Keith D. Sales

Semiquinone anion radicals formed by the reaction of quinones with glutathione or amino acids

Quinone Thiol Amino acid ESR Semiquinone anion radical

Preparation of the bis(trimethylsilyl)amido lanthanide chlorides [{Ln[N(SiMe3)2]2(µ-Cl)(thf)}2](thf = tetrahydrofuran), and the crystal and molecular structures of the gadolinium and ytterbium complexes

Reaction of LnCl3 with 2 equivalents of LiL [L = N(SiMe3)2] in tetrahydrofuran (thf) at –5 °C yields [{LnL2Cl(thf)}2] for Ln = Eu, Gd, or Yb or [YL2Cl(thf)2] for Ln = Y. X-Ray crystal structure analysis of the compounds of Gd and Yb confirms the formation of halide-bridged dimers. In the M2Cl2 bridging unit the M–Cl bond lengths differ slightly and the angle at the metal (ca. 74°) is much smaller than at chlorine (ca. 106°). The metal co-ordination geometries are irregular but can be best described as distorted trigonal bipyramidal with the bridging chlorines spanning one axial and one equatorial site. The bond lengths M–Cl and M–N indicate a Gd–Yb radius difference of ca. 0.07 A, but the M–O distances differ by 0.093 A. Variable-temperature n.m.r. studies of [{EuL2Cl(thf)}2] and [{YbL2Cl(thf)}2] show that there is free rotation about Ln–N and Ln–O bonds at room temperature. These rotations are frozen out at low temperatures to give a solution structure having higher symmetry (C2h) than that found in the crystal.

Effects of preparation parameters on oxygen stoichiometry in Bi4V2O11–δ

The effects of various preparation parameters on vanadium reduction in Bi4V2O11–δhave been investigated using EPR, 51V MAS solid state NMR and UV diffuse-reflectance spectroscopies and also SQUID magnetometry and powder neutron diffraction. The results confirm that a greater amount of vanadium reduction is observed in rapidly quenched samples and that significant oxidation occurs when samples are slow cooled. Evidence for spin–spin dipolar coupling is seen in the EPR patterns while uncoupled V4+ spins contribute to weak paramagnetic behaviour. Band gaps of around 2 eV from the UV data suggest there may be a significant electronic component to low temperature conductivities. The 51V NMR data are not inconsistent with the presence of mainly distorted octahedral and tetrahedral coordinations for vanadium.

Formalisms for the description of multiple-pulse NMR experiments

During the last few years, the product operator formalism has provided the most popular theoretical basis for the description and analysis of multiple-pulse NMR experiments, having all but replaced the more cumbersome density matrix method. However, those unacquainted with the detailed theory behind the formalism experience difficulty in arriving at a working understanding of the method. In this paper an introductory treatment of the theory for spin-12 systems with weak coupling using results from both the product operator and density matrix approaches is provided. Based on a handful of results from quantum mechanics and matrix algebra a small number of physical ideas are introduced which allow description of the evolution of product operators during any multiple-pulse experiment. These ideas are illustrated with an analysis of a COSY experiment.

Lanthanide thiolate complexes; synthesis of [Ln{N(SiMe3)2}(µ-SBut)]2(Ln = Eu, Gd, Y) and the X-ray crystal structure of the Gd complex

The lanthanide thiolate complexes [LnL2(µ-SBut)]2[L = N(SiMe3)2; Ln = Eu, Gd, Y] have been prepared by reaction of [LnL2Cl(THF)]2(Ln = Eu, Gd; THF = tetrahydrofuran) or LnL2(THF)2(Ln = Y) with LiSBut; the crystal structure of [GdL2(µ-SBut)]2 is reported.

Electro-organic reactions: Part 34. Kinetic basicities of some electrogenerated organic dianions and the competition between protonation and reproportionation

Abstract Convenient and reliable double potential step chronamperometry methods are described for measurement of the rates of protonation of 9-fluorenylidene dianions by carbon acids. The rates of protonation are shown to be insensitive to competing base-catalysed enolisation of the carbon acid. Comparison of experimental i - t transients with those simulated for plausible mechanisms shows that although the measured protonation rate constants are affected by allowing for rapid reproportionation, they are all affected in a similar manner. In contrast, ion-pairing or complexation effects are significant; protonation rate differences caused by varying cation: dianion interactions have been measured and explained. For the dianion of diethyl(4,5-diazafluoren-9-ylidene)malonate a 1:2 complex is formed with Na + and its formation constant in DMSO has been measured as 1.7 × 10 8 .

