Stephen P. Best

Non-destructive pigment analysis of artefacts by Raman microscopy

Abstract When integrated with a microscope, Raman spectroscopy provides a uniquely specific and sensitive means of identifying pigment grains of sub-micrometre dimensions in a broad range of artefacts. This article reviews the methodology of Raman microscopy and its application to the study of mediaeval manuscripts.

Structural studies on the caesium alums, CsMIII[SO4]2·12H2O

Structure determinations of the caesium alums CsMIII[SO4]2·12H2O, M = V, Cr, Mn, Fe, Co, Al, Ga, or In, have been carried out at 295(1) K by X-ray diffraction methods. The data obtained include (i) MIII–O distances for the metals in the [M(OH2)6]3+ species [V–O, 1.992(6); Cr–O, 1.959(3); Mn–O, 1.991(6); Fe–O, 1.995(4); Co–O, 1.873(5); Al–O, 1.877(3); Ga–O, 1.944(3); and In–O, 2.112(4)A], (ii) an estimate of 1.34A for the effective co-ordinated radius for the water molecule in the cation, and (iii) the proposal of a new criterion for the classification of the different alum types, based on the disposition of the water molecules about the univalent cation.

A di-iron dithiolate possessing structural elements of the carbonyl/cyanide sub-site of the H-centre of Fe-only hydrogenase

The synthesis and characterisation of the first {2Fe2S}-cluster bearing both CO and CN ligands is described; the iron atoms are linked by the bridging 1,3-propanedithiolate unit that has been identified in the crystallographic structure of the {2Fe2S} sub-unit of the H-centre of the all-iron hydrogenase from Desulfovibrio desulfuricans.

Vibrational spectroscopic studies of trivalent hexa-aqua-cations: single-crystal Raman spectra between 275 and 1 200 cm–1 of the caesium alums of titanium, vanadium, chromium, iron, gallium, and indium

Oriented single-crystal Raman spectra have been recorded at 80 or 90 K for CsM(SO4)2·12H2O (M = Ti, V, Cr, Fe, Ga, or In), CsM(SeO4)2·12H2O (M = Cr or Fe), and CsFe(SO4)2·12D2O. A complete assignment of the spectra between 275 and 1 200 cm–1 has been made. The total symmetric metal–water stretching modes of the trivalent hexa-aqua-cations occur at 517 (Ti), 525 (V), 540 (Cr), 523 (Fe), 537 (Ga), and 505 cm–1(In), significantly higher than values previously reported. The wavenumbers of these modes vary with the reciprocal of the metal(III)–water bond lengths.

Infrared reflection absorption spectro‐electrochemical cell for the in situ study of redox‐active species at variable temperature

The design of an improved infrared reflection–absorption cell which facilitates the study of redox‐active chemical species is described. The modifications to existing cell designs permit (i) the sample to be cooled to temperatures limited by the characteristics of the solvent, (ii) the sample to be maintained under an inert atmosphere, (iii) the study of small (1–2‐cm3) volumes of solution, and (iv) the entire optical path to be maintained under vacuum. These features of the cell design facilitate the study of small quantities of compounds which are unstable with respect to reaction with oxygen, moisture, and redox partners. The study of tris‐maleonitriledithiolato complexes of chromium in formal oxidation states iv, v, and vi demonstrates the advantages of the cell.

Structures and spectroscopy of hexaaquametal(III) ions

Abstract The hexaaquametal(III) complex ions of transition and Group 13 metals can be isolated in a highly symmetric environment in alum crystals with various monovalent cations and sulphate or selsnate anions, M I M III (XO 4 ) 2 ·12H 2 O. Many of these have been investigated by X-ray crystallography and some by neutron scattering. The relationships among the structures are described, with emphasis on the way in which the water molecule is coordinated to the terviilent metal ion. Both trigonal-planar and trigonal-pyramidal coordination geometries arc observed. The structural studies are correlated with electron spin distributions obtained from polarised neutron diffraction, with aspects of the electronic spectra of some of the metal ions, with vibrational spectra, with electron paramagnetic resonance spectroscopy of the titanium alum, and with theoretical calculations. It is concluded that the observed stereochemistry reflects a subtle interplay between the electronic structure of the tervalent and the hydrogen bonding requirements of the lattice.

