Resolved Hyperfine Research Articles

Gradual transition from insulator to semimetal ofCa1−xEuxB6with increasing Eu concentration

The local environment of ${\mathrm{Eu}}^{2+}$ $(4{f}^{7},S=7∕2)$ in ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Eu}}_{x}{\mathrm{B}}_{6}$ $(0.003\ensuremath{\leqslant}x\ensuremath{\leqslant}1.00)$ is investigated by means of electron spin resonance (ESR). For $x\ensuremath{\lesssim}0.003$ the spectra show resolved fine and hyperfine structures due to the cubic crystal electric field and nuclear hyperfine field, respectively. The resonances have Lorentzian line shape, indicating an insulating environment for the ${\mathrm{Eu}}^{2+}$ ions. For $0.003\ensuremath{\lesssim}x\ensuremath{\lesssim}0.07$, as $x$ increases, the ESR lines broaden due to local distortions caused by the $\mathrm{Eu}∕\mathrm{Ca}$ ions substitution. For $0.07\ensuremath{\lesssim}x\ensuremath{\lesssim}0.30$, the lines broaden further and the spectra gradually change from Lorentzian to Dysonian resonances, suggesting a coexistence of both insulating and metallic environments for the ${\mathrm{Eu}}^{2+}$ ions. In contrast to ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Gd}}_{x}{\mathrm{B}}_{6}$, the fine structure is still observable up to $x\ensuremath{\approx}0.15$. For $x\ensuremath{\gtrsim}0.30$ the fine and hyperfine structures are no longer observed, the line width increases, and the line shape is purely Dysonian, anticipating the semimetallic character of ${\mathrm{EuB}}_{6}$. This broadening is attributed to a spin-flip scattering relaxation process due to the exchange interaction between conduction and ${\mathrm{Eu}}^{2+}$ $4f$ electrons. High-field ESR measurements for $x\ensuremath{\gtrsim}0.15$ reveal smaller and anisotropic linewidths, which are attributed to magnetic polarons and Fermi surface effects, respectively.

Crystal structure and spectral studies of a novel manganese(II) complex of 4-phenyl-2-acetylpyridine thiosemicarbazone: Extended network of [Mn(Ac4Ph) 2] via hydrogen bond linkages and π–π interactions

The reaction of Mn(CH 3COO) 2 · 4H 2O with 4-phenyl-2-acetylpyridine thiosemicarbazone, HAc4Ph, yielded the novel six-coordinate complex [Mn(Ac4Ph) 2]. The complex has been characterized by spectral (IR and EPR) and X-ray diffraction methods. EPR spectra were recorded with the external magnetic field oriented either perpendicular or parallel to the microwave magnetic field. In perpendicular mode, signals are observed in a wide spectral region. Of importance are the broad feature in the low field region and the broad absorption peaks at g ∼ 5.3 and g ∼ 3.07. The lack of resolved hyperfine patterns is attributed to a distribution on the zero field splitting parameters. The X-ray structural analysis showed that [Mn(Ac4Ph) 2] has a centrosymmetric distorted octahedral geometry with the MnN 4S 2 chromophore. Intermolecular hydrogen bonds of the DA AD type and π–π interactions stabilize the structure and result in the formation of infinite layers.

The redox pair {(μ 4-TCNE)[Fe(CO) 2(C 5H 5)] 4} 4+/3+: Surprisingly small metal–ligand interaction and the first resolved EPR hyperfine structure ( 13C, 14N, 57Fe) of a tetranuclear complex with TCNE [rad

Electrochemistry and absorption spectroscopy (UV/Vis, IR) of the complex {(μ 4-TCNE)[Fe(CO) 2(C 5H 5)] 4}(BF 4) 4 reveal only a small degree of metal-to-ligand π back donation, in contrast to previously characterized compounds [(TCNE)(ML n ) 4]. Accordingly, the one-electron reduced {(μ 4-TCNE)[Fe(CO) 2(C 5H 5)] 4} 3+ exhibits a very well resolved EPR spectrum at g = 1.9965 with detectable coupling of 0.055 mT for 57Fe in natural abundance from four coordinated [Fe(CO) 2(C 5H 5)] + groups.

