Quadrupole Shift Research Articles

The electric quadrupole moment of molecular hydrogen ions and their potential for a molecular ion clock

The systematic shifts of the transition frequencies in the molecular hydrogen ions are of relevance to ultra-high-resolution radio-frequency, microwave and optical spectroscopy of these systems, performed in ion traps. We develop the ab-initio description of the interaction of the electric quadrupole moment of this class of molecules with the static electric field gradients present in ion traps. In good approximation, it is described in terms of an effective perturbation hamiltonian. An approximate treatment is then performed in the Born-Oppenheimer approximation. We give an expression of the electric quadrupole coupling parameter valid for all hydrogen molecular ion species and evaluate it for a large number of states of H2+, HD+, and D2+. The systematic shifts can be evaluated as simple expectation values of the perturbation hamiltonian. Results on radio-frequency (M1), one-photon electric dipole (E1) and two-photon E1 transitions between hyperfine states in HD+ are reported. For two-photon E1 transitions between rotationless states the shifts vanish. For a subset of rovibrational one-photon transitions the quadrupole shifts range from 0.2 to 10 Hz for an electric field gradient of 0.1 GV/m2. We point out an experimental procedure for determining the quadrupole shift which will allow reducing its contribution to the uncertainty of unperturbed rovibrational transition frequencies to the 1.10^(-15) relative level and, for selected transitions, even below it. The combined contributions of black-body radiation, Zeeman, Stark and quadrupole effects are considered for a large set of transitions and it is estimated that the transition frequency uncertainty of selected transitions can be reduced below the 1.10^(-15) level.

Mechanosynthesis, magnetic and Mössbauer characterization of pure and Ti4+-doped cubic phase BiFeO3 nanocrystalline particles

The mechanosynthesis of cubic γ -phase pure BiFeO3 and Ti 4+ -doped BiFeO3 nanocrystalline particles and their preliminary characterization with magnetic measure- ments and Mossbauer spectroscopy are reported. The BiFeO3 nanoparticles (5-40 nm) were prepared by heating a 48 h pre-milled 1:1 molar mixture of α-Bi2O3 and α-Fe2O3 at 400 ◦ C for (1 h). Doping α-Fe2O3in the initial mixture of reactants with Ti 4+ was found to lead to the formation of Ti 4+ -doped BiFeO3 nanoparticles by milling the reac- tants for 32 h. The magnetization of the BiFeO3 nanoparticles is found to be tripled under a maximum external field of 1.35 T and their magnetic hardness increases by ∼15 times relative to those of the corresponding bulk. The Ti 4+ -doped BiFeO3 nanoparticles exhibit higher magnetization relative to the pure ones. These observations are related to the spi- ral modulated spin structure of the compound. The Mossbauer data show ∼12 % of the BiFeO3 nanocrystalline particles to be superparamagnetic having blocking temperatures of less than 78 K. The quadrupole shift values of the magnetic spectral component favor the cubic structural symmetry. These observations were mainly associated with possible col- lective magnetic excitations as well as transverse relaxation of canted surface spins. The Ti-doped BiFeO3 nanoparticles gave statistically-poor Mossbauer spectra with no signs of a superparamagnetic behavior.

On the Track to Silica-Supported Tungsten Oxo Metathesis Catalysts: Input from 17O Solid-State NMR

The grafting of an oxo chloro trisalkyl tungsten derivative on silica dehydroxylated at 700 °C was studied by several techniques that showed reaction via W-Cl cleavage, to afford a well-defined precatalyst for alkene metathesis. This was further confirmed by DFT calculations on the grafting process. (17)O labeling of the oxo moiety of a series of related molecular and supported tungsten oxo derivatives was achieved, and the corresponding (17)O MAS NMR spectra were recorded. Combined experimental and theoretical NMR studies yielded information on the local structure of the surface species. Assessment of the (17)O NMR parameters also confirmed the nature of the grafting pathway by ruling out other possible grafting schemes, thanks to highly characteristic anisotropic features arising from the quadrupolar and chemical shift interactions.

