ENDOR Research Articles

Strong isotopic effect in the electron-mediated nuclear-nuclear interaction in solids

We studied the nuclear-nuclear spin interaction mediated by an unpaired electron spin, focusing on an isotopic effect by electron nuclear double resonance (ENDOR) spectroscopy. We investigated a linear cluster Ga-Ti3+-Ga in titanium-doped gallium oxyde β − Ga2O3, whereby the unpaired electron spin density of Ti3+ is equally delocalized on the nuclear spins I = 3/2 of nearest-neighboring 69Ga and 71Ga nuclei. The linear geometry of the spin arrangement allowed us to easily identify the ENDOR spectra for the three possible isotopic configurations: 69Ga-Ti-71Ga 69Ga-Ti-69Ga and 71Ga-Ti-71Ga. Despite the magnetic moments of 71Ga and 69Ga nuclei differing by only by 27%, the experimental effect of the electron-mediated nuclear-nuclear interaction (pseudodipolar interaction) on the ENDOR spectra is one order of magnitude larger ( 1 MHz) for the symmetrical clusters (69Ga-Ti-69Ga and 71Ga-Ti-71Ga, respectively) than for the asymmetrical cluster 69Ga-Ti-71Ga (<0.1 MHz). This important isotopic effect in the internuclear interaction is a consequence of the cluster symmetry with a local inversion center for the symmetrical configurations, which is lacking in the asymmetrical configuration. These symmetrical clusters thus combine a resolved nuclear-nuclear spin interaction, a nuclear spin monitoring by an unpaired electron, and a large nuclear spin quantum register, which make them attractive for quantum information processing whereby nuclear qubits can be monitored by short selective radiofrequency pulses.

ISC-like [2Fe–2S] ferredoxin (FdxB) dimer from Pseudomonas putida JCM 20004: structural and electron–nuclear double resonance characterization

The crystal structure of the ISC-like [2Fe-2S] ferredoxin (FdxB), probably involved in the de novo iron-sulfur cluster biosynthesis (ISC) system of Pseudomonas putida JCM 20004, was determined at 1.90-Å resolution and displayed a novel tail-to-tail dimeric form. P. putida FdxB lacks the consensus free cysteine usually present near the cluster of ISC-like ferredoxins, indicating its primarily electron transfer role in the iron-sulfur cluster. Orientation-selective electron-nuclear double resonance spectroscopic analysis of reduced FdxB in conjunction with the crystal structure has identified the innermost Fe2 site with a high positive spin population as the nonreducible iron retaining the Fe(3+) valence and the outermost Fe1 site as the reduced iron with a low negative spin density. The average g (max) direction is skewed, forming an angle of about 27.3° (±4°) with the normal of the [2Fe-2S] plane, whereas the g (int) and g (min) directions are distributed in the cluster plane, presumably tilted by the same angle with respect to this plane. These results are related to those for other [2Fe-2S] proteins in different electron transport chains (e.g. adrenodoxin) and suggest a significant distortion of the electronic structure of the reduced [2Fe-2S] cluster under the influence of the protein environment around each iron site in general.

An ENDOR and HYSCORE Investigation of a Reaction Intermediate in IspG (GcpE) Catalysis

IspG is a 4Fe-4S protein that carries out an essential reduction step in isoprenoid biosynthesis. Using electron-nuclear double resonance (ENDOR) and hyperfine sublevel correlation (HYSCORE) spectroscopies on labeled samples, we have specifically assigned the hyperfine interactions in a reaction intermediate. These results help clarify the nature of the reaction intermediate, supporting a direct interaction between the unique fourth Fe in the cluster and C2 and O3 of the ligand.

Electrical Detection of Coherent Nuclear Spin Oscillations in Phosphorus-Doped Silicon using Pulsed ENDOR

We demonstrate the electrical detection of pulsed X-band electron nuclear double resonance (ENDOR) in phosphorus-doped silicon at 5 K. A pulse sequence analogous to Davies ENDOR in conventional electron spin resonance is used to measure the nuclear spin transition frequencies of the (31)P nuclear spins, where the (31)P electron spins are detected electrically via spin-dependent transitions through Si/SiO(2) interface states, thus not relying on a polarization of the electron spin system. In addition, the electrical detection of coherent nuclear spin oscillations is shown, demonstrating the feasibility to electrically read out the spin states of possible nuclear spin qubits.

