Superhyperfine Lines Research Articles

Mixed ligand copper(II) chelates derived from an O, N, S- donor tridentate thiosemicarbazone: Synthesis, spectral aspects, FMO, and NBO analysis

Five new copper(II) chelates [(Cu(bmct))2] (1), [Cu(bmct)(phen)] (2), [Cu(bmct)(bipy)] (3), [Cu(bmct)(4,4′-dmbipy)] (4) and [Cu(bmct)(5,5′-dmbipy)] (5) with 5-bromo-3-methoxysalicylaldehyde-N(4)-cyclohexylthiosemicarbazone (H2bmct) as the chelating ligand and 1,10-phenanthroline, 2,2′-bipyridine, 4,4′-dimethylbipyridine, 5,5′-dimethylbipyridine as coligands have been synthesized and characterized by different physicochemical techniques like CHNS analysis, molar conductivity and magnetic studies, IR, UV/Vis and EPR spectral studies. In all the complexes, the thiosemicarbazone exists in thioiminolate form and coordinates to the metal through azomethine nitrogen, thioiminolate sulfur, and phenolate oxygen. EPR spectra in polycrystalline state at 298 K showed that compounds 1, 4, and 5 are isotropic, 2 is axial and 3 is rhombic in nature. In DMF at 77 K, compound 1 showed hyperfine lines in the parallel and perpendicular regions as well as superhyperfine lines due to the interaction of copper center with azomethine nitrogen of the ligand. Complex 2, in which g||>g⊥> 2.0023 suggests a distorted square pyramidal structure. To analyze the stability of the complexes, quantum chemical parameters like hardness, softness, polarizability, electrophilicity, electronegativity, and dipole moment were calculated and discussed within the framework of electronic structure principles known as Maximum Hardness, Minimum Polarizability and Minimum Electrophilicity Principles. Besides, the intramolecular donor–acceptor interactions for all complexes were evaluated by using NBO analysis. All calculations proved that Compound 3 is the most stable chelate among them.

Superhyperfine Interactions of the Nitrogen Donors in 4H SiC Studied by Pulsed ENDOR and TRIPLE ENDOR Spectroscopy

The nitrogen donors residing at quasi-cubic lattice site (Nk) in 4H SiC were investigated by field sweep electron spin echo (FS ESE), pulsed electron nuclear double resonance (ENDOR) and pulsed General TRIPLE ENDOR spectroscopy. The 29Si and 13C superhyperfine lines observed in the FS ESE and ENDOR spectra of Nk in n-type 6H SiC were assigned by pulsed General TRIPLE resonance spectroscopy to the specific carbon (C) and silicon (Si) atoms located in the nearest environment of Nk in 4H SiC. The superhyperfine interaction constants and their relative signs for Nk with 29Si and 13C nuclei located in the nearest-neighbor shells are found from the General TRIPLE ENDOR spectra to be positive for C atoms and negative for Si atoms.

Electronic structure of the nitrogen donors in 6H SiC as studied by pulsed ENDOR and TRIPLE ENDOR spectroscopy

AbstractThe nitrogen (N) donors in 6H SiC were investigated by field sweep electron spin echo (FS ESE), pulsed electron nuclear double resonance (ENDOR) and pulsed General TRIPLE ENDOR spectroscopy. The 29Si and 13C superhyperfine (shf) lines observed in the FS ESE and ENDOR spectra of N in n‐type 6H SiC were assigned by pulsed General TRIPLE resonance spectroscopy to the specific carbon (C) and silicon (Si) atoms located in the environment of N donors residing at two quasi‐cubic lattice sites (Nk1, Nk2) in 6H SiC. As a result, the largest value of the shf interaction was found with Si atoms for the N donors at the hexagonal (Nh) and quasi‐cubic site Nk1, while for Nk2 the largest value of the shf interaction was found with C atoms. It gives us the argument to consider that N substitute different lattice sites in 6H SiC lattice. The relative signs of the shf interaction (shfi) constants for Nk1 and Nk2 with 29Si and 13C nuclei located in the nearest‐neighbor, next nearest‐neighbor, and outer shells are found from the TRIPLE ENDOR spectra to be positive for C atoms and negative for Si atoms. From the comparison of the experimentally obtained shfi constants with the theory, the electronic spin‐density distribution over the 29Si and 13C nuclei located in the nearest neighbor shells of N donors has been obtained taking into account the Kohn–Luttinger interference effect. The position of the conduction band minimum along the ML‐line was determined to be at k0z/kmax = 0.2 ± 0.05.