Influence of RE3+ co-substitution on the structure and magnetic properties of Sr0.82RE0.18Fe12O19 (RE: La0.18−xPrx) ferrites

The influence of replacing Sr2+ ions by RE3+ ions (RE: La and Pr) on the structural and magnetic properties of Sr0.82(La0.18−xPrx)Fe12O19 hexaferrite prepared via autocombustion method is presented in this study. The effect of codoping La3+ (non-magnetic) and Pr3+ (magnetic) ions for Sr2+ in Sr-Ferrite is studied using X-ray diffraction, magnetometry, and 57Fe Mössbauer spectroscopy. The X-ray diffraction patterns show the samples are single phase hexaferrites with magnetoplumbite structure without other secondary phases. A continuous decline in saturation magnetization and an increase in coercivity is observed upon replacing La with Pr. Maximum coercivity, Hc∼5.97kOe, and Curie temperature, Tc∼732K was obtained for x=0.18 samples. A decline in hyperfine field values at all Fe3+ sites is observed with Pr3+ substitution. The quadrupole shift and isomer shift values of Fe3+ sites reflect subtle charge variations resulting upon Pr3+ substitution. It is found that the appropriate amount of La increases the saturation magnetization while Pr improves Hc and Tc as compared to undoped Sr-Ferrites.

Characterization and spectral studies of Co3+-doped Cd0.4Mn0.6Fe2O4 ferrites

Abstract A series of the Cd0.4Mn0.6CoxFe2−xO4 ferrites, 0≤x≤1, were prepared and studied by using X-ray patterns and Mossbauer and FMR spectra. The lattice parameter, mean ionic radii and hopping and bond lengths and edges are determined and discussed as functions of Co3+ content x. The obtained value of oxygen positional parameter was 0.397 for all samples. The recorded FMR and Mossbauer spectra indicated a ferrimagnetic nature of all the samples. The spin–spin relaxation time, the Lande factor (g), line width ∆H and the resonance field are affected by the Co3+ additions x. The Mossbauer spectra were analyzed to two sextets attributed to the Fe3+ ions at A- and B-sites. The quadrupole and isomer shift values are found to be independent on x, whereas the hyperfine magnetic fields, HA and HB, and Mossbauer line widths at the A- and B-sites, respectively, showed dependence on x. Also HA and HB were affected by the hopping and bond lengths at these sites.

Solid‐state NMR Spectroscopy of Quadrupolar Nuclei in Inorganic Chemistry

AbstractWe here review the principles and applications of solid‐state NMR spectroscopy of quadrupolar nuclei, with a special emphasis on structural studies of inorganic solids. Most NMR‐observable nuclei have spin I > 1/2, and possess a quadrupole moment. The resulting quadrupolar interaction severely broadens the resonances, but also encapsulates valuable information about the symmetry of the electronic surroundings of the observed nucleus. The effect of the quadrupolar interaction, as well as that of the chemical shift and dipolar interaction, on solid‐state NMR spectra is examined in this article. To regain good resolution, specifically designed NMR techniques exist to remove the quadrupolar broadening, i.e. overtone and MQMAS spectroscopy, the principles of which are outlined here. In addition, the possibility of distance measurements via the dipolar interaction using the REDOR technique is discussed. The combined information derived from distance measurements, quadrupolar and chemical shift parameters can be helpful for determination of the crystal structure, or for detection of impurity phases, as illustrated by surveying a number of case studies covering spin I = 1, 3/2, 5/2 and 7/2.

Evaluation of systematic shifts of the88Sr+single-ion optical frequency standard at the10−17level