Synthesis, EPR and biological activities of bis(1- n-butylamidino- O-alkylurea)copper(II)chloride complexes: EPR evidence for binuclear complexes in frozen DMF solution

Copper(II) complexes of the type [Cu(1-Bu n AAUH) 2]Cl 2, where 1-Bu n AAUH = 1- n-butylamidino- O-alkylurea and alkyl = methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl have been synthesized and characterized by elemental analyses, magnetic moments, conductivity measurements, electronic absorption, IR and EPR spectroscopy. The EPR spectra of these complexes in frozen solutions of DMF showed a signal at g ca. 2. In addition to this, a signal at half the resonance field, having well resolved seven-line hyperfine structure, was observed. The EPR signals at g ca. 2 and at the half-field have been interpreted as arising from Δ M s = ±1 and Δ M s = ±2 transitions, respectively, occurring as a result of the formation of a binuclear complex. The trend in g value ( g ∥ > g ⊥ > 2.00) suggests that the unpaired electron on copper has d x 2 - y 2 character and the complex has a square planar structure. Based on the EPR and electronic spectral data, the bonding parameters of these complexes in MeOH have been calculated. The metal-ligand bonding parameters evaluated showed strong in plane σ and in plane π bonding. The bis(1- n-butylamidino- O-alkylurea)copper(II)chloride complexes show growth inhibitory activity against pathogenic fungi, viz., Ceratocystis paradoxa, Aspergillus niger, Trichoderma koningii and pathogenic bacteria, viz., Bacillus sps(1), Bacillus sps(3).

Influence of mixed alkali on the spectral properties of vanadyl ions doped xNa 2O–(30 − x)K 2O–60B 2O 3 glasses—an EPR and optical study

Electron paramagnetic resonance (EPR) and optical investigations have been performed in the mixed alkali borate xNa 2O–(30 − x)K 2O–60B 2O 3 (5 ≤ x ≤ 25) glasses doped with 10 mol% of vanadyl ions in order to look for the effect of ‘mixed alkalis’ on the spectral properties of the glasses. The observed EPR spectra have structures for x > 5 mol% which are characteristic of a hyperfine interaction arising from an unpaired electron with the 51V nucleus and it builds up in intensity as x increases. It is observed that the mixed alkali play a significant role in accommodating the vanadyl ions in these mixed alkali glasses and for x > 5 mol%, shows a well resolved hyperfine structure typical for isolated VO 2+ ions. The spin-Hamiltonian parameters ( g and A), the dipolar hyperfine coupling parameter ( P) and Fermi contact interaction parameter ( k) have been evaluated. It is observed that the spin-Hamiltonian parameters do not vary much with the change in composition. It is observed that with increase of x, an increase occurs in tetragonal distortion for VO 2+. The number of spins ( N) participating in resonance and the paramagnetic susceptibility ( χ) have been calculated. It is observed that N and χ increase with x. The optical bandgap energies evaluated for these glasses slightly increase with x and reach a maximum around x = 20 and thereafter decrease showing the mixed alkali effect. Optical band gap energies obtained in the present work vary from 2.73 to 3.10 eV for both the direct and indirect transitions. The physical parameters of the glasses are also determined with respect to the composition.

Silicon vacancy annealing andDIluminescence in6H−SiC

Combining electron paramagnetic resonance measurements with ab initio calculations, we identify the ${V}_{\mathrm{C}}{\mathrm{C}}_{\mathrm{Si}}({\mathrm{Si}}_{\mathrm{C}}{\mathrm{C}}_{\mathrm{Si}})$ complex as a second annealing product of the silicon vacancy via an analysis of resolved carbon hyperfine interactions and of the zero-field splitting parameter of its excited triplet state. At high temperatures the carbon vacancy can dissociate from this complex leaving behind a diamagnetic ${\mathrm{Si}}_{\mathrm{C}}{({\mathrm{C}}_{\mathrm{Si}})}_{2}$ complex, which is proposed to be an excellent candidate to cause the ${D}_{\mathrm{I}}$ photoluminescence spectrum: the calculated local vibration modes fit very well with the characteristic phonon assisted structure of the photoluminescence-spectra. Furthermore, our model provides a detailed explanation for the excitonic electron-hole recombination responsible for the ${D}_{\mathrm{I}}$ luminescence.