EPR Study of 5-Chlorosalicylate-Cu(II)-3-Pyridylmethanol Ternary Complex Systems in Frozen Water–Methanol Solutions

Two copper(II) ternary complex systems containing 5-chlorosalicylic acid (5-ClsalH) and different copper(II) salts with varying 3-pyridylmethanol (ron = ronicol) concentration, system I [CuSO4 (aq) +2(5-ClsalH(solv)) + xron(l)] and system II [Cu(ac)2(aq) + 2(5-ClsalH(solv)) + xron(l)], where x = 0, 2, 4, 6 and 8, were prepared and studied by electron paramagnetic resonance (EPR) spectroscopy in frozen water–methanol solutions to observe the effects of different copper(II) salts and varying neutral ligand concentration on the formation of resulting complexes in solution. The trend in g-values (g|| > g⊥ > 2.0023) indicates that the unpaired electron on the copper ion is localized in the \( d_{{x}^{\rm{2}} - {y}^{\rm{2}}} \) orbital. The detailed analysis of the second-derivative Cu(II) EPR spectra has shown well-resolved 14N superhyperfine splitting in the perpendicular part of the axially symmetric spectra. The resolution of nitrogen superhyperfine multiplet patterns increased with increase in the ronicol concentration (ligand-to-metal ratio x). The number of superhyperfine lines was found to be constant (nonet) when x > 4 for system I and x ≥ 4 for system II. This fact indicates that for these x-values four equivalent nitrogen atoms could be coordinated to the central copper atom in the equatorial plane of both systems.

EPR and optical absorption studies of VO2+ doped l-alanine (C3H7NO2) single crystals

Abstract VO 2+ doped l -alanine (C 3 H 7 NO 2 ) single crystals and powders are examined by electron paramagnetic resonance (EPR) and optical absorption spectroscopy. Three magnetically different sites are resolved from angular variations of l -alanine single crystal EPR spectra. In some specific orientations each VO 2+ line splits into three superhyperfine lines with intensities of 1:2:1 and maximum splitting value of 2.23 mT. The local symmetries of VO 2+ complex sites are nearly axial. The optical absorption spectra show three bands. Spin Hamiltonian parameters are measured and molecular orbital coefficients are calculated by correlating EPR and optical absorption data for the central vanadyl ion.

A stable oxoverdazyl free radical: Structural and magnetic characterization

The structure and magnetic properties (susceptibility and ESR) of the stable oxoverdazyl free radical 6-(4-acetamidophenyl)-1,4,5,6-tetrahydro-2,4-dimethyl-1,2,4,5-tetrazin-3(2 H)-one are presented. The crystal structure consists of chains of parallel planar molecules running along the b-axis. These chains are formed by dimers with a ring-over-bond overlap and a significant offset between dimers, although with similar inter- and intradimer distances. The susceptibility measurements show that this compound is an S = 1/2 paramagnet with weak antiferromagnetic interactions. The magnetic susceptibility can be very well reproduced with an antiferromagnetic regular chain model with g = 2.012(1) and an intrachain exchange parameter J = −1.54(1) K = −1.07(1) cm −1. Solid-state Q-band ESR spectra confirm the presence of an unpaired electron in the oxoverdazyl radical and the thermal behavior observed in the susceptibility measurements. Solution ESR spectrum show a very complicated spectrum with up to 13 hyperfine lines, some of them further splitted into three or more superhyperfine lines. A very satisfactory simulation of the shape, intensity and position of all the lines in this spectrum could be obtained with hyperfine coupling constants of 6.5 and 5.4 G for the two groups of two equivalent N atoms and 5.3 G for the six H atoms of the two methyl groups bonded to the verdazyl ring.

