Fluorine Hyperfine Interactions Research Articles

EPR spectra of the Mn2+ ion in the oxyfluoride glass ceramics containing BaF2 nanocrystalline phase

Electron paramagnetic resonance (EPR) spectroscopy has been used to investigate structure of the oxyfluoride glass ceramics containing BaF2 fluoride component. Mn2+ ion has been used as a paramagnetic probe. After successful material synthesis and thermal treatment at temperatures above first crystallization temperature transparent glass ceramics samples were obtained. For these samples superhyperfine (SHF) structure in the X-band EPR spectra taken at 77 K has been observed. Such SHF structure is characteristic to fluorine hyperfine interaction observed in the fluoride crystals. Obtained data allow analyzing the structure of the BaF2 crystallites and possible location of the Mn2+ ion in the BaF2 crystalline lattice.

Velocity modulation spectroscopy of molecular ions II: The millimeter/submillimeter-wave spectrum of TiF + ( X3Φr)

The pure rotational spectrum of the molecular ion TiF + in its 3 Φ r ground state has been measured in the range 327–542 GHz using millimeter-wave direct absorption techniques combined with velocity modulation spectroscopy. TiF + was made in an AC discharge from a mixture of TiCl 4, F 2 in He, and argon. Ten transitions of this ion were recorded. In every transition, fluorine hyperfine interactions, as well as the fine structure splittings, were resolved. The fine structure pattern was found to be regular with almost equal spacing in frequency between the three spin components, in contrast to TiCl +, which is perturbed in the ground state. The data were fit with a case ( a) Hamiltonian and rotational, fine structure, and hyperfine constants were determined. The bond length established for TiF +, r 0 = 1.7775 Å, was found to be shorter than that of TiF, r 0 = 1.8342 Å—also established from mm-wave data. The hyperfine parameters determined are consistent with a δ 1 π 1 electron configuration with the electrons primarily located on the titanium nucleus. The nuclear spin–orbit constant a indicates that the unpaired electrons are closer to the fluorine nucleus in TiF + relative to TiF, as expected with the decrease in bond length for the ion. The shorter bond distance is thought to arise from increased charge on the titanium nucleus as a result of a Ti 2+F − configuration. A similar decrease in bond length was found for TiCl + relative to TiCl.

Perturbed hyperfine doubling in the A 2Π1/2 and [18.6]0.5 states of YbF

We report a spectroscopic hyperfine structure study of the YbF174 molecule in the excited states A 2Π1/2(v=0), A 2Π1/2(v=1), and [18.6]0.5(v=0). The fluorine hyperfine interaction constant A∥ in each state is, respectively, −1.6±2 MHz, 48.8±3 MHz, and 63.3±3 MHz. The corresponding values of A⊥ are 4.6±1 MHz, −24.7±1 MHz, and −31.9±1 MHz. The very small hyperfine interaction characteristic of A 2Π1/2(v=0) is not seen in A 2Π1/2(v=1) because this state is strongly mixed with [18.6]0.5(v=0). Analysis of our results shows that the lower-lying of the two mixed states is predominantly of [18.6]0.5(v=0) character, not A 2Π1/2(v=1) as had previously been thought. The deperturbed hyperfine constants for [18.6]0.5 are A∥=114±5 MHz and A⊥=−61±2 MHz, comparable with those in the X state and much stronger than in the unperturbed A 2Π1/2 state. This supports the view that the state [18.6]0.5 is of 4f136s2 character.

Laser-rf double resonance spectroscopy of 174YbF in theX 2Σ+ state: Spin-rotation, hyperfine interactions, and the electric dipole moment