An ion trap of the end-cap design was built recently at the National Research Council of Canada for improved control of the ${}^{88}$Sr${}^{+}$ single-ion optical frequency standard systematic shifts. The uncertainty on the micromotion-induced shifts is smaller by more than four orders of magnitude when compared to our previous trap system and reaches a fractional frequency uncertainty of $1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}18}$. To obtain this low uncertainty level, the micromotion is minimized with trim electrodes and the trap is operated at a special frequency at which there is anticorrelation between the second-order Doppler and Stark shifts. This choice of operating frequency, determined by the differential scalar polarizability of the clock transition, yields a suppression by a factor of $\ensuremath{\approx}$28 in the combined micromotion shifts. Like many optical frequency standards, the dominant source of uncertainty in the new trap is the blackbody radiation shift. Its uncertainty has been reduced by an order of magnitude with a recent theoretical evaluation of the differential scalar polarizability of the clock transition. The fractional blackbody shift uncertainty, estimated using a model of the blackbody field at the ion, is $2.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}17}$. The otherwise dominant electric quadrupole shift is reduced to below the $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}19}$ level with a cancellation method based on the average frequency of several pairs of Zeeman components. This method also cancels the tensor Stark shift and simplifies the description of the frequency shifts that are quantization-axis dependent. This paper provides a detailed description of the ${}^{88}$Sr${}^{+}$ optical frequency standard uncertainty evaluation and the methods used to make the standard robust against changes in the trap environment. The total fractional frequency uncertainty of the ${}^{88}$Sr${}^{+}$ ion for our current system is estimated at $2.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}17}$. We also discuss the uncertainty evaluation of a recently reported measurement of the ${}^{88}$Sr${}^{+}$ $S$-$D$ clock transition made over a 2-month period by comparison with a maser referenced to the SI second. The frequency measured is $444\phantom{\rule{0.16em}{0ex}}779\phantom{\rule{0.16em}{0ex}}044\phantom{\rule{0.16em}{0ex}}095\phantom{\rule{0.16em}{0ex}}485.5(9)$ Hz, with an uncertainty limited by the evaluation of the maser frequency.

14N–1H Heteronuclear Multiple-Quantum Correlation Magic-Angle Spinning NMR Spectroscopy of Organic Solids

Abstract 14N–1H heteronuclear multiple-quantum correlation (HMQC) solid-state magic-angle spinning (MAS) NMR spectra recorded at a 1H Larmor frequency of 850 MHz are presented for the dipeptide β-AspAla. A modified version of the pulse sequence presented by Gan et al. (Chem. Phys. Lett. 435 (2007) 163) that utilises rotary resonance recoupling (R3) at the n = 2 condition (ν 1 = 2ν R ) is employed. Spectra recorded with a short recoupling period (under 200 μs) show two correlation peaks corresponding to the NH and NH3 moieties in the dipeptide. The quadrupolar product, P Q = C Q √ [1 + (η Q 2/3)], is determined experimentally as 3.1 MHz (NH) and 1.0 MHz (NH3) by a comparison of the 14N and 15N isotropic chemical shifts which differ due to the isotropic second-order quadrupolar shift for the spin I = 1 14N nucleus. It is shown that the peak sensitivities increase markedly upon increasing the MAS frequency from 30 to 45 to 60 kHz due to a combination of the reduced residual dipolar broadening of the 1H resonances and a lengthening of the coherence lifetimes under R 3 recoupling. Increasing the recoupling period leads to the observation of additional peaks corresponding to longer range intra- and intermolecular NH proximities. Reasonable agreement is evident upon comparing the experimental build-up of correlation peak intensity to that observed for eight-spin density-matrix simulations.

Mössbauer studies of the magnetic phase transition in La(Fe0.88Si x Al0.12 − x )13 compounds

Spectra of the nuclear gamma resonance for La(Fe0.88Si x Al0.12 − x )13 compounds with different x values have been measured at 100 K. The compounds exhibit an antiferromagnetic ordering in the range of silicon concentrations x ≤ 0.024 and a ferromagnetic ordering at x > 0.024. Analysis of the spectra has been carried out with the use of a two-core distribution of quadrupole shifts. It has been demonstrated that upon a transition from antiferromagnetic to ferromagnetic ordering, the average hyperfine field at the Fe nuclei increases by almost 40 kOe. It has been found that antiferromagnetic compounds are mainly characterized by a positive quadrupole shift, and ferromagnetic ones by a negative quadrupole shift. A model of the formation of a magnetic structure that explains the appearance of a layered antiferromagnetic ordering in La(Fe0.88Si x Al0.12 − x )13 compounds has been proposed.

The Magnetic Behaviors of Spin-Glass $\hbox{FeGa}_{2}\hbox{O}_{4}$ System

We present the investigation of magnetic properties of spin-glass FeGa2O4 system. From X-ray diffraction patterns of FeGa2O4, refined with Rietveld's refinement method, its structure is determined to be cubic spinel with space group Fd - 3m and the lattice parameter of α0 = 8.385 Å. From temperature-dependent magnetization curves under 1000 Oe, the Neel temperature is found to be TN = 14 K, which coincides with the value obtained from the Mossbauer spectrum. The freezing temperature Tf of the sample shifts to higher temperature with increasing frequency, as seen in conventional metallic spin glasses. Also, we have determined the small activation energy, Ea of 1.04266 × 10-4 meV from Arrhenius law v = v0 exp(-En/kBTf), where kB is Boltzmann constant, and Ea is activation energy. The Mossbauer spectrum at 4.2 K shows severely distorted 8-line shape coming from frozen spin-disorder state and an incommensurate spin structure, as in spin glasses. The change in the electric quadrupole shift above Tf is caused by the presence of the maximum electric dipole interaction among frozen disordered spins around Tf as in spin-glass material, and charge re-distribution from spin-relocation arising above TN.