EPR Characterisation of CuII Complexes of Poly(propylene imine) Dendromesogens: Using the Orienting Effect of a Magnetic Field

Liquid-crystalline derivatives of poly(propylene imine)dendrimers of the 0th, 1st and 2nd generations, complexed with copper(II) ions, were studied by EPR spectroscopy. The structures of copper (II) complexes with different Cu(II) loadings x per dendrimer ligand L (x = Cu/L) were determined. At the lowest concentration, the Cu(II) ions form monomeric complexes with approximately square-planar N2O2 coordination of both carbonyl oxygen and amido nitrogen atoms. At higher copper content, two kinds of Cu(II) complex sites with different geometries exist. The orienting effect of a high magnetic field was used to investigate the structure and magnetic properties of the copper(II) complexes. This effect, for the first time in dendrimers, allowed the resolution of five nitrogen super-hyperfine lines on g(z) components with the unusual coupling constant of a(Nz)= 35.9 x 10(-4) cm(-1). The combination of the magnetic parameters and the orienting effect indicates the presence of a monomeric complex with pseudotetrahedral N2O2 coordination of the Cu(II) ion, as well as a "dimer" structure with fivefold coordination, presumably due to an N3O2 environment. Higher copper loadings lead to increased exchange coupling between the complex sites.

A spectroscopic investigation of the interaction between nitrogen monoxide and copper sites of the fungal laccase from Rigidoporus lignosus.

The interaction of NO, with the copper centres of the laccase secreted by Rigidoporus lignosus was studied under both aerobic or anaerobic conditions. The reduction of the T1 site was always observed, as detected by the disappearance of the characteristic optical band at 604 nm (T3 presents probably the same behaviour because of the decreasing of the band at 330 nm) and the absence of its characteristic EPR signal, while T2 undergoes an initial partial and transitory reduction, its EPR signal intensity totally restoring after 24 h interaction. Different magnetic parameters of the T2 site have been detected, evidencing an increase of the hyperfine coupling constant. Furthermore, the number of superhyperfine lines on the fourth line of T2 copper was also found to increase from seven in the native to nine in the NO-treated laccase, this fact implying the coordination of a nitrogenous species to the T2 site. It was also shown that nitrite can be a source of NO, thus, paralleling the behaviour of NO-donor molecules or NO gas, but after longer interaction times. The nitrogenous species coordinated to T2 site is probably NO2-, which arises indirectly by NO oxidation. In order to understand the mechanistic pathway of this interaction, some experiments were also carried out in the presence of azide to study the interaction of NO with this laccase having its trinuclear cluster blocked by the presence of an exogenous ligand as N3-. After the addition of NO-donor molecules to the azide-treated laccase, a new EPR signal appeared at low temperatures, which is ascribable to the partially reduced T3 site, while the T1 and T2 sites were found to be totally reduced. The mechanistic pathway of the NO interaction seems to proceed through the reduction of T1 and T3 copper sites, followed by the coordination of nitrogenous species to T2.

EPR of Cu(II) in sarcosine cadmium chloride: probe into dopant site – symmetry and copper–sarcosine interaction

The electron paramagnetic resonance spectra of Cu(II) doped sarcosine cadmium chloride single crystals have been investigated at room temperature. Experimental results reveal that the Cu(II) ion enters the lattice interstitially. The observed superhyperfine lines indicate the superposition of two sets of quintet structure with interaction of nitrogen atoms and the two isotopes of copper. The spin Hamiltonian parameters are evaluated by Schonland method and the electric field symmetry around the copper ion is rhombic. An admixture of d z 2 orbital with the d x 2− y 2 ground state is observed. Evaluation of MO coefficients reveals that the in-plane interaction between copper and nitrogen is strong in this lattice.

Electrochemical Properties and ESR Characterization of Mixed Valence alpha-[XMo(3)(-)(x)()V(x)()W(9)O(40)](n)()(-) Heteropolyanions with X = P(V) and Si(IV), x = 1, 2, or 3.

Electrochemical behavior of the alpha-[SiMo(3)(-)(x)()V(x)()W(9)O(40)]((4+)(x)()())(-) and alpha-[PMo(3)(-)(x)()V(x)()W(9)O(40)]((3+)(x)()())(-) anions with x = 1, 2, or 3 were studied. Electrochemical reduction of each compounds was consistent with its Mo/V ratio, reduction of vanadium and molybdenum atoms occurring in the +0.6 to -0.6 V potential range. The one-electron-reduced species were prepared by electrolysis and then characterized by ESR spectroscopy. The g and A values for V(4+)ions appeared to depend on the nature of the surrounding atoms (Mo(VI), W(VI), and V(V)). In solution at 330 K, the ESR spectrum of the protonated alpha-H[SiMoV(IV)VW(9)O(40)](6)(-) anion displayed 29 superhyperfine lines which were related to the partial localization of the electron on one vanadium nucleus. The ESR spectra at room temperature for the divanadium-substituted anions showed a strong anisotropy of the A tensor which would be related to the electron transfer along a preferential axis. An isolated V(4+) signal was not observed, even at 12 K, indicating that the electron is never firmly trapped on one single vanadium atom.