We report the first laser-radio-frequency double resonance spectrum of a simple lanthanide compound, the paramagnetic radical 174YbF. Measurements of the rf intervals as a function of the rotational quantum number N allow us to determine precise spin-rotation and fluorine hyperfine interaction coupling constants for the X 2Σ+(v=0 and v=1) ground states of 174YbF. The results for v=0 are γ0=−13.424 00(16) MHz, γ1=3.982 3(11) kHz, γ2=−25(1) mHz, b0=141.795 6(5) MHz, b1=−0.510(11) kHz, c=85.402 6(14) MHz, C=20.38(13) kHz. For v=1 they are γ0=−33.811 8(7) MHz, γ1=4.323(6) kHz, γ2=−28(9) mHz, b0=139.89(4) MHz, b1=−0.7(4) kHz, c=86.75(5) MHz, C=18.3(1) kHz. A direct microwave measurement of the first rotational interval in X 2Σ+(v=0) gives the rotational constant B0=0.241 292 7(7) cm−1. Finally, the Stark shift of hyperfine transitions in the first two rotational states of X 2Σ+(v=0) are analyzed to determine the electric dipole moment μe=3.91(4) D. We find that although the gross structure of YbF in its ground state resembles that of an alkaline earth monofluoride, the 4f shell is evidently important even in the low-lying excited states and leads to unusual behavior in the ground-state magnetic structure.

Valence bond study of fluorine hyperfine interactions near trapped hydrogen atoms in the alkaline earth fluorides

Hyperfine interactions of fluorine nuclei near trapped hydrogen atoms in CaF2, SrF2, and BaF2 are computed using a valence-bond model that includes electronic charge transfer from the nearest-neighbour fluorine ions to the trapped hydrogen atoms at both interstitial and substitutional sites, termed Hi0 and H20 centres respectively. The best agreement between theory and experiment is obtained by fitting the computed and experimental anisotropic hyperfine constants (HFCs) of the nearest-neighbour fluorine ions in order to assess the amount of charge transfer. The isotropic HFCs are then computed with the resulting wavefunction. In all cases, the amount of charge transfer is very small but nevertheless essential for explaining the anisotropic HFCs and the g-shift. In the case of the Hi0 centre the theoretical isotropic HFCs are smaller than the corresponding experimental values by 20% for SrF2 and BaF2, and by 17% for CaF2. For the H20 centre, on the other hand, quantitative agreement between theory and experiment can be achieved by including a small inward displacement of the nearest-neighbour F- ions, which reduces the undistended F--H distance by about 10%.

ChemInform Abstract: HYPERFINE COUPLING CONSTANTS AND ANGLES OF TWIST OF PHENYL AND PENTAFLUOROPHENYL SUBSTITUTED METHYL RADICALS

AbstractAnhand der Literaturdaten für das Benzyl‐, Diphenylmethyl‐ und Triphenylmethyl‐ Radikal sowie für die entsprechenden Perfluorphenyl‐Analogen und anhand eigner Untersuchungen am Bis‐(pentafluorphenyl)‐methyl‐Radikal wird das aF/aH‐Verhältnis für die o‐, m‐ und p‐Stellung berechnet.

Hyperfine coupling constants and angles of twist of phenyl and pentafluorophenyl substituted methyl radicals

The ring proton hyperfine coupling constants of the benzyl, diphenylmethyl and triphenylmethyl radicals are compared with fluorine splittings in the corresponding perfluorophenyl radicals. Trends in the ratio of aF/aH are rationalised in terms of twisting of the phenyl rings; INDO calculations on model systems support this interpretation and suggest that a comparison of ring proton and fluorine hyperfine interactions, particularly at the meta positions, can provide useful stereochemical information.

Study of Fluorine Hyperfine Interactions in KZnF3: V2+ and Cr3+ by ENDOR

Fluorine ENDOR measurements have been made on first and second fluorine neighbours to V 2+ and Cr 3+ in KZnF 3 in order to clarify the role of the intervening Zn 2+ ions. The isotropic hf interaction parameters for the second fluorine neighbours have been obtained as +79 kHz and -22 kHz for v 2+ and Cr 3+ respectively. The negative sign of the result for the Cr 3+ strongly suggests that the cation-cation charge transfer interaction between the 3d 3 impurity and Zn 2+ ions can play important roles in these systems. Differences between the KMgF 3 and KZnF 3 for both V 2+ and Cr 3+ are discussed from the same standpoint.