High-frequency EPR of Tb3+-doped KPb2Cl5 crystal

High-frequency electron paramagnetic resonance (EPR) spectra of the KPb2Cl5:Tb3+ crystal have been investigated. Three types of spectra were observed in the frequency range of 74–200 GHz. The most intensive spectrum with the resolved hyperfine structure corresponded to transitions between sublevels of the159Tb3+ ground quasi-doublet with the zero-field splitting (ZFS) close to 48 GHz. Experimental results were analyzed by the exchange charge model of the crystal field affecting terbium ions in low-symmetry Pb2+ positions with the chlorine sevenfold coordination and the charge compensating vacancy in the nearest potassium site. The calculated values ofg-factors and ZFS were in agreement with the experimental data. The nature of a broad EPR line with ZFS of about 180 GHz and of additional weak EPR lines observed as satellites of the main Tb3+ lines was discussed.

59Co NMR observation of monolayer resolved hyperfine fields in ultrathin epitaxial FCT-Co(0 0 1) films

59Co nuclear magnetic resonance measurements have been performed on thin epitaxial FCT-Co(001) films and the effect of an interface on the Co hyperfine field has been measured with monolayer resolution. Four different sub-lines could be measured within the bulk resonance line and were attributed to Co atoms at various distances from the interface. This effect is observable because of the virtually perfect structural quality of the Co layer, which reduces the width of the resonance lines. In less perfect systems the line width obliterates the effect.

Microwave investigation of the CO-CH4 van der Waals complex.

Rotational spectra of eight isotopomers of the weakly bound van der Waals complex CO-CH4 were recorded in the frequency range from 4 to 19 GHz using a pulsed molecular beam Fourier transform microwave spectrometer. For the isotopomers containing methane monomers of Td symmetry, namely, 12C16O-12CH4, 12C16O-13CH4, 12C16O-12CD4, 13C16O-12CH4, and 13C18O-12CH4, three rotational progressions were observed that correlate to the jm=0, 1, and 2 rotational levels of free methane. For those containing partially deuterated methane monomers with C3V symmetry, namely, 12C16O-12CH3D and 12C16O-12CHD3, only two progressions were recorded, correlating to the jk=0(0) and 1(1) rotational levels of free CH3D and CHD3, respectively. The van der Waals bond distance R, intermolecular stretching frequency nus, and the corresponding stretching force constant ks were derived from the obtained spectroscopic results. The results obtained for the jm=0 ground state are compared to the previous infrared and millimeter wave data. A 17O nuclear quadrupole coupling constant was determined from the resolved hyperfine structure of 13C17O-12CH4 and was used to obtain angular information about the carbon monoxide subunit. A Coriolis interaction was deduced from the irregular spectral pattern involving levels with jm=1. Qualitative information about the extent of the perturbation was obtained from a comparison of spectroscopic constants of different isotopomers.

Silver(II) Complexes of Tetraazamacrocycles: Studies on e.p.r. and Electron Transfer Kinetics with Thiosulfate Ion

The e.p.r. studies of AgII complexes of three tetraazamacrocycles, 1,4,8,11-tetraazacyclo-tetradecane (cyclam)(I), 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclo-teradecane-4,11-diene (Me6CD)(II) and 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclo-teradecane (Me6Cy)(III) and the electron transfer between [AgII(cyclam)]2+ and thiosulfate ion are described. The e.p.r. studies reveal that the spectra are almost the same as those reported earlier, particularly, for polycrystalline material and are typical of a d 9 planar AgII complexes. Previous e.p.r. studies on these square planar polycrystalline complexes showed no ligand hyperfine splitting. Reinvestigation of the e.p.r. spectra of these complexes in both the solid state and in solution at room (T∼ 297 K) and at low (T= 120 K) temperature reveals resolved hyperfine structures in solution for [Ag(Me6CD)]2+ and [Ag(Me6Cy)]2+ complexes. Surprisingly, such a structure was not observed in solutions of [Ag(cyclam)]2+. Computer simulations of the hyperfine structure observed in solutions are in good agreement with structural formulae proposed. The oxidation of thiosulfate ion by [Ag(cyclam)]2+ follows the rate law:−d/dt[AgII(cyclam)2+]=(k 1 Q 1[H+]+k 2 Q 2 K a2)/([H+]+K a2)[AgII(cyclam)2+][S2O3 2−] and an inner-sphere mechanism is proposed, based on the spectral and kinetic evidence.

Phosphorus hyperfine structure in the electronic spectrum of the HPCl free radical.