Al-O--Al paramagnetic defects in kaolinite

Trapped holes located on Al-O-Al bonds in kaolinite were studied by electron paramagnetic resonance spectroscopy (EPR) at 9.3 and 35 GHz applied to well-crystallized, X-ray irradiated and oriented samples. The Q-band EPR spectrum is characterized by three clearly separated groups of 11 quasi-equidistant superhyperfine lines centered at gxx=2.040±0.0005, gyy=2.020±0.0005 and gzz=2.002±0.001. In each of these groups, the 11 superhyperfine lines exhibit intensities according to the ratios 1∶2∶3∶4∶5∶6∶5∶4∶3∶2∶1. An angular dependence of the Q-band EPR spectrum with respect to the magnetic field is demonstrated by measurements on oriented films of kaolinite. An appropriate numerical treatment of the EPR spectra is described, which allowed extraction of the SuperHyperfine Structures (SHFS). X-and Q-band spectra have also been simulated. It is concluded from these experiments that only one type of center is present. This center, labelled the B-center in the literature, is very probably a hole trapped on oxygen (O- center) atoms coupled to two octahedral aluminium.

Magnetic interactions in compounds based on nickel phthalocyanines and pyridine

The nickel(II) complexes of phthalocyanine (four-coordinate, low spin) in pyridine were investigated by EPR spectroscopy and by magnetic susceptibility. The ESR spectra of Ni(II) complexes in frozen (77 K) pyridine solution exhibit well-resolved superhyperfine lines. We attributed this to the formation of six-coordinate high-spin Ni(II) complexes and the interaction of the magnetic moment of the Ni(II) ions with the in-plane and out-of-plane (solvating molecules) nitrogen atoms. Due to the aggragation of molecules, no resolved spectrum could be obtained. The EPR spectrum of the solid-state compound, which was obtained by evaporation of the solution, exhibits a single broad line. The magnetic susceptibility of this compound obeys the Curie-Weiss law with a Weiss constant of θ = −82.9 K. It prefers Ni(↑↑)…N(↓)N(↑)…Ni(↓↓) types of interaction.

Ni3+-Ni3+ pair transitions in highly doped LiNbO3:Ni.

We report on the EPR ${\mathrm{Ni}}^{3+\mathrm{\ensuremath{-}}}$${\mathrm{Ni}}^{3+}$ pair transitions in highly doped ${\mathrm{LiNbO}}_{3}$:Ni. It is found that there are nine absorption peaks at 640, 1200, 1460, 1520, 3040, 3160, 3380, 7800, and 10 600 G in the range 0--12 000 G in highly doped ${\mathrm{LiNbO}}_{3}$:Ni (0.8 wt. %). The spectra were interpreted as both the ${\mathrm{Ni}}^{3+}$ single-ion transitions and ${\mathrm{Ni}}^{3+\mathrm{\ensuremath{-}}}$${\mathrm{Ni}}^{3+}$ pair transitions. In addition, it is found that many superhyperfine lines exist in the range 3000--3500 G which is not yet understood. Their origin is possibly an electron-phonon disturbance.

Copper(II) and Cobalt(II) Complexes with Derivatives of Salen and Tetrahydrosalen: An Electron Spin Resonance, Magnetic Susceptibility, and Quantum Chemical Study

