from fitting of the simulated EPR spectra line shapes to experimental spectra. The data obtained demonstrated the advantages of the isotopically substituted spin pH probes in spectral resolution and sensitivity which can be an important factor particularly for applications in vivo where the fundamental sensitivity is much lower. The sensitivity of EPR spectra of these spin probes to the buffer capacity could be of practical importance taking into account the biological relevance of monitoring this parameter in some pathological states.     

EPR/ENDOR, Mössbauer, and Quantum-Chemical Investigations of Diiron Complexes Mimicking the Active Oxidized State of [FeFe]Hydrogenase

Understanding the catalytic process of the heterolytic splitting and formation of molecular hydrogen is one of the key topics for the development of a future hydrogen economy. With an interest in elucidating the enzymatic mechanism of the [Fe(2)(S(2)C(2)H(4)NH)(CN)(2)(CO)(2)(μ-CO)] active center uniquely found in [FeFe]hydrogenases, we present a detailed spectroscopic and theoretical analysis of its inorganic model [Fe(2)(S(2)X)(CO)(3)(dppv)(PMe(3))](+) [dppv = cis-1,2-bis(diphenylphosphino)ethylene] in two forms with S(2)X = ethanedithiolate (1edt) and azadithiolate (1adt). These complexes represent models for the oxidized mixed-valent Fe(I)Fe(II) state analogous to the active oxidized "H(ox)" state of the native H-cluster. For both complexes, the (31)P hyperfine interactions were determined by pulse electron paramagnetic resonance and electron nuclear double resonance (ENDOR) methods. For 1edt, the (57)Fe parameters were measured by electron spin-echo envelope modulation and M?ssbauer spectroscopy, while for 1adt, (14)N and selected (1)H couplings could be obtained by ENDOR and hyperfine sublevel correlation spectroscopy. The spin density was found to be predominantly localized on the Fe(dppv) site. This spin distribution is different from that of the H-cluster, where both the spin and charge densities are delocalized over the two Fe centers. This difference is attributed to the influence of the "native" cubane subcluster that is lacking in the inorganic models. The degree and character of the unpaired spin delocalization was found to vary from 1edt, with an abiological dithiolate, to 1adt, which features the authentic cofactor. For 1adt, we find two (14)N signals, which are indicative for two possible isomers of the azadithiolate, demonstrating its high flexibility. All interaction parameters were also evaluated through density functional theory calculations at various levels.     

An ESEEM investigation of single crystals and powders of copper-dopedl-histidine hydrochloride monohydrate

In this paper we present an investigation of the remote nitrogen of imidazole in copper-dopedl-histidine hydrochloride monohydrate, which is a model system for a wide range of biologically important copper-imidazole complexes. Since these systems are mostly not available in a crystalline form it is interesting to investigate to what extent information that can be obtained from these disordered systems using ESEEM techniques is reliable. From single crystal ESEEM and HYSCORE experiments we have determined the hyperfine and quadrupole coupling tensors of the remote nitrogen in this model system. Additionally, we determined information on these tensors from powdered material, independent of the single crystal results, using orientation selective multifrequency and two-dimensional experiments. The availability of the coupling tensors from the single crystal investigation allowed an assessment of the applicability of the employed powder techniques to this important class of systems. The two-dimensional stimulated echo experiment was found to be a very useful spectroscopic tool for the study of these systems when combined with simulations.