Electronic Structure of the Aqueous Vanadyl Ion Probed by 9 and 94 GHz EPR and Pulsed ENDOR Spectroscopies and Density Functional Theory Calculations

In situ electron paramagnetic resonance spectroscopy using single nanodiamond sensors

An ultimate goal of electron paramagnetic resonance (EPR) spectroscopy is to analyze molecular dynamics in place where it occurs, such as in a living cell. The nanodiamond (ND) hosting nitrogen-vacancy (NV) centers will be a promising EPR sensor to achieve this goal. However, ND-based EPR spectroscopy remains elusive, due to the challenge of controlling NV centers without well-defined orientations inside a flexible ND. Here, we show a generalized zero-field EPR technique with spectra robust to the sensor's orientation. The key is applying an amplitude modulation on the control field, which generates a series of equidistant Floquet states with energy splitting being the orientation-independent modulation frequency. We acquire the zero-field EPR spectrum of vanadyl ions in aqueous glycerol solution with embedded single NDs, paving the way towards in vivo EPR.

Exploiting clock transitions for the chemical design of resilient molecular spin qubits

Molecular spin qubits are chemical nanoobjects with promising applications that are so far hampered by the rapid loss of quantum information, a process known as decoherence. A strategy to improve this situation involves employing so-called Clock Transitions (CTs), which arise at anticrossings between spin energy levels. At CTs, the spin states are protected from magnetic noise and present an enhanced quantum coherence. Unfortunately, these optimal points are intrinsically hard to control since their transition energy cannot be tuned by an external magnetic field; moreover, their resilience towards geometric distortions has not yet been analyzed. Here we employ a python-based computational tool for the systematic theoretical analysis and chemical optimization of CTs. We compare three relevant case studies with increasingly complex ground states. First, we start with vanadium(iv)-based spin qubits, where the avoided crossings are controlled by hyperfine interaction and find that these S = 1/2 systems are very promising, in particular in the case of vanadyl complexes in an L-band pulsed EPR setup. Second, we proceed with a study of the effect of symmetry distortions in a holmium polyoxotungstate of formula [Ho(W₅O₁₈)₂]^9- where CTs had already been experimentally demonstrated. Here we determine the relative importance of the different structural distortions that causes the anticrossings. Third, we study the most complicated case, a polyoxopalladate cube [HoPd₁₂(AsPh)₈O₃₂]^5- which presents an unusually rich ground spin multiplet. This system allows us to find uniquely favorable CTs that could nevertheless be accessible with standard pulsed EPR equipment (X-band or Q-band) after a suitable chemical distortion to break the perfect cubic symmetry. Since anticrossings and CTs constitute a rich source of physical phenomena in very different kinds of quantum systems, the generalization of this study is expected to have impact not only in molecular spin science but also in other related fields such as molecular photophysics and photochemistry.

Benchmark study of the performance of density functional theory for reduction potentials of vanadium compounds

A systematic benchmark study was performed for the first time to investigate the performance of density functional theory for calculation of reduction potentials of vanadium compounds. Six density functionals of different types were selected for testing: local OLYP and M06L, global hybrid O3LYP and B3LYP, as well as, meta-hybrid functionals TPSSh and M06. Local and hybrid functionals with a relatively high contribution of Hartree-Fock exchange showed unsatisfactory results. In particular, the widely used hybrid functional B3LYP for the transformation VIII→VII occurring in the vanadium redox flow battery yields a negative value of the standard potential instead of a positive one. Among the tested functionals the smallest deviation from the experimental data provides the meta-hybrid functional TPSSh with a 10% contribution of the Hartree-Fock exchange. The computational protocol to calculate redox potentials of vanadium compounds is suggested.

First-Principles Calculation of Transition Metal Hyperfine Coupling Constants with the Strongly Constrained and Appropriately Normed (SCAN) Density Functional and its Hybrid Variants

Density functional theory (DFT) is used extensively for the first-principles calculation of hyperfine coupling constants in both main-group and transition metal systems. As with many other properties, the performance of DFT for hyperfine coupling constants is of variable quality, particularly for transition metal complexes, because it strongly depends on the nature of the chemical system and the type of approximation to the exchange-correlation functional. Recently, a meta-generalized-gradient approximation (mGGA) functional was proposed that obeys all known exact constraints for such a method, known as the Strongly Constrained and Appropriately Normed (SCAN) functional. In view of its theoretically superior formulation a benchmark set of complexes is used to assess the performance of SCAN for the challenging case of transition metal hyperfine coupling constants. In addition, two global hybrid versions of the functional, SCANh and SCAN0, are described and tested. The values computed with the new functionals are compared with experiment and with those of other DFT approximations. Although the original SCAN and the SCAN-based hybrids may offer improved hyperfine coupling constants for specific systems, no uniform improvement is observed. On the contrary, there are specific cases where the new functionals fail badly due to a flawed description of the underlying electronic structure. Therefore, despite these methodological advances, systematically accurate and system-independent prediction of transition metal hyperfine coupling constants with DFT remains an unmet challenge.