2,3-diamino-2,3-dimethylbutane; H2[I&]L1 = N,N-bis(2-hydroxy-3-tert-butyl-5-methylbenzyl)-2,3-diamino2,3-dimethylbutane; H2L2 = N,~-bis(3-tert-butyl-5-chlorosalicylidene)-2,3-di~no-2,3-dimethylbutane; H2[H4]L2 = N,~-bis(2-hydroxy-3-tert-butyl-5-chlorobenzyl)-2,3-diamino-2,3-dimethylbutane). The ESR spectra of Cu(I1) complexes in frozen (100 K) toluene solution exhibit a well-resolved perpendicular part. In addition to hyperfine structure, superhyperfine lines are also seen. The superhyperfine structure in the perpendicular region for both CuL' and CuL2 complexes could be well accounted for by the interaction of two equivalent protons along with the two nitrogen nuclei. The protons here belong to the carbon atoms adjacent to the nitrogen nuclei. In the presence of pyridine (5% v/v) there is a considerable shift in both gll and gL values. The higher gll values compared with those of the parent complexes are consistent with the square pyramidal geometry implying axial (py) coordination. An almost negligible effect of an electronwithdrawing substituent (X5 = C1) on spin Hamiltonian parameters was observed. Due to the aggregation of molecules, no resolved spectrum could be obtained from frozen toluene solutions of both CoL' and CoL2 complexes. The addition of an axial pyridine leads to a better resolution of the spectra. In the presence of dioxygen and pyridine (5% v/v) the frozen (100 K) toluene solution of both CoL' and CoL2 exhibits rhombic symmetry with well-resolved hyperfine structures in all three directions. The shape of the spectra and spin Hamiltonian parameters indicate the interaction of the square pyramidal cobalt core with dioxygen. The interaction of the complexes CoL(py) with molecular oxygen leads to a spin-spin pairing process which results in a partial ligand to metal charge transfer and a large spin density on the oxygen moiety. CoL complexes are low-spin d7 systems with peff = 2.49,~~ for CoL'. The copper complexes CuL and Cu[&]L are magnetically normal = 1.81,~~ for CuL2). The calculated spin densities show that the unpaired electron is localized on the molecular orbital of b2 symmetry which is almost the dv orbital of the central atom. Only negligible spin density appears at the pyridine nitrogen atom, which is in agreement with the ESR measurements. The [CoL'] system exhibits its unpaired electron at the molecular orbital of a1 symmetry which is the net d,z metal orbital. The IND0/2 method yields the description of the dioxygen adduct which matches well with the generally accepted MO model. The QR-INDO/I failed in the prediction of the spin pairing process. It prefers either Co(t). * -02(?4) or Co(4). -02(tt) types of interaction. This may be due to an improper balance of the resonance and exchange contributions to the magnetic coupling.

P mevalonii 3-Hydroxy-3-Methylglutaryl-CoA Lyase: Electron Paramagnetic Resonance Investigation of the Copper Binding Site

The copper binding site in Pseudomonas mevalonii 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) lyase has been investigated by microltifrequency electron spin resonance spectroscopy. Methodology has been developed to introduce copper in vitro into the isolated apoenzyme. The X-band EPR of Cu 2+ (mixed isotopes or 63Cu 2+) introduced in this way is very similar to the EPR spectra of samples in which copper is introduced during protein expression. g || and A || values in the X-band spectra support the prediction of nitrogen ligands to the tightly bound copper. In the g || region of S-band EPR spectra ( m I = − 1 2 ) of lyase-bound 63Cu 2+, superhyperfine interactions due to nitrogen ligands are observed. Computer simicrolation with appropriate g values and A values and strain parameters was used to satisfactorily model both the X-band and the S-band spectra. The g || value of 2.282 and the A || value of 470 MHz are consistent with two nitrogen and two oxygen donor atoms for a square planar type 2 copper center. By simicrolating the m I = − 1 2 line of the S-band spectrum, the superhyperfine features could be well modeled. This simicrolation approach was also used to distinguish between two and three nitrogen donor atoms. Based on the intensity patterns of the superhyperfine lines and the estimated coupling constants, it is concluded that at least two (and probably only two) nitrogen donor atoms are liganded to the tightly bound copper in HMG-CoA lyase. Additionally, kinetic experiments demonstrate that a spin-labeled substrate analog (R . CoA) is a competitive inhibitor of HMG-CoA lyase ( K I = 98 μM). ESR titration experiments indicate that R . CoA binds to lyase with an equilibrium dissociation constant of 103 μM. Bound spin label exhibits a rotational correlation time, τ c, of 20 ns, in agreement with the value predicted for immobilization on a protein composed of two 32-kDa subunits.

Porous Silicon Nanostructure Revealed By Electron Spin Resonance

AbstractElectron spin resonance (ESR) study of annealed porous Si (PS) samples shows the existence of 20% to 30% of oxygen related Pbl (.Si-Si2O) centers, in addition to the Pb0 (·Si-Si3) centers already reported by other groups. The 29Si hyperfine and superhyperfine lines are well-defined at room temperature. We find that the nanostructure of PS, although twisted as appeared in scanning electron microscopy images, is well aligned with the underlying crystal lattice, and the ESR line broadening is mainly due to the local strain around the dangling bonds and not to the nanoscale column or particle misalignment.