Perylene Diimide: Versatile Building Blocks for Molecular Self-Assemblies, Folding Motifs, and Reaction-Directing Codes

Perylene diimides (PDIs) have been playing important roles in several active fields. Therefore, we will first review the current methods to modify the skeleton of perylene diimides. Next, PDI self-assemblies and PDI folded nanostructures and chemically restrained PDI nanostructures are presented. Unusual physical properties due to nanostructures formation are discussed including the absorption band intensity reversal and π-stack red-fluorescence emission. Because of interesting π-stacked structures, PDI derivatives are considered as potential candidates for photovoltaic applications and organic electronics. More importantly, π-stacking can be tuned when the dihedral angle of the two naphthalene planes varies, thus resulting in adjusted interacting forces. Such cooperative forces lead to the discovery of molecular code phenomena, which will have profound impact an future bi-molecular reaction designs. Keywords: Perylene, self-assembly, folding, Franck-Condon, fluorescence, PDI aromatic core, dibenzocoronene tetracarboxydiimide, nitrogen-fused heterocoronene tetracarboxydiimide, palladium-catalyzed dehydrohalogenation, PDI-DNA Conjugates, Duplex Formation, Hairpin Formation, organic thin-film transistors, photochromic spiropyran, photoswitchable nanoprobes, electron nuclear double resonance (ENDOR)

Oxygen vacancies adjacent to Cu2+ ions in TiO2 (rutile) crystals

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) are used to characterize Cu2+ ions substituting for Ti4+ ions in nominally undoped TiO2 crystals having the rutile structure. Illumination at 25 K with 442 nm laser light reduces the concentration of Cu2+ ions by more than a factor of 2. The laser light also reduces the EPR signals from Fe3+ and Cr3+ ions and introduces signals from Ti3+ ions. Warming in the dark to room temperature restores the crystal to its preilluminated state. Monitoring the recovery of the photoinduced changes in the Cu2+ ions and the other paramagnetic electron and hole traps as the temperature is raised from 25 K to room temperature provides evidence that the Cu2+ ions have an adjacent doubly ionized oxygen vacancy. These oxygen vacancies serve as charge compensators for the substitutional Cu2+ ions and lead to the formation of electrically neutral Cu2+-VO complexes during growth of the crystals. The Cu2+-VO complexes act as electron traps and convert to nonparamagnetic Cu+-VO complexes when the crystals are illuminated at low temperature. Complete sets of spin-Hamiltonian parameters describing the electron Zeeman, hyperfine, and nuclear electric quadrupole interactions for both the 63Cu and 65Cu nuclei are obtained from the EPR and ENDOR data. This study suggests that other divalent cation impurities in TiO2 such as Co2+ and Ni2+ may also have an adjacent oxygen vacancy for charge compensation.

Electron nuclear double resonance study of photostimulated luminescence active centers in CsBr:Eu2+medical imaging plates

${\text{CsBr:Eu}}^{2+}$ needle image plates exhibit an electron-paramagnetic-resonance (EPR) spectrum at room temperature (RT), whose intensity is correlated with the photostimulated luminescence sensitivity of the plate. This EPR spectrum shows a strong temperature dependence: At RT it is owing to a single ${\text{Eu}}^{2+}$ ($S$ $=7/2$) center with axial symmetry, whereas at $Tl35$ K the spectra can only be explained when two distinct centers are assumed to be present, a minority axial center and a majority center with nearly extremely rhombic symmetry. In this paper these low-temperature centers are studied with electron nuclear double resonance (ENDOR) spectroscopy, which reveals the presence of $^{1}\mathrm{H}$ nuclei close to the central ${\text{Eu}}^{2+}$ ions in the centers. Analysis of the angular dependence of the ENDOR spectra allows to propose models for these centers, providing an explanation for the observed difference in intensity between the spectral components and for their temperature dependence.