Weak Halogen Bonding in Solid Haloanilinium Halides Probed Directly via Chlorine-35, Bromine-81, and Iodine-127 NMR Spectroscopy

A series of monohaloanilinium halides exhibiting weak halogen bonding (XB) has been prepared and characterized by 35Cl, 81Br, and 127I solid-state nuclear magnetic resonance (SSNMR) spectroscopy in magnetic fields of up to 21.1 T. The quadrupolar and chemical shift (CS) tensor parameters for halide ions (Cl–, Br–, I–) which act as electron density donors in the halogen bonds of these compounds are measured to provide insight into the possible relationship between halogen bonding and NMR observables. The NMR data for certain series of related compounds are strongly indicative of when such compounds pack in the same space group, thus providing practical structural information. Careful interpretation of the NMR data in the context of novel and previously available X-ray crystallographic data, and new gauge-including projector-augmented-wave density functional theory (GIPAW DFT) calculations has revealed several notable trends. When a series of related compounds pack in the same space group, it has been possible to interpret trends in the NMR data in terms of the strength of the halogen bond. For example, in isostructural series, the halide quadrupolar coupling constant was found to increase as the halogen bond weakens. In the case of a series of haloanilinium bromides, the 81Br isotropic chemical shift and CS tensor span both decrease as the bromide–halogen XB is weakened. These trends were reproduced using both GIPAW DFT and cluster-model calculations of the bromide ion magnetic shielding tensor. Such trends are particularly exciting given the well-known role that NMR has played historically in the characterization of hydrogen bonding.

Crystal structure and spin dynamics for the Li9V3P8−δO29−δ′ insertion electrode system with a multiple-electron reaction

The crystal structure, the electron configurations of V ions and the spin dynamics for the Li9V3P8−δO29−δ′ insertion electrode system that exhibits a multiple-electron reaction are explored through measurements of x-ray four-circle diffraction, magnetization and nuclear magnetic resonance. The structure for the stoichiometric composition with δ = δ′ = 0 is refined precisely and the effective valences of all ions are determined. The electron configurations of V ions for the system annealed at various temperatures are considered in the framework of intermediate crystal field approach. For the system that often exhibits the charge–discharge performance better than the double-electron reaction, the spin dynamics mainly for the tetrahedral Li3 ions is understood on the basis of the relaxation mechanisms of quadrupole coupling and resonance shift anisotropy caused by the dipolar field of V ions. The correlation time of fluctuating fields is proportional to the temperature above 30 K, below which it follows the fractional Debye–Stokes–Einstein relation.

Oxidation of olefins catalyzed by Iron (III) complexes of bis-benzimidazolyl diamide ligands

Abstract Iron (III) complexes of bis-benzimidazole based diamide ligands N,N′-bis(1-butyl-benzimidazol-2-yl-methyl)-hexane-1,6-dicarboxamide (L1 = b-GBSA) and N,N′-bis(benzimidazol-2-yl-methyl)-benzene-1,4-dicarboxamide (L2 = p-GBBA) have been synthesized. These analyze for the stoichiometry [Fe(L)X2]X where (X = Cl− and NO 3 - ). Complexes have been characterized by elemental analysis, UV–Visible, IR, Cyclic Voltammetry, conductance measurements, EPR, Mossbauer, Magnetic susceptibility studies. Isomer shift values (δ) lie in the range of 0.29–0.36 mm/s and Quadrupole shift values (ΔEq) lie in the range of 0.73–0.81 mm/s. Mossbauer spectroscopy confirms them to be high spin Iron (III) complexes with distorted octahedral geometry, a result which is also confirmed by low temperature EPR spectroscopy. Cyclic voltammograms have been obtained for the Iron (III) complexes. E1/2 values are quite cathodic. The magnetic moment values are found to be in the range for high spin Iron (III) complexes. The oxidation of electron deficient olefins has been studied using [Fe(b-GBSA)Cl2]Cl and [Fe(p-GBBA)Cl2]Cl as a catalyst and TBHP as an alternate source of oxygen. Products have been characterized by GCMS analysis.

