Richard Bramley

On standardization of the spin Hamiltonian and the ligand field Hamiltonian for orthorhombic symmetry

The idea that the ratio E/D ≡ λ can be confined by a proper choice of the axis system to the range (0,±1/3) serves as a basis for the standardization of the spin Hamiltonian and the ligand field Hamiltonian for orthorhombic symmetry. Due to recent results on the transformation of the Stevens operators Oqk, relations are given which allow for extension of the standardization to the spin Hamiltonian for paramagnetic centers with spin S≥2. Several EPR studies of 3dn and 4 f n ions in crystals are reconsidered. The results prove the usefulness of the clear choice of the parameters Bqk suggested by the standardization idea. Application of the standardization in the area of the ligand field theory, which seems rather novel, is also considered. Crystal‐field parameters for several rare earth ions in garnets are recalculated. An arbitrary choice from among the equivalent sets of the crystal‐field parameters used in the review by Morrison and Leavitt (1982) should be replaced by a transformation to the ‘‘standard’...

Spin-orbit coupling effects in EPR spectra of lowest triplet states of aromatic carbonyl compounds

Abstract The assignment of 3nπ* or 3ππ* character to the lowest triplet states of aromatic carbonly compounds can be made from their EPR spectra on the basis of spin-orbit induced changes to both the fine-structure parameters and theg-values.

EPR characterisation of the CrV intermediates in the CrVI/V oxidations of organic substrates and of relevance to Cr-induced cancers

An empirical method has been developed for determining the coordination number of CrV complexes (five- or six-coordinate) and the donor groups bound to CrV from the measured giso and Aiso values. The giso values depend only on the nature of the donor groups and the number of each group, which is very useful in the characterisation of the coordination number and donors. By contrast, the Aiso values appear to be sensitive to the nature of the equatorial donor atoms and the coordination geometry. Strong correlations have been found between the giso values of isoelectronic CrV and VIV complexes and between the Aiso values of analogous complexes of these two metal ions. These, combined, have been used to understand the solution chemistry of CrV using EPR spectroscopy. Such studies have also been invaluable in unravelling the biochemistry that occurs in cell media and cells exposed to carcinogenic CrVI and, hence, possible mechanisms of Cr-induced cancers.

High-field NMR spectroelectrochemistry of spinning solutions: simultaneous in situ detection of electrogenerated species in a standard probe under potentiostatic control

Abstract This report summarizes the design and performance of the first practicable in situ electrogenerative high-field solution-NMR cell, designed for simultaneous NMR spectroelectrochemistry in a conventional 10- or 16-mm 300 MHz probe. The axially symmetric three-electrode assembly is stationary and self-aligning. It dips into a standard spinning 10-mm sample tube and is adapted for convenient operation in any commercial high-field, multi-nuclear, variable-temperature FT NMR spectrometer. The tubular, rf-transparent working electrode (presently consisting of a gold film, coated on glass) completely spans the detection region; the recommended coating thickness is ≤0.01 δ , where δ is the rf-absorbing conductor skin depth. With careful shimming, very narrow 1 H signals (∼1 Hz) can be obtained. The performance of the in situ NMR-electrochemical cell is illustrated by monitoring the progressive reduction of ‘ p -quinone’ (1,4-benzoquinone) to its dihydro-form by controlled-potential electrolysis in an acidic medium.

Highly resolved dual phosphorescence of xanthone in several hosts

Abstract The energies and orbital natures of the lowest two triplet states of xanthone and the temperature dependence of the phosphorescence spectra have been investigated in six hosts. The energy separation between T 1 ≈ 3 ππ * and T 2 ≈ 3 Ππ * varies sensitively with the solvent and accounts for the dual phosphorescence.