Hydration Structure of the Ti(III) Cation as Revealed by Pulse EPR and DFT Studies: New Insights into a Textbook Case

The (17)O and (1)H hyperfine interactions of water ligands in the Ti(III) aquo complex in a frozen solution were determined using Hyperfine Sublevel Correlation (HYSCORE) and Pulse Electron Nuclear Double Resonance (ENDOR) spectroscopies at 9.5 GHz. The isotropic hyperfine interaction (hfi) constant of the water ligand (17)O was found to be about 7.5 MHz. (1)H Single Matched Resonance Transfer (SMART) HYSCORE spectra allowed resolution of the hfi interactions of the two inequivalent water ligand protons and the relative orientations of their hfi tensors. The magnetic and geometrical parameters extracted from the experiments were compared with the results of DFT computations for different geometrical arrangements of the water ligands around the cation. The theoretical observable properties (g tensor (1)H and (17)O hfi tensors and their orientations) of the [Ti(H(2)O)(6)](3+) complex are in quantitative agreement with the experiments for two slightly different geometrical arrangements associated with D(3d) and C(i) symmetries.

Dynamic Hydrogen-Bonding Network in the Distal Pocket of the Nitrosyl Complex of Pseudomonas aeruginosa cd1 Nitrite Reductase

cd(1) nitrite reductase (NIR) is a key enzyme in the denitrification process that reduces nitrite to nitric oxide (NO). It contains a specialized d(1)-heme cofactor, found only in this class of enzymes, where the substrate, nitrite, binds and is converted to NO. For a long time, it was believed that NO must be released from the ferric d(1)-heme to avoid enzyme inhibition by the formation of ferrous-nitroso complex, which was considered as a dead-end product. However, recently an enhanced rate of NO dissociation from the ferrous form, not observed in standard b-type hemes, has been reported and attributed to the unique d(1)-heme structure (Rinaldo, S.; Arcovito, A.; Brunori, M.; Cutruzzolà, F. J. Biol. Chem. 2007, 282, 14761-14767). Here, we report on a detailed study of the spatial and electronic structure of the ferrous d(1)-heme NO complex from Pseudomonas aeruginosa cd(1) NIR and two mutants Y10F and H369A/H327A in solution, searching for the unique properties that are responsible for the relatively fast release. There are three residues at the "distal" side of the heme (Tyr(10), His(327), and His(369)), and in this work we focus on the identification and characterization of possible H-bonds they can form with the NO, thereby affecting the stability of the complex. For this purpose, we have used high field pulse electron-nuclear double resonance (ENDOR) combined with density functional theory (DFT) calculations. The DFT calculations were essential for assigning and interpreting the ENDOR spectra in terms of geometric structure. We have shown that the NO in the nitrosyl d(1)-heme complex of cd(1) NIR forms H-bonds with Tyr(10) and His(369), whereas the second conserved histidine, His(327), appears to be less involved in NO H-bonding. This is in contrast to the crystal structure that shows that Tyr(10) is removed from the NO. We have also observed a larger solvent accessibility to the distal pocket in the mutants as compared to the wild-type. Moreover, it was shown that the H-bonding network within the active site is dynamic and that a change in the protonation state of one of the residues does affect the strength and position of the H-bonds formed by the others. In the Y10F mutant, His(369) is closer to the NO, whereas mutation of both distal histidines displaces Tyr(10), removing its H-bond. The implications of the H-bonding network found in terms of the complex stability and catalysis are discussed.

Identification of shallow Al donors in ZnO

AbstractA combined magnetic resonance, photoluminescence, photoconductivity, and Raman scattering study of ZnO is presented. Electron paramagnetic resonance (EPR) and electron‐nuclear double resonance (ENDOR) spectroscopy identify substitutional Al as a binding core of a shallow, effective‐mass‐like donor in ZnO. Based on the correlation between the EPR and photoluminescence data it is shown that recombination of an exciton bound to Al gives rise to the 3360.7 meV photoluminescence line (I6). A $1{\rm s}\to 2{\rm p}$ donor transition at 316 cm−1 is detected in photoconductivity and Raman spectra.