The 444 nm 2 0 (1) bands of the A 2A'-X 2A" transition of the jet-cooled HP 35Cl and HP 37Cl radicals have been studied at high resolution using the pulsed electric discharge technique with a precursor mixture of PCl3 and H2. Spectra recorded with linewidths of approximately 360 MHz revealed resolved hyperfine structure in both isotopomers arising from the excited state Fermi contact interaction of the unpaired electron with the magnetic moment of the 31P nucleus, with aF'=0.0641(10) cm(-1) and 0.0636(31) cm(-1) for HP 35Cl and HP 37Cl, respectively. No contribution from the ground state, or excited state contributions from the hydrogen or chlorine nuclei were resolved, confirming ab initio predictions that HPCl is a p pi radical in the X state, and an s sigma radical with a substantial contribution from the phosphorus 3s atomic orbital in the A state. The free atom comparison method has been used to estimate that the singly occupied molecular orbital in the excited state has 14% phosphorus 3s character.

Single crystal X- and Q-band EPR spectroscopy of a binuclear Mn(2)(III,IV) complex relevant to the oxygen-evolving complex of photosystem II.

The anisotropic g and hyperfine tensors of the Mn di-micro-oxo complex, [Mn(2)(III,IV)O(2)(phen)(4)](PF(6))(3).CH(3)CN, were derived by single-crystal EPR measurements at X- and Q-band frequencies. This is the first simulation of EPR parameters from single-crystal EPR spectra for multinuclear Mn complexes, which are of importance in several metalloenzymes; one of them is the oxygen-evolving complex in photosystem II (PS II). Single-crystal [Mn(2)(III,IV)O(2)(phen)(4)](PF(6))(3).CH(3)CN EPR spectra showed distinct resolved (55)Mn hyperfine lines in all crystal orientations, unlike single-crystal EPR spectra of other Mn(2)(III,IV) di-micro-oxo bridged complexes. We measured the EPR spectra in the crystal ab- and bc-planes, and from these spectra we obtained the EPR spectra of the complex along the unique a-, b-, and c-axes of the crystal. The crystal orientation was determined by X-ray diffraction and single-crystal EXAFS (Extended X-ray Absorption Fine Structure) measurements. In this complex, the three crystallographic axes, a, b, and c, are parallel or nearly parallel to the principal molecular axes of Mn(2)(III,IV)O(2)(phen)(4) as shown in the crystallographic data by Stebler et al. (Inorg. Chem. 1986, 25, 4743). This direct relation together with the resolved hyperfine lines significantly simplified the simulation of single-crystal spectra in the three principal directions due to the reduction of free parameters and, thus, allowed us to define the magnetic g and A tensors of the molecule with a high degree of reliability. These parameters were subsequently used to generate the solution EPR spectra at both X- and Q-bands with excellent agreement. The anisotropic g and hyperfine tensors determined by the simulation of the X- and Q-band single-crystal and solution EPR spectra are as follows: g(x) = 1.9887, g(y) = 1.9957, g(z) = 1.9775, and hyperfine coupling constants are A(III)(x) = |171| G, A(III)(y) = |176| G, A(III)(z) = |129| G, A(IV)(x) = |77| G, A(IV)(y) = |74| G, A(IV)(z) = |80| G.

Doppler-limited rotational spectrum of the NH radical in the 2 THz region

The Doppler-limited rotational spectrum of the NH radical in its electronic ( X 3 Σ − ) and vibrational ground state has been measured using the frequency stabilized Cologne side-band spectrometer in the frequency region near 2 THz. The nitrogen 14N nuclear hyperfine patterns have been observed accompanying the resolved fine ( J ′← J″) structure of the N=2←1 rotational transition. The observed peak frequencies were analyzed in detail together with the previously measured hyperfine frequencies of the N=1←0 rotational transition and with combination differences obtained from the high-resolution electronic spectra to derive precise rotational, centrifugal distortion, fine, and hyperfine parameters. In the numerical analysis the essential attention has been paid to partly resolved and unresolved hyperfine structures. The peak positions of the partly or fully overlapped lines were analyzed with the help of a profile simulation with estimated half-widths and calculated relative intensities and in this manner the least square fit of the unresolved and partly resolved lines was significantly improved. The NH radical is an extremely important species in nitrogen chemical reaction networks in the interstellar medium and atmospheric chemistry.