Studies on the Magnetic Ground State of a Spin Möbius Strip

Here we report the synthesis, structure and detailed characterisation of three n-membered oxovanadium rings, Nan [(V=O)n Nan (H2 O)n (α, β, or γ-CD)2 ]⋅m H2 O (n=6, 7, or 8), prepared by the reactions of (V=O)SO4 ⋅x H2 O with α, β, or γ-cyclodextrins (CDs) and NaOH in water. Their alternating heterometallic vanadium/sodium cyclic core structures were sandwiched between two CD moieties such that O-Na-O groups separated the neighbouring vanadyl ions. Antiferromagnetic interactions between the S=1/2 vanadyl ions led to S=0 ground states for the even-membered rings, but to two quasi-degenerate S=1/2 states for the spin-frustrated heptanuclear cluster.

VO2+-hydroxyapatite complexes as models for vanadyl coordination to phosphate in bone

We describe a 1D and 2D ESEEM investigation of VO2+ adsorbed on hydroxyapatite (HA) at different concentrations and compare with VO2+-triphosphate (TPH) complexes studied previously in detail, in an effort to provide more insight into the structure of VO2+coordination in bone. Structures of this interaction are important because of the role of bone in the long-term storage of administered vanadium, and the likely role of bone in the steady-state release of vanadium leading to the chronic insulin-enhancing anti-diabetic effects of vanadyl complexes. Three similar sets of cross-peaks from phosphorus nuclei observed in the 31P HYSCORE spectra of VO2+-HA, VO2+-TPH, and VO2+-bone suggest a common tridentate binding motif for triphosphate moieties to the vanadyl ion. The similarities between the systems present the possibility that in vivo vanadyl coordination in bone is relatively uniform. Experiments with HA samples containing different amounts of adsorbed VO2+ demonstrate additional peculiarities of the ion-adsorbent interaction which can be expected in vivo. HYSCORE spectra of HA samples show varying relative intensities of 31P lines from phosphate ligands and 1H lines, especially lines from protons of coordinated water molecules. This result suggests that the number of equatorial phosphate ligands in HA could be different depending on the water content of the sample and the VO2+ concentration; complexes of different structure probably contribute to the spectra of VO2+-HA. Similar behavior can be also expected in vivo during VO2+ accumulation in bones.

Structure and stability of hexa-aqua V(III) cations in vanadium redox flow battery electrolytes

The vanadium(III) cation structure in mixed acid based electrolyte solution from vanadium redox flow batteries is studied by (17)O and (35/37)Cl nuclear magnetic resonance (NMR) spectroscopy, electronic spectroscopy and density functional theory (DFT) based computational modelling. Both computational and experimental results reveal that the V(III) species can complex with counter anions (sulfate/chlorine) depending on the composition of its solvation sphere. By analyzing the powder precipitate it was found that the formation of sulfate complexed V(III) species is the crucial process in the precipitation reaction. The precipitation occurs through nucleation of neutral species formed through deprotonation and ion-pair formation process. However, the powder precipitate shows a multiphase nature which warrants multiple reaction pathways for precipitation reaction.

Monomeric Oxovanadium(IV) Compounds of the General Formula cis-[VIV(O)(X)(LNN)2]+/0 {X = OH-, Cl-, SO42- and LNN = 2,2‘-Bipyridine (Bipy) or 4,4‘-Disubstituted Bipy}