Identification of primary free radicals in trehalose dihydrate single crystals X-irradiated at 10 K

Primary free radical formation in trehalose dihydrate single crystals X-irradiated at 10 K was investigated at the same temperature using X-band Electron Paramagnetic Resonance (EPR), Electron Nuclear Double Resonance (ENDOR) and ENDOR-induced EPR (EIE) techniques. The ENDOR results allowed the unambiguous determination of six proton hyperfine coupling (HFC) tensors. Using the EIE technique, these HF interactions were assigned to three different radicals, labeled R1, R2 and R3. The anisotropy of the EPR and EIE spectra indicated that R1 and R2 are alkyl radicals (i.e. carbon-centered) and R3 is an alkoxy radical (i.e. oxygen-centered). The EPR data also revealed the presence of an additional alkoxy radical species, labeled R4. Molecular modeling using periodic Density Functional Theory (DFT) calculations for simulating experimental data suggests that R1 and R2 are the hydrogen-abstracted alkyl species centered at C5' and C5, respectively, while the alkoxy radicals R3 and R4 have the unpaired electron localized mainly at O2 and O4'. Interestingly, the DFT study on R4 demonstrates that the trapping of a transferred proton can significantly influence the conformation of a deprotonated cation. Comparison of these results with those obtained from sucrose single crystals X-irradiated at 10 K indicates that the carbon situated next to the ring oxygen and connected to the CH(2)OH hydroxymethyl group is a better radical trapping site than other positions.

ChemInform Abstract: EPR and ENDOR Studies of Shallow Donors in SiC

AbstractReview: 85 refs.

Structural Characterization of a High Affinity Mononuclear Site in the Copper(II)-α-Synuclein Complex

Human α-Synuclein (aS), a 140 amino acid protein, is the main constituent of Lewy bodies, the cytoplasmatic deposits found in the brains of Parkinson's disease patients, where it is present in an aggregated, fibrillar form. Recent studies have shown that aS is a metal binding protein. Moreover, heavy metal ions, in particular divalent copper, accelerate the aggregation process of the protein. In this work, we investigated the high affinity binding mode of truncated aS (1-99) (aS99) with Cu(II), in a stoichiometric ratio, to elucidate the residues involved in the binding site and the role of copper ions in the protein oligomerization. We used Electron Paramagnetic Resonance spectroscopy on the Cu(II)-aS99 complex at pH 6.5, performing both multifrequency continuous wave experiments and pulsed experiments at X-band. The comparison of 9.5 and 95 GHz data showed that at this pH only one binding mode is present. To identify the nature of the ligands, we performed Electron Spin Echo Envelope Modulation, Hyperfine Sublevel Correlation Spectroscopy, and pulsed Davies Electron-Nuclear Double Resonance (Davies-ENDOR) experiments. We determined that the EPR parameters are typical of a type-II copper complex, in a slightly distorted square planar geometry. Combining the results from the different pulsed techniques, we obtained that the equatorial coordination is {N(Im), N(-), H(2)O, O}, where N(im) is the imino nitrogen of His50, N(-) a deprotonated amido backbone nitrogen that we attribute to His50, H(2)O an exchangeable water molecule, and O an unidentified oxygen ligand. Moreover, we propose that the free amino terminus (Met1) participates in the complex as an axial ligand. The MXAN analysis of the XAS k-edge absorption data allowed us to independently validate the structural features proposed on the basis of the magnetic parameters of the Cu(II)-aS99 complex and then to further refine the quality of the proposed structural model.

Clock Shift and Interstate Coherence of Multi-Level Atoms

In contrast to two-level atoms, the interstate coherence of both three-level bosons and fermions contributes to the collisional frequency shift of internal transitions in a cold homogeneous gas. In particular, the measured contact shift of the b−c (a−d) hyperfine transition in 3D atomic hydrogen in the presence of incoherently populated state a (b) yields the difference at−as=30(5) pm between the triplet and singlet s-wave scattering lengths, which is a factor of 2 smaller than the previous result. The dynamic contact shift of the b−c resonance due to the Rabi oscillations between the coherently populated states a and b under the simultaneous continuous excitation of the a−b transition can explain the low-temperature electron-nuclear double resonance spectra of 2D hydrogen. The role of field inhomogeneity, spin diffusion, relaxation and recombination is discussed.