Nitrosation and nitrosylation of haemoproteins and related compounds. Part 4. Pentaco-ordinate nitrosylprotohaem as the pigment of cooked cured meat. Direct evidence from e.s.r. spectroscopy

The pigment of cured meat, nitrosylprotohaem, was obtained by synthesis and by extractionn from nitrosylhaemoglobin and from cured meats. The e.s.r. spectrum of nitrosylprotohaem was observed in two types of solvent; those which do, and those which do not, provide an effective nitrogenous sixth ligand : spectra characteristic of hexa- and penta-co-ordination are observed, respectively. Similar e.s.r. spectra were detected in the direct examination of a variety of cured meats, and are interpreted in terms of pigment structure.

Solid-state 13C NMR spectra of iron carbonyl complexes: shielding tensor and error analyses

The high-resolution solid-state 13C NMR (nuclear magnetic resonance) spectra are reported of the iron carbonyl complexes Fe3(CO)8 (PhCCPh)2(I) and Fe3(CO)12(II), 13C-enriched at the carbonyl carbons. The spectra were recorded with slow magic-angle spinning, and the intensities of the spinning side bands analysed by a computerized adaptation of the method of Herzfeld & Berger to give values for the principal components of the carbonyl carbon shielding tensors. Complex (I) includes two asymmetric double-bridging carbonyl groups, and the 13C shielding-tensor components assigned to the C—O bond axis (σ33) are intermediate between the values obtained for symmetric double-bridging carbonyl groups and linear M—C≡0 terminal carbonyls. It is proposed that the σ33 shielding component may therefore be used to monitor the degree of asymmetric bridging of a carbonyl group. All 13C resonances of complex (II) exhibit a degree of asymmetry in the shielding tensor and possible reasons for this are discussed. Three methods of estimating errors in the derived shielding-tensor components are considered. One method use calculated variances based upon differences between the experimental and calculated side-band intensities; this gives unreasonably large errors in the cases of axial symmetry in the shielding tensor. A second method explores different shielding components for values of ɸ2 (the sum of the squares of the differences between the experimental and calculated side-band intensity ratios) which exceed the best minimum by an arbitrary percentage. The third method considers errors due to the signal: noise ratios in the experimental spectra. The relative merits of these three approaches of error estimation are examined.

Quantitative analysis of 1D and 2D magnetization transfer experiments and the mechanism of rearrangement of [Re3(μ-H)4(CO)9NCMe]−

Abstract In the context of slowly chemically exchanging systems, we show how the same theory may be applied to the analysis of 2D NOESY data and 1D selective perturbation data. Both an iterative and a noniterative method may be used to analyze the same NOESY data and yield relaxation rates and exchange rate coefficients. In application to the intramolecular rearrangement of [Re 3 (μ-H) 4 (CO) 9 NCMe] − , the use of 13 C experimental NOESY data yields rate coefficients which support a concerted mechanism for the rearrangement, the first step of which is the dissociation of the acetonitrile ligand. The temperature dependence of the kinetics gives an activation energy of 63.1 ± 17.1 kJ mol −1 for the process. The 13 C relaxation is shown to be dominated by scalar coupling of the second kind to the quadrupolar rhenium. In the analysis of the 1 D selective perturbation 1 H data, using an iterative method, a nonzero value for the 1 H 1 H cross-relaxation rate (σ) means that it is not possible to obtain a reliable value for the exchange rate coefficients. The 1 H relaxation of the hydride ligands is dominated by dipole-dipole interactions with the rhenium nuclei.

Solid state 13C CP-MAS NMR spectra of iron (alkyne) carbonyl compounds

Abstract The solid state 13 C NMR spectra of a series of iron (alkyne) carbonyl complexes, enriched in 13 C at the carbonyl groups, have been obtained using 13 C-{ 1 H} cross-polarisation (CP) and slow magic angle spinning (MAS). In general, more resonances are observed from the solid state than from samples in solution; this may be due either to crystal packing effects lifting the degeneracy of certain signals or to a slowing of carbonyl fluxionality in the solid state. The intensities of the spinning side bands in the 13 C spectra have been used to derive values for the components of the 13 C shielding tensor. The component of the shielding tensor assigned to the CO bond axis (σ 33 ) is sensitive to the bonding of the CO group, i.e. whether it is a terminal or asymmetrically bridging group.