Nitric oxide adsorbed on zeolites: EPR studies

CW-EPR studies of NO adsorbed on sodium ion-exchanged zeolites were focused on the geometrical structure of NO monoradical and (NO) 2 biradical formed on zeolites. The EPR spectrum of NO monoradical adsorbed on zeolite can be characterized by the three different g-tensor components and the resolved y-component hyperfine coupling with the 14 N nucleus. Among the g-tensor components, the value of g zz is very sensitive to the local environment of zeolite and becomes a measure of the electrostatic field in zeolite. The temperature dependence of the g-tensor demonstrated the presence of two states of the Na–NO adduct, in rigid and rotational states. The EPR spectra of NO adsorbed on alkaline metal ion-exchanged zeolite and their temperature dependency are essentially the same as that on sodium ion-exchanged zeolite. On the other hand, for NO adsorbed on copper ion-exchanged zeolite it is known that the magnetic interaction between NO molecule and paramagnetic copper ion are observable in the spectra recorded at low temperature. The signals assigned to (NO) 2 biradical were detected for EPR spectrum of NO adsorbed on Na-LTA. CW-EPR spectra as well as their theoretical calculation suggested that the two NO molecules are aligned along their NO bond axes. A new procedure for automatical EPR simulation is described which makes it possible to analyze EPR spectrum easily. In the last part of this paper, some instances when other nitrogen oxides were used as a probe molecule to characterize the zeolite structure, chemical properties of zeolites, and dynamics of small molecules were described on the basis of selected literature data reported recently.

Evolution of the Eu2+ local environment in Ca1−Eu B6

The local environment of Eu 2+ (4 f 7 ,S= 7 2 ) in Ca 1−x Eu x B 6 (0.003⩽x⩽0.10 and x =1.00) was studied by electron spin resonance. For x =0.003 the spectra show resolved fine and hyperfine structure due to the electric cubic crystal field and nuclear hyperfine field, respectively. The observed Lorentzian line shape indicates an insulating environment for the Eu 2+ ions. As x increases ( x >0.003) the lines broaden and became Dysonian in shape, suggesting a metallic environment for the Eu 2+ ions. In contrast to our previous work in Ca 1− x Gd x B 6 , the resolved fine structure can be observed in the metallic regime up to x ≃0.10 and no coexistence of insulating/metallic-like spectra was found.

Identification of the carbon dangling bond center at the 4H-SiC/SiO(2) interface by an EPR study in oxidized porous SiC.

We report the observation of a paramagnetic interface defect in thermally oxidized porous n-type doped 4H-SiC/SiO(2). Based on its axial symmetry and resolved hyperfine interactions it is attributed to an sp(3) carbon dangling bond center situated at the SiC side of the interface. This center is electrically active and pins the Fermi level in the oxidized samples. No silicon related paramagnetic dangling bond centers are observed. The formation of dangling bond centers seems to be related to interstitial oxygen diffusion at the interface during the oxidation process.

Electron paramagnetic resonance study of Ag0 atoms and Ag2+ ions in β-BaB2O4 nonlinear optical crystals

Silver ions have been diffused into single crystals of β-BaB2O4 (commonly referred to as BBO). The crystals were wrapped in silver foil and then held in air at 760°C for periods of time ranging from 1 to 7 h. This treatment resulted in Ag+ ions occupying both Ba2+ sites and interstitial sites within the crystal. After the diffusion, the crystal was irradiated at room temperature with 60 kV x-rays. Three distinct paramagnetic defects, one holelike and two electronlike, were produced by the x-rays and observed with electron paramagnetic resonance (EPR) at temperatures between 15 and 80 K. The Ag+ ions substituting for Ba2+ ions trapped radiation-induced holes and became paramagnetic Ag2+ ions. At the same time, interstitial Ag+ ions trapped electrons and became paramagnetic Ag0 atoms. There are two distinct versions of these latter centres. The EPR spectra of the Ag0 atoms show resolved hyperfine from the 107,109Ag isotopes, as well as from a neighbouring nucleus (possibly a Na+ impurity substituting for a Ba2+ ion). All the radiation-induced EPR signals were stable at room temperature for hours.