Electron−Nuclear and Electron−Electron Double Resonance Spectroscopies Show that the Primary Quinone Acceptor QA in Reaction Centers from Photosynthetic Bacteria Rhodobacter sphaeroides Remains in the Same Orientation Upon Light-Induced Reduction

Reaction centers (RCs) from the photosynthetic bacterium Rhodobacter (Rb.) sphaeroides R-26 exhibit changes in the recombination kinetics of the charge-separated radical-pair state, P(·+) Q(A)(·-), composed of the dimeric bacteriochlorophyll donor P and the ubiquinone-10 acceptor Q(A), depending on whether the RCs are cooled to cryogenic temperatures in the dark or under continuous illumination (Kleinfeld et al. Biochemistry 1984, 23, 5780-5786). Structural changes near redox-active cofactors have been postulated to be responsible for these changes in kinetics and to occur in the course of light-induced oxidation and reduction of the cofactors thereby assuring a high quantum yield. Here we investigated such potential light-induced structural changes, associated with the formation of P(·+) Q(A)(·-), via pulsed electron-nuclear double resonance (ENDOR) at Q-band (34 GHz) and pulsed electron-electron double resonance (PELDOR) at W-band (95 GHz). Two types of light excitation have been employed for which identical RC samples were prepared: (a) one sample was frozen in the dark and then illuminated to generate transient P(·+) Q(A)(·-), and (b) one was frozen under illumination which resulted in both trapped and transient P(·+) Q(A)(·-) at 80 K. The hyperfine interactions between Q(A)(·-) and the protein were found to be the same in RCs frozen in the dark as in RCs frozen under illumination. Furthermore, these interactions are completely consistent with those observed in RC crystals frozen in the dark. Thus, QA remains in its binding site with the same position and orientation upon reduction. This conclusion is consistent with the result of our orientation-resolving PELDOR experiments on transient P(·+) Q(A)(·-) radical pairs. However, these findings are incompatible with the recently proposed ~60° reorientation of Q(A) upon its photoreduction, as deduced from an analysis of Q-band quantum-beat oscillations (Heinen et al. J. Am. Chem. Soc. 2007, 129, 15935-15946). Such a large reorientation appears improbable, and our objections against this proposition are substantiated here in detail. Our results show that Q(A) is initially in an orientation that is favorable for its light-driven reduction. This diminishes the reorganization requirements for fast electron reduction and high quantum efficiency.

Giant titanium electron wave function in gallium oxide: A potential electron-nuclear spin system for quantum information processing

The hyperfine interactions of the unpaired electron with eight surrounding $^{69}\text{G}\text{a}$ and $^{71}\text{G}\text{a}$ nuclei in Ti-doped $\ensuremath{\beta}{\text{-Ga}}_{2}{\text{O}}_{3}$ were analyzed by electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopies. They are dominated by strong isotropic hyperfine couplings due to a direct Fermi contact interaction with Ga nuclei in octahedral sites of rutile-type chains oriented along $\mathbf{b}$ axis, revealing a large anisotropic spatial extension of the electron wave function. Titanium in $\ensuremath{\beta}{\text{-Ga}}_{2}{\text{O}}_{3}$ is thus best described as a diffuse $({\text{Ti}}^{4+}\text{\ensuremath{-}}{e}^{\ensuremath{-}})$ pair rather than as a localized ${\text{Ti}}^{3+}$. Both electron and $^{69}\text{G}\text{a}$ nuclear spin Rabi oscillations could be observed by pulsed EPR and pulsed ENDOR, respectively. The electron spin decoherence time is about $1\text{ }\ensuremath{\mu}\text{s}$ (at 4 K) and an upper bound of $520\text{ }\ensuremath{\mu}\text{s}$ (at 8 K) is estimated for the nuclear decoherence time. Thus, $\ensuremath{\beta}{\text{-Ga}}_{2}{\text{O}}_{3}:\text{Ti}$ appears to be a potential spin-bus system for quantum information processing with a large nuclear spin quantum register.

Electron Magnetic Resonance and Density Functional Theory Study of Room Temperature X-Irradiated β-d-Fructose Single Crystals