Classical toy models for the monopole shift and the quadrupole shift

The penetration of s- and p(1/2)-electrons into the atomic nucleus leads to a variety of observable effects. The presence of s-electrons inside the nucleus gives rise to the isotope shift in atomic spectroscopy, and to the isomer shift in Mössbauer spectroscopy. Both well-known phenomena are manifestations of the more general monopole shift. In a recent paper (Koch et al., Phys. Rev. A, 2010, 81, 032507), we discussed the existence of the formally analogous quadrupole shift: a tensor correction to the electric quadrupole interaction due to the penetration of relativistic p(1/2)-electrons into the nucleus. The quadrupole shift is predicted to be observable by high-accuracy molecular spectroscopy on a set of 4 molecules (the quadrupole anomaly). The simple physics behind all these related phenomena is easily obscured by an elaborate mathematical formalism that is required for their derivation: a multipole expansion in combination with perturbation theory, invoking quantum physics and ideally relativity. In the present paper, we take a totally different approach. We consider three classical 'toy models' that can be solved by elementary calculus, and that nevertheless contain all essential physics of the monopole and quadrupole shifts. We hope that this intuitive (yet exact) analysis will increase the understanding about multipole shift phenomena in a broader community.

Phase transition and relaxation processes by 87Rb and 39K nuclear magnetic measurements of RbKSO4 single crystals

Abstract 87 Rb and 39 K nuclear magnetic resonance (NMR) spectra of RbKSO 4 single crystals were measured at room temperature. 87 Rb central line has the angular dependences of second-order quadrupolar shifts. From these results, the quadrupole coupling constant and the asymmetry parameter were determined at room temperature. In addition, the spin–lattice relaxation rate, 1/ T 1 , and the spin–spin relaxation rate, 1/ T 2 , were measured as a function of temperature. The values of 1/ T 1 for the 87 Rb and 39 K nuclei were found to increase with increasing temperature, and 1/ T 1 was determined to be proportional to T n . Therefore, for the 87 Rb and 39 K nuclei, Raman processes with n =2 are more significantly in nuclear quadrupole relaxation than direct processes.

Mössbauer study on Y-type hexaferrite Ba2Mg2Fe12O22

Mossbauer study on a Y-type hexaferrite Ba2Mg2Fe12O22 has been conducted by using a single crystal specimen. The spins are in the c-plane down to 60 K. For 18h VI site Fe, the quadrupole shifts and the outermost line width change around 195 K, where the transition from ferrimagnetic to proper screw spin structure takes place. Below 50 K, the spin reorientation transition to a longitudinal conical structure was also recognized. At 16 K, the spins incline about 15° from the c-plane.

Ab Initio Study of the Structural, Electronic, Magnetic, and Hyperfine Properties of GaxFe4–xN (0.00 ≤ x ≤ 1.00) Nitrides

The dependence with Ga content (x) of structural, magnetic, and hyperfine properties of GaxFe4–xN compounds has been studied using the full-potential linearized augmented planewave method based on density functional theory. A nonlinear increase of the lattice parameter with x was observed, ascribed to the different metallic radii of Fe and Ga atoms and a magneto-volumetric effect. The magnetic moment per formula unit (Mfu) decreases linearly with x, where the Mfu of GaFe3N is almost half for the corresponding value of γ′-Fe4N, mainly due to the decreasing of magnetic moment of the FeII (3c Wyckoff position) atoms with the number of Ga next nearest neighbors (nnn). The hyperfine parameters of the FeI (1a Wyckoff position) do not change with x, whereas the hyperfine field (Bhf), isomer shift (δ), and quadrupole shift (ε) of the FeII atoms depend linearly with nnn Ga atoms. Furthermore, by use of fixed spin moment calculation to obtain the curve Mfu vs total energy, it was possible to determine that the ground state of GaFe3N belongs to a ferrimagnetic configuration with a lattice parameter similar to the experimental value.