Spectroscopic investigation of hard and soft metal binding sites in synthetic melanin

Understanding the interaction of metal ions with melanin structures is essential to know some intricate functions of this kind of compounds in the living systems. The coordination chemistry of vanadyl oxycation, VO(II), and copper, Cu(II), in a synthetic l-dopa melanin (SM) was investigated using electron paramagnetic resonance (EPR), Fourier transform infrared (FT IR) and ultraviolet–visible (UV–Vis) spectroscopy. The EPR and FT IR spectra indicated axially coordinated VO(II) ions bound to catecholate functional groups, and a fraction of the Cu(II) ions in a square planar structure partially coordinated to carboxylate functional groups. The resolved EPR perpendicular super hyperfine splitting constant, A N⊥, and the Cu(II) complex EPR parallel parameters g ‖ and A ‖ values indicated two magnetically equivalent nitrogen atoms in a plane with the coordination sphere, CuN 2O 2. The ultraviolet–visible (UV–Vis) spectra of solid samples indicate supramolecular structure constitution for the synthesized VO(II)SM and Cu(II)SM complexes.

High-frequency and -field EPR spectroscopy of tris(2,4-pentanedionato)manganese(III): investigation of solid-state versus solution Jahn-Teller effects.

High-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy of a classical coordination complex, Mn(acac)(3) (Hacac = 2,4-pentanedione), has been performed on both solid powder and frozen solution (in CH(2)Cl(2)/toluene, 3:2 v/v) samples. Parallel mode detection X-band EPR spectra exhibiting resolved (55)Mn hyperfine coupling were additionally obtained for frozen solutions. Magnetic susceptibility and field-dependent magnetization measurements were also made on powder samples. Analysis of the entire EPR data set for the frozen solution allowed extraction of the relevant spin Hamiltonian parameters: D = -4.52(2); |E| = 0.25(2) cm(-1); g(iso) = 1.99(1). The somewhat lower quality solid-state HFEPR data and the magnetic measurements confirmed these parameters. These parameters are compared to those for other complexes of Mn(III) and to previous studies on Mn(acac)(3) using X-ray crystallography, solution electronic absorption spectroscopy, and powder magnetic susceptibility. Crystal structures have been reported for Mn(acac)(3) and show tetragonal distortion, as expected for this Jahn-Teller ion (Mn(3+), 3d(4)). However, in one case, the molecule exhibits axial compression and, in another, axial elongation. The current HFEPR studies clearly show the negative sign of D, which corresponds to an axial (tetragonal) elongation in frozen solution. The correspondence among solution and solid-state HFEPR data, solid-state magnetic measurements, and an HFEPR study by others on a related complex indicates that the form of Mn(acac)(3) studied here exhibits axial elongation in all cases. Such tetragonal elongation has been found for Mn(3+) and Cr(2+) complexes with homoleptic pseudooctahedral geometry as well as for Mn(3+) in square pyramidal geometry. This taken together with the results obtained here for Mn(acac)(3) in frozen solution indicates that axial elongation could be considered the "natural" form of Jahn-Teller distortion for octahedral high-spin 3d(4) ions. The previous electronic absorption data together with current HFEPR and magnetic data allow estimation of ligand-field parameters for Mn(acac)(3).

Laser spectroscopy of VS: hyperfine and rotational structure of the C4Σ −– X4Σ − transition

The (0,0) and (0,1) bands of the C 4Σ −– X 4Σ − electronic transition of VS (near 809 and 846 nm, respectively) have been recorded at high resolution by laser-induced fluorescence, following the reaction of laser-ablated vanadium atoms with CS 2 under supersonic free-jet conditions. A least squares fit to the resolved hyperfine components of the rotational lines gives the rotational constants and bond lengths as C 4Σ −: B 0=0.188898±0.000027 cm −1 , r 0=2.1315 A ̊ ; X 4Σ −: B 0=0.203684±0.000025 cm −1 , r 0=2.0526 A ̊ . The electron spin parameters for the two states show that there are some similarities between the states of VS and those of VO, but the hyperfine parameters show that the compositions of the partly filled molecular orbitals are by no means the same. The ground state Fermi contact parameter of VS, b( X 4Σ −), is only 58% of that of the ground state of VO, which implies that the σ orbital of the ground σδ 2 electron configuration has less than 50% vanadium 4 s character. Similarly, the excited state Fermi contact parameter, b( C 4Σ −), is very much smaller than that of VO. No local rotational perturbations have been found in the C 4Σ − state of VS, though an internal hyperfine perturbation between the F 2 and F 3 electron components at low N confuses the hyperfine structure and induces some forbidden (Δ J=±2) rotational branches.