Stable free radical formation in fructose single crystals X-irradiated at room temperature was investigated using Q-band electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and ENDOR induced EPR (EIE) techniques. ENDOR angular variations in the three main crystallographic planes allowed an unambiguous determination of 12 proton HFC tensors. From the EIE studies, these hyperfine interactions were assigned to six different radical species, labeled F1-F6. Two of the radicals (F1 and F2) were studied previously by Vanhaelewyn et al. [Vanhaelewyn, G. C. A. M.; Pauwels, E.; Callens, F. J.; Waroquier, M.; Sagstuen, E.; Matthys, P. J. Phys. Chem. A 2006, 110, 2147.] and Tarpan et al. [Tarpan, M. A.; Vrielinck, H.; De Cooman, H.; Callens, F. J. J. Phys. Chem. A 2009, 113, 7994.]. The other four radicals are reported here for the first time and periodic density functional theory (DFT) calculations were used to aid their structural identification. For the radical F3 a C3 carbon centered radical with a carbonyl group at the C4 position is proposed. The close similarity in HFC tensors suggests that F4 and F5 originate from the same type of radical stabilized in two slightly different conformations. For these radicals a C2 carbon centered radical model with a carbonyl group situated at the C3 position is proposed. A rather exotic C2 centered radical model is proposed for F6.

Structure and properties of CO2– centers in biological and synthetic hydroxyapatite

Biological and synthetic carbonate-containing hydroxyapatites (HAP) were exposed to γ- rays and studied by the electron paramagnetic resonance (EPR) and the electron-nuclear double resonance (ENDOR). Two types of CO2− radicals - axial and orthorhombic ones - were shown to be the dominating γ-induced paramagnetic centers in the abovementioned materials. The study of metastable radiation-induced centers allowed to identify these defects with molecular ions CO33− in B position. The mechanism of CO2− radical formation was proposed to be CO32−→ CO33−→ CO2−.

Self-Trapped Excitons in Ionic-Covalent Silver Halide Crystals and Nanostructures: High-Frequency EPR, ESE, ENDOR and ODMR Studies.

Silver halides have unique features in solid state physics because their properties are considered to be of borderline nature between ionic and covalent bonding. In AgCl, the self-trapped hole (STH) is centered and partly trapped in the cationic sublattice, forming an Ag2+ ion inside of a (AgCl6)4− complex as a result of the Jahn–Teller distortion. The STH in AgCl can capture an electron from the conduction band forming the self-trapped exciton (STE). Recent results of a study of STE by means of high-frequency electron paramagnetic resonance, electron spin echo, electron–nuclear double resonance (ENDOR) and optically detected magnetic resonance (ODMR) are reviewed. The properties of the STE in AgCl crystals, such as exchange coupling, the ordering of the triplet and singlet sublevels, the dynamical properties of the singlet and triplet states, and the hyperfine interaction with the Ag and Cl (Br) nuclei are discussed. Direct information about the spatial distribution of the wave function of STE unpaired electrons was obtained by ENDOR. From a comparison with the results of an ENDOR study of the shallow electron center and STH, it is concluded that the electron is mainly contained in a hydrogen-like 1s orbital with a Bohr radius of 15.1 ± 0.6 Å, but near its center the electron density reflects the charge distribution of the hole. The hole of the STE is virtually identical to an isolated STH center. For AgCl nanocrystals embedded into the KCl crystalline matrix, the anisotropy of the g-factor of STE and STH was found to be substantially reduced compared with that of bulk AgCl crystals, which can be explained by a considerable suppression of the Jahn–Teller effect in nanoparticles. A study of ODMR in AgBr nanocrystals in KBr revealed spatial confinement effects and allowed estimating the nanocrystal size from the shape of the ODMR spectra.

Structural Phase Transitions and Thermal Hysteresis in the Metal−Organic Framework Compound MIL-53 As Studied by Electron Spin Resonance Spectroscopy

The dehydrated aluminum form of the metal−organic framework compound MIL-53 shows a temperature-driven phase transition with pronounced structural hysteresis as recently shown by neutron diffraction and scattering experiments (Liu et al. J. Am. Chem. Soc. 2008, 130, 11813). Thereby, the structure of the corner-sharing metal AlO4(OH)2 octahedra differs in terms of local symmetry for the dehydrated MIL-53(Al) material in its high- and low-temperature form with open or closed pore structure, respectively. In this work, some of the framework aluminum ions were exchanged by chromium(III) to introduce an electron spin resonance (ESR) active probe ion. The resulting material was investigated by means of ESR and electron nuclear double resonance (ENDOR) spectroscopy to verify the incorporation of chromium at the octahedral framework sites. In addition, variable-temperature ESR measurements were performed to analyze the temperature-dependent phase behavior of the doped MIL-53 material. The Cr(III) ions have an ele...