EPR studies of iron substitution in the molecular sieve VPI-5

The X-band EPR spectra of iron-containing VPI-5 were recorded at room temperature and liquid nitrogen temperature. An intense signal at g/sub ///=2.0 and gperpendicular to =6.0 characteristic of high-spin Fe3+ ions in axial symmetry was observed at both temperatures. A splitting of these components due to superhyperfine interaction indicated the presence of these Fe3+ ions in the network-forming position. An anisotropic line at G=4.3 and weak superhyperfine lines centred at g=1.7 characteristic of high-spin Fe3+ ions in orthorhombic symmetry were also observed at liquid nitrogen temperature. These signals may also be attributed to the presence of Fe3+ ions, most probably in the network-forming position. Lastly, the very weak signal at g=3.1 was tentatively assigned to traces of occluded compounds of iron outside the framework.

Superhyperfine structure in the EPR spectra of Gd 3+ ions in PbTe

X-band EPR measurements have been made from 4.2K up to 300K on single crystals of PbTe doped with a nominal Gd content of 0.04 and 0.1 at.%. A multi-fold splitting of the ( − 1 2 , 1 2 ) fine structure transition was recorded along the [100] and [111] crystallographic directions. The splitting is shown to arise from a superhyperfine interaction between gadolinium ions and the lead isotope 207Pb. The superhyperfine coupling constant is temperature independent within experimental error. At 24K we obtain a Pb = 2.026±0.006G or (1.883 ± 0.005). 10 −4 cm −1. The superhyperfine lines broaden with increasing temperature due to a localized spin-free carrier interaction and cannot be resolved above 80K. These results show that the interactions of the 4f electrons, while somewhat smaller than those of Mn 3d electrons, are not negligible. The superhyperfine structure shows that the Gd ions occupy cation sites. No evidence of interstitial sites was seen.

Synthesis and copper(II)-binding properties of the N-terminal peptide of human α-fetoprotein

The N-terminal native sequence tripeptide of alpha-fetoprotein, L-threonyl-L-leucyl-L-histidine N-methylamide, was synthesized and its interaction with Cu(II) ions was investigated by potentiometric titration at 25 degrees C in 0.15 M-NaCl and by visible-absorption, e.p.r. and n.m.r. spectroscopy. Analyses of the results in the pH range 4-10 indicated the presence of multiple complex species in solution: MHL, MH-2L, MHL2, ML2 and MH-1L2, where M, H and L represent metal ion, proton and ligand anion respectively. Only the species MH-2L and MH-1L2 are present in significant amounts at physiological pH. The results of the visible-absorption spectroscopy are consistent with the findings of species distribution that MH-2L is the major complex species detected above physiological pH that has the spectral characteristics of lambda max. = 523 nm and epsilon max. = 98 M-1.cm-1. The nine superhyperfine lines in e.p.r. spectra of the major species MH-2L strongly support the co-ordination of four nitrogen atoms by Cu(II). Both 1H- and 13C-n.m.r. studies suggest that the species MH-2L is a square-planar complex. The results from the equilibrium-dialysis experiments showed that this peptide is able to compete with albumin for Cu(II) ions. At equimolar concentrations of albumin and the peptide, about 52% of the Cu(II) was bound to the peptide. The possibility that alpha-fetoprotein plays an important role as the Cu(II)-transport protein in fetal life is discussed.

Electron spin resonance studies in an irradiated single crystal of hexakis(ammonium) diformylated octamolybdate dihydrate

X-Band e.s.r studies on Mov centres formed by an u.v.-irradiated single crystal of the title compound, (NH4)6[(CHO)2Mo8O28]·2H2O, have been carried our at room temperature.Analyses of the e.s.r. spectra show that there are two different paramagnetic species in the unit cell. One may be resolved superhyperfine lines due to the interaction of the Mov centre with a 1H atom; the other can only be resolved hyperfine lines due to the Mov centre.The spin-Hamiltonian parameters have been rigorously calculated using a least-squares-fitting technique, specially adapted to non-coincident g- and A-tensor systems. The principal values of the g, AMo and AH tensors for the two species, as well as the direction cosines of their principal axes with respect to the reference system, are obtained. The spin density and bond nature of the MoV—O bond are estimated according to the principal values of the A tensors. The results indicate the main component of the ground -state molecular orbital is the atomic orbital 4dxy of the MoV paramagnetic centre. The MoV—O bond is an ionic bond with a covalent component.