NMR study of size effects in relaxor PMN nanoparticles

93Nb − NMR line shape and spin–lattice measurements show that microcrystalline PbMg1/3Nb2/3O3 (PMN) powder is dynamically disordered at room temperature, whereas nanocrystalline PMN powder is orientationally frozen out and long-range ordered at room temperature. The dynamical disorder of the microcrystalline powder results in a motional averaging of the anisotropic part of the 93Nb chemical shift tensor and second order quadrupole shift, whereas this averaging is absent in the nanocrystalline powder, resulting in a broader central line and a longer spin–lattice relaxation time. This seems to be the first observation of such size effects in a relaxor.

Water molecular reorientation in ice and tetrahydrofuran clathrate hydrate from lineshape analysis of 17O spin-echo NMR spectra

Lineshape analysis of 17O spin-echo NMR spectra has been used to study water molecular reorientations in ice-Ih and THF Structure II clathrate hydrate. The kinetics was determined by the changes of the lineshapes of the 17O central transitions at different temperatures. A model involving 12 orientations and 4-step jumps of water molecular orientations was proposed. Semi-classical exchange formalism was employed to simulate the lineshape of the central transitions of the 17O nuclei. Lineshape analysis gave the quadrupolar coupling constant CQ = 6.43 MHz and the asymmetry parameter η = 0.935, for both ice-Ih and THF gas hydrate. The theoretical lineshape simulations resulted in activation energies of water molecular reorientations Ea = 55.2 ± 2.1 kJ mol–1 and Ea = 30.5 ± 0.8 kJ mol–1 for ice-Ih and THF hydrate, respectively. The range of dynamic rates in THF clathrate hydrate is such that before melting, a pseudo-isotropic lineshape is observed that retains a second-order quadrupolar shift. The water reorientation process is discussed in light of recent results on Bjerrum defect injection obtained from molecular dynamics simulation and structural studies.

Characterization of Noninnocent Metal Complexes Using Solid-State NMR Spectroscopy: o-Dioxolene Vanadium Complexes

(51)V solid-state NMR (SSNMR) studies of a series of noninnocent vanadium(V) catechol complexes have been conducted to evaluate the possibility that (51)V NMR observables, quadrupolar and chemical shift anisotropies, and electronic structures of such compounds can be used to characterize these compounds. The vanadium(V) catechol complexes described in these studies have relatively small quadrupolar coupling constants, which cover a surprisingly small range from 3.4 to 4.2 MHz. On the other hand, isotropic (51)V NMR chemical shifts cover a wide range from -200 to 400 ppm in solution and from -219 to 530 ppm in the solid state. A linear correlation of (51)V NMR isotropic solution and solid-state chemical shifts of complexes containing noninnocent ligands is observed. These experimental results provide the information needed for the application of (51)V SSNMR spectroscopy in characterizing the electronic properties of a wide variety of vanadium-containing systems and, in particular, those containing noninnocent ligands and that have chemical shifts outside the populated range of -300 to -700 ppm. The studies presented in this report demonstrate that the small quadrupolar couplings covering a narrow range of values reflect the symmetric electronic charge distribution, which is also similar across these complexes. These quadrupolar interaction parameters alone are not sufficient to capture the rich electronic structure of these complexes. In contrast, the chemical shift anisotropy tensor elements accessible from (51)V SSNMR experiments are a highly sensitive probe of subtle differences in electronic distribution and orbital occupancy in these compounds. Quantum chemical (density functional theory) calculations of NMR parameters for [VO(hshed)(Cat)] yield a (51)V chemical shift anisotropy tensor in reasonable agreement with the experimental results, but surprisingly the calculated quadrupolar coupling constant is significantly greater than the experimental value. The studies demonstrate that substitution of the catechol ligand with electron-donating groups results in an increase in the HOMO-LUMO gap and can be directly followed by an upfield shift for the vanadium catechol complex. In contrast, substitution of the catechol ligand with electron-withdrawing groups results in a decrease in the HOMO-LUMO gap and can directly be followed by a downfield shift for the complex. The vanadium catechol complexes were used in this work because (51)V is a half-integer quadrupolar nucleus whose NMR observables are highly sensitive to the local environment. However, the results are general and could be extended to other redox-active complexes that exhibit coordination chemistry similar to that of the vanadium catechol complexes.