Stereochemical analysis of ferrocene and the uncertainty of fluorescence XAFS data

Methods for the quantification of statistically valid measures of the uncertainties associated with X-ray absorption fine structure (XAFS) data obtained from dilute solutions using fluorescence measurements are developed. Experimental data obtained from 10 mM solutions of the organometallic compound ferrocene, Fe(C(5)H(5))(2), are analysed within this framework and, following correction for various electronic and geometrical factors, give robust estimates of the standard errors of the individual measurements. The reliability of the refinement statistics of standard current XAFS structure approaches that do not include propagation of experimental uncertainties to assess subtle structural distortions is assessed in terms of refinements obtained for the staggered and eclipsed conformations of the C(5)H(5) rings of ferrocene. Standard approaches (XFIT, IFEFFIT) give refinement statistics that appear to show strong, but opposite, preferences for the different conformations. Incorporation of experimental uncertainties into an IFEFFIT-like analysis yield refinement statistics for the staggered and eclipsed forms of ferrocene which show a far more realistic preference for the eclipsed form which accurately reflects the reliability of the analysis. Moreover, the more strongly founded estimates of the refined parameter uncertainties allow more direct comparison with those obtained by other techniques. These XAFS-based estimates of the bond distances have accuracies comparable with those obtained using single-crystal diffraction techniques and are superior in terms of their use in comparisons of experimental and computed structures.

Stereochemistry of tervalent aqua ions: low temperature neutron diffraction structures of CsFe(SO4)2•12H2O and CsFe(SeO4)2•12H2O

The structures of CsFe(SO4)2·12H2O and CsFe(SeO4)2·12H2O have been investigated by neutron diffraction at 15 K. Both alums crystallise in the cubic space group Pa3, Z= 4, with a= 12.354(6)A for the sulphate and 12.593(10)A for the selenate. The structures were refined using 802 [CsFe(SO4)2·12H2O] and 1 059 [CsFe(SeO4)2·12H2O] inequivalent reflections to give weighted R factors of 0.029 and 0.028 respectively. In agreement with prior X-ray crystallographic determinations, CsFe(SO4)2·12H2O gives the β alum modification whereas CsFe(SeO4)2·12H2O forms an α alum. The stereochemistry of water co-ordination to FeIII differs in the two alums, with the angle between the metal (III)–water bond and the plane of the co-ordinated water molecule being 0.6(10)° for the sulphate and 18.6(10)° for the selenate. This tilting of the plane of the co-ordinated water molecule is shown to be related to the alum type. The iron(III)–water bond distance shows some sensitivity to the tilt of the co-ordinated water molecule, lengthening from 1.994(1) to 2.002(1)A as the plane of the water molecule is tilted by 18.6°. The sensitivity of the alum type to the size of the constituent ions, and to the stereochemistry of water co-ordination to the tervalent cation, is discussed in light of the hydrogen bonding in the alum structures.

Transient FTIR spectroelectrochemical and stopped-flow detection of a mixed valence {Fe(I)–Fe(II)} bridging carbonyl intermediate with structural elements and spectroscopic characteristics of the di-iron sub-site of all-iron hydrogenase

Iron(I) in biology?: one-electron oxidation of an (Fe(I)-Fe(I)) carbonyl cyanide precursor bearing a proximal thioether group leads to an (Fe(I)-Fe(II)) bridging carbonyl transient with spectral features similar to the di-iron sub-site of the CO inhibited paramagnetic centre of all-iron hydrogenase.

Infrared spectroscopic studies of aqueous solutions of dioxouranium(VI) and its hydrolysed products and of in situ electro-generated dioxouranium(V)

Aqueous solutions of dioxouranium(VI) (pH range 0 to 4) give rise to bands at 954 and 938 cm−1 attributable to the v3(MO2) stretching modes of the UO22+ and (UO2)2(OH)22+ cations, respectively. A shoulder at 916 cm−1 is assigned to the v3(MO2) mode of hydrolysed dioxouranium(VI) species of higher nuclearity. Infrared spectro-electrochemical studies using a thin-layer reflection-absorption cell have facilitated the study of the reduction of aqueous solutions of dioxouranium(VI) to yield dioxouranium(V) which may be further reduced to uranium(IV). The electrogeneration of dioxouranium(V) is monitored by following the increase in intensity of a band at 914 cm−1 which is present in the spectra at potentials between −0.2 and −0.8 V. The dioxouranium(V) species is predominantly in the form UO2+, which may be in solution or incorporated into an insoluble phase of uranium oxides which deposit onto the working electrode. The UVO bond length is estimated to be 1.76 A, 0.03 A longer than the UVIO bond in aqueous solution. The maximum concentration of UO2+able to be achieved is highly dependent on the pH and is optimum at pH 3.4. Changes in the pH of the solution under study can be monitored by infrared spectroscopy during the course of the reduction by determining the relative concentrations of hydrolysed dioxouranium(VI) species.