Reaction of [V(IV)OCl(2)(THF)(2)] in aqueous solution with 2 equiv of AgBF(4) or AgSbF(6) and then with 2 equiv of 2,2'-bipyridine (bipy), 4,4'-di-tert-butyl-2,2'-bipyridine (4,4'-dtbipy), or 4,4'-di-methyl-2,2'-bipyridine (4,4'-dmbipy) affords compounds of the general formula cis-[V(IV)O(OH)(L(NN))(2)]Y [where L(NN) = bipy, Y = BF(4)(-) (1), L(NN) = 4,4'-dtbipy, Y = BF(4)(-) (2.1.2H(2)O), L(NN) = 4,4'-dmbipy, Y = BF(4)(-) (3.2H(2)O), and L(NN) = 4,4'-dtbipy, Y = SbF(6)(-) (4)]. Sequential addition of 1 equiv of Ba(ClO(4))(2) and then of 2 equiv of bipy to an aqueous solution containing 1 equiv of V(IV)OSO(4).5H(2)O yields cis-[V(IV)O(OH)(bipy)(2)]ClO(4) (5). The monomeric compounds 1-5 contain the cis-[V(IV)O(OH)](+) structural unit. Reaction of 1 equiv of V(IV)OSO(4).5H(2)O in water and of 1 equiv of [V(IV)OCl(2)(THF)(2)] in ethanol with 2 equiv of bipy gives the compounds cis-[V(IV)O(OSO(3))(bipy)(2)].CH(3)OH.1.5H(2)O (6.CH(3)OH.1.5H(2)O) and cis-[V(IV)OCl(bipy)(2)]Cl (7), respectively, while reaction of 1 equiv of [V(IV)OCl(2)(THF)(2)] in CH(2)Cl(2) with 2 equiv of 4,4'-dtbipy gives the compound cis-[V(IV)OCl(4,4'-dtbipy)(2)]Cl.0.5CH(2)Cl(2) (8.0.5CH(2)Cl(2)). Compounds cis-[V(IV)O(BF(4))(4,4'-dtbipy)(2)]BF(4) (9), cis-[V(IV)O(BF(4))(4,4'-dmbipy)(2)]BF(4) (10), and cis-[V(IV)O(SbF(6))(4,4'-dtbipy)(2)]SbF(6) (11) were synthesized by sequential addition of 2 equiv of 4,4'-dtbipy or 4,4'-dmbipy and 2 equiv of AgBF(4) or AgSbF(6) to a dichloromethane solution containing 1 equiv of [V(IV)OCl(2)(THF)(2)]. The crystal structures of 2.1.2H(2)O, 6.CH(3)OH.1.5H(2)O, and 8.0.5CH(2)Cl(2) were demonstrated by X-ray diffraction analysis. Crystal data are as follows: Compound 2.1.2H(2)O crystallizes in the orthorhombic space group Pbca with (at 298 K) a = 21.62(1) A, b = 13.33(1) A, c = 27.25(2) A, V = 7851(2) A(3), Z = 8. Compound 6.CH(3)OH.1.5H(2)O crystallizes in the monoclinic space group P2(1)/a with (at 298 K) a = 12.581(4) A, b = 14.204(5) A, c = 14.613(6) A, beta = 114.88(1) degrees, V = 2369(1), Z = 4. Compound 8.0.5CH(2)Cl(2) crystallizes in the orthorhombic space group Pca2(1) with (at 298 K) a = 23.072(2) A, b = 24.176(2) A, c = 13.676(1) A, V = 7628(2) A(3), Z = 8 with two crystallographically independent molecules per asymmetric unit. In addition to the synthesis and crystallographic studies, we report the optical, infrared, magnetic, conductivity, and CW EPR properties of these oxovanadium(IV) compounds as well as theoretical studies on [V(IV)O(bipy)(2)](2+) and [V(IV)OX(bipy)(2)](+/0) species (X = OH(-), SO(4)(2)(-), Cl(-)).

Two-dimensional (2D) pulsed electron paramagnetic resonance study of VO(2+)-triphosphate interactions: evidence for tridentate triphosphate coordination, and relevance to bone uptake and insulin enhancement by vanadium pharmaceuticals

Two- and four-pulse electron spin echo envelope modulation (ESEEM) and four-pulse two-dimensional hyperfine sublevel correlation (HYSCORE) spectroscopies have been used to determine the solution structure of a 3:1 triphosphate:vanadyl solution at pH 5.0. Limited quantitative data were extracted from the two pulse spectra; however, HYSCORE proved to be more useful in the detection and interpretation of the (31)P and (1)H couplings. Three sets of cross-peaks were observed for each nucleus. For the (31)P couplings, three sets of cross-peaks were observed in the HYSCORE spectrum, and contour line shape analysis yielded coupling constants of approximately 15, 9, and 1 MHz. HYSCORE cross-peaks in the proton region were partially overlapping; however, interpretation of the proton coupling was simplified through the use of one-dimensional four-pulse ESEEM and subsequent analysis of the sum combination peaks. Comparison of the derived isotropic and anisotropic coupling constants with results from earlier ESEEM and electron nuclear double resonance (ENDOR) studies was consistent with the presence of at least one, and most likely two, water molecules coordinated in the equatorial plane of the vanadyl cation. The vanadyl-triphosphate system was shown to be an accurate model of the in vivo vanadyl-phosphate coupling constants determined in an earlier study (Dikanov, S. A.; Liboiron, B. D.; Thompson, K. H.; Vera, E.; Yuen, V. G.; McNeill, J. H.; Orvig, C. J. Am. Chem. Soc. 1999, 121, 11004.) Comparison of these values to those found in previous spectroscopic studies of vanadyl-triphosphate interactions, along with a detailed structural interpretation, are presented. This work represents the first detection of tridentate polyphosphate coordination to the vanadyl ion, and the first observation of an axial phosphate interaction not previously reported in earlier ENDOR and pulsed electron paramagnetic resonance studies.