Xanthone. II. Vibronic coupling analysis from high resolution phosphorescence spectra

Abstract High resolved phosphorescence spectra of xanthone have been recorded in four host matrices in order to study vibronic coupling between the lowest tripl origin in all hosts, and the energetically close-lying second triplet state T 2 , which is of nπ* orbital origin. In three hosts, there is thermall to phosphorescence from T 1 . Vibrational analyses of the two emissions are reported. The vibrational structure of both emissions depends little on t vibronic mixing between the lowest two triplet states is weak, in spite of the small energy separation in some hosts. The importance of the different i 1 and its sublevels is discussed, and it is concluded that across energy separations smaller than about 200 cm −1 spin—orbit mixing is more orbital mixing between the 3 ππ* and 3 nπ* configurations of xanthone.

Improved spin hamiltonian parameters for Mn2+ determined by EPR at zero magnetic field

Abstract EPR has been studied at zero magnetic field for high-spin Mn 2+ in MgSO 4 ·7H 2 O and NH 4 Cl. Significant differences have been found between high-field EPR spin hamiltonmn parameters in the literature and those necessary to fit the complex zero-field spectra. Sources of the discrepancies are discussed in relation to the Mn 2+ systems and more generally

Zero-field EPR of Mn2+ in tutton salts

Abstract Accurate values of the main spin Hamiltonian parameters for Mn 2+ in four Tutton salts have been obtained using the technique of zero-field EPR (ZFR). Anomalies in the sign of the second-order axial fine-structure term found in previous published data are shown from single-crystal EPR measurements to be due to orthogonal transformations of the axis system. Analysis in terms of superposition theory shows that the intrinsic second-order parameter for the ligand oxygen atoms probably falls within the normal range for oxygen. It is also found that the hydrogen atoms may have a significant effect on the second-order tensor.

Xanthone. I. optical detection of an extremely large sublevel splitting the lowest triplet state

Abstract The phosphorescence spectrum of xanthone in n -hexane is resolved into three components, one of which is due to thermally activated emission from a n π * origin. The other two emissions originate from extremely widely split sublevels of the lowest triplet state, which is of 3 ππ* origin. higher energy of T L x , and T 1 y , and thus below 4K there is thermal depletion of T 1 z which has the largest radiative strength. Th consequence of spin-orbit coupling between the T x and T y sublevels of the-two very close-lying, lowest triplet states. There are signif mixing of orbital character, a mixing which is governed by spin-orbit coupling rather than vibronic coupling.

Synthesis and characterization of a (Zn, Ti)-substituted layered silicate

The incorporation of TiIV into the framework of zinc-substituted fluorohectorite is reported. Various spectroscopic techniques point to the substitution of TiIV into predominantly the octahedral rather than tetrahedral sublattice of the smectite framework. For clay particles prepared from a reactant composition with Ti/Ti + Si = 0.063 (ZTFH), energydispersive X-ray analysis of the particles observed by transmission electron microscopy indicates that TiIV accounts for ca. 6% of the Ti + Si. X-Ray absorption near-edge spectra indicate that the TiIV in these clay particles is predominantly in an octahedral coordination. This is supported by electron paramagnetic resonance (EPR) spectra of an oriented film sample in which TiIV is reduced to TiIII by heating the sample to 450 °C in 30 Torr of CO. The observed single-ion TiIII EPR spectrum shows dramatic dependence of the orientation of the clay film in the magnetic field. This orientation dependence is consistent with the principal g-tensor axis of the TiIII site being perpendicular to the clay layers. The EPR parameters of this TiIII species are consistent with TiIII occupying an octahedral site. Three-pulse electron spin-echo modulation data taken at the TiIII EPR line position shows modulation at the 7Li nuclear frequency and simulation indicates that TiIII ions are at a distance of 0.30–0.35 nm from LiI ions, a distance that is consistent with these two ions occupying adjacent sites in the octahedral sheet. The MAS NMR spectrum of the zinc/titanium-substituted fluorhectorite consists of an intense Q3 resonance at –96.4 ppm and a weak shoulder at ca.–92.5 ppm. These two resonances are interpreted as due to variations in the octahedral sheet composition. Therefore TiIV appears to have a strong preference for occupation of the octahedral sites in the zinc-substituted fluorohectorite framework.