ESR study of F32− in γ-irradiated KAsF6

An ESR study of single crystals of KAsF6, γ irradiated and observed at 77°K, has been made. A three-fluorine center similar to the Vt center, F32−, has been identified, and the principal components and directions for the g tensor, and for the fluorine hyperfine interaction tensors, have been obtained. The center is nonlinear with bond angle F–F–F = 150° ± 3°, the odd-electron density being larger on the central fluorine atom and smaller on the two equivalent end fluorine atoms of the radical. INDO calculations suggest that it is a σ-type radical with the unpaired spin density distributed in the fluorine orbitals lying in the plane of the three atoms.

Theory of F centres in alkaline earth fluorides

The point symmetry of the anion site in alkaline earth fluorides is T3d and the F centre (an electron in an anion vacancy) in these crystals lacks inversion symmetry. Point ion calculations have been performed for the F centre in alkaline earth fluorides with trial functions of sufficient flexibility to incorporate the anisotropy of the potential. The predicted strong admixture of s and f orbitals in the ground Gamma 1 state yields calculated fluorine hyperfine interactions in substantial agreement with results of Endor measurements. An even stronger admixture of p and d orbitals in the first excited state leads to a linear Stark effect and the magnitude of this effect has been calculated for the F band. Experimental investigation of the Stark effect of the F band in SrF2 provides an upper bound to the linear Stark splitting which is consistent with the predicted value. The quadratic Stark effect is discussed.

Fluorine Hyperfine Interaction in Aromatic Anion Radicals

The relation between the fluorine hyperfine splittings in about 20 fluorine substituted aromatic radicals and the π spin densities calculated according to McLachlans method is analysed with an expression of the type aF = QF FF ρπ FF + QF CC ρπ CC + (QF FC + QF FC) ρπ FC . For the Q factors the following values are obtained: QF FF =931 gauss; QF CC =86.6 gauss and (QF FC + QF FC) =345 gauss. Two simpler expressions with one and two parameters respectively are also tested. A theoretical estimate is made for the values of the Q factors and the results are compared with the values obtained semi-empirically.

The calculation of fluorine hyperfine interactions in π-electron radicals

Q factors have been calculated for fluorine hyperfine interaction in π-electron radicals using an independent-electron model for the σ orbitals. It is shown that the off-diagonal elements give an important contribution to fluorine coupling constants. Combining these Q factors with calculated π-electron spin densities of fluoro-nitrobenzenes gives good agreement with experimental coupling constants.

Electron paramagnetic resonance spectra of the ·O 2F radical in liquid and solid O 2F 2

It has been established that the paramagnetism of O 2F 2 is due to the ·O 2F radical. The electron paramagnetic resonance spectrum of liquid dioxygen difluoride (O 2F 2) contains an isotropic doublet as a result of a hyperfine interaction with one fluorine nucleus. The doublet has a splitting of 15.9 gauss, and is centered at a g-value of 2.0033. Liquid O 2F 2 prepared with oxygen enriched to 51.2% with 17O exhibited a complex isotropic spectral pattern also centered at g = 2.0033 with hyperfine interactions of 15.9 gauss from a fluorine nucleus and 22.4 gauss and 14.9 gauss from two nonequivalent oxygen nuclei. The observed pattern is a result of four spectra superimposed one upon the other. The following isotopic species contributed to the paramagnetism: ·OOF, 24.4% abundant; · 17OOF, 24.4%; ·O 17OF, 24.4%; and · 17O 17OF, 26.8%. The EPR spectrum of solid O 2F 2 and solid 17O 2F 2 consisted of an anisotropic powder pattern characteristic of axial symmetry. The values of the fluorine hyperfine interaction observed were A ⊥ = −24.0 gauss, and A ∥ = 98.8 gauss. The average g-value was 2.0033. The values of the hyperfine interactions were used to calculate the unpaired electron density in various orbitals. The unpaired electron density in the radical was high in the 2 p orbitals of the oxygen atoms and low in orbitals of the fluorine atom.

Electron resonance studies of fluorine hyperfine interactions

Radicals derived from five fluoronitrobenzenes have been prepared and their E.S.R. spectra measured. The hyperfine splittings are discussed and evidence is presented for steric hindrance of a nitro group when it has two orthofluorine neighbours.