Enhanced and normal Raman scattering from pyridine adsorbed on rough and smooth silver electrodes

A multiplex spectrograph has been used to record potential difference and modulation Raman spectra of pyridine adsorbed on silver electrodes in an electrochemical cell. Spectra have been obtained from rough silver surfaces which give SERS and from surfaces where SERS has been diminished by prolonged cathodic polarisation (DSERS). Raman scattering from pyridine at smooth silver surfaces in potassium perchlorate and fluoride solutions has been distinguished from solution scatter by a potential modulation technique. The results show that the enhanced scattering caused by silver atom or cluster sites is respresentative of the surface as a whole as similar Raman spectra are obtained on smooth surfaces at a count rate as low as ?1.4 photons s^?1 (incident laser power 500 mW).Correlation of simultaneous differential capacitance data and “snapshot” SER spectra indicate that pyridine molecules in aqueous chloride ion solutions adsorb on silver in a flat π-bonded configuration at potentials markedly positive to the point of zero charge and exhibit specific reorientation at ?0.3 V and ?0.45 V (vs. SCE) to become N-bonded, perpendicular to the surface. Results also show that the adsorption behaviour of pyridine in chloride and fluoride ion solutions is largely similar.     

Chemical bonding interactions in Zr2Al

The electronic structure of Zr₂Al with the Ni₂In-type structure has been calculated by the method of W. Kohn and N. Rostoker (Phys. Rev. 91, 1111 (1954)). The results include densities of states, both total and partial, and resolved according to the angular momentum quantum number, and calculated electron densities presented so as to display directional bonding characteristics of electrons in the valence and conduction regions of energy. It is concluded that: (1) the principal bonding involves aluminum s-type orbitals; (2) the aluminum p-type orbitals are principally nonbonding; and (3) the metallic interactions are principally between zirconium atoms.     

Determination of cyanide by a highly sensitive indirect spectrophotometric method

Complexation of Pd(2+) with cyanide inhibits the extraction of the palladium complex of 5-phenylazo-8-aminoquinoline. This effect is used for the indirect spectrophotometric determination of cyanide at the mug level. Cyanide in industrial waste water and in sea-water is determined after distillation as HCN from the sample and collection in sodium hydroxide solution.     

Chemical syntheses with metal atoms

Metal evaporation syntheses of [Cr(C₇H₇)(C₇H₁₀)], [Cr(C₇H₈)(PF₃)₃], metal carbonyls, and anhydrous metal acetylacetonates are described.     

Reversed-phase partition high-pressure liquid chromatography of trace amounts of inorganic and organic mercury with silver diethyldithiocarbamate

Inorganic and organic mercury diethyldithiocarbamates have been separated by reversedphase partition high-pressure liquid chromatography. The mercury chelates were formed by an exchange reaction with silver diethyldithiocarbamate in chloroform, in the presence of acetate buffer (pH 5.0). The inorganic and organic mercury chelates in the extract were separated within 30 min on a 3.9 x 300 mm mu-Bondapak C(18) column. EDTA (10(-4)M) in methanol-water mixture (78:22 v v ) was used as eluent at a flow-rate of 0.5 ml min .     

Magnetic interactions in ternary cobalt oxides with cubic perovskite structure

Magnetic properties were measured on the cubic perovskite systems SrCoO_3?δ, (La_1?xSr_x)CoO₃ (0.5 ≦ x ≦ 1.0), and Sr(Co_1?xMn_x)O₃ (0 ≦ x ≦ 1.0). It is found that S²⁺ and La³⁺ ions strongly affect the spin state of the Co²⁺ ion and that the Mn⁴⁺ ion located at the octahedral site affects the spin state of Co⁴⁺ ion. The magnetic properties (T_c, T_θ, and σ) are explained by the magnetic interaction Co³⁺OCo³⁺, Co³⁺OCo⁴⁺, Co⁴⁺OCo⁴⁺, Mn⁴⁺OMn⁴⁺, and Mn⁴⁺OCo⁴⁺ in these systems.     

Determination of silver in dore metal by weight titration with equivalence-point detection by differential electrolytic potentiometry

Use of weight titrimetry, with differential electrolytic potentiometry for detection of the equivalence point, instead of gravimetry, has been critically tested for the determination of silver in doré metal. The electrode response in the presence of possible interferences (Sb, As, Se, Cu, Ni, Fe, Pb and Co) was studied by voltammetry. Titration curves for six different samples were obtained and their differences from those for pure silver were interpreted. Positive errors were found to occur in the presence of 0.1% Fe(III) and Se(IV) but not in the presence of 1% Cu(II). The results obtained by gravimetry and weight titrimetry were compared. The imprecision of the proposed method was between 0.03 and 0.05% (relative standard deviation); that of gravimetry was between 0.05 and 0.08%. The absolute difference from the gravimetric results was between + 0.05 and + 0.08%. The time for analysis of one sample was between 60 and 80 min.     

An ab initio study of the interaction of atomic hydrogen with cluster models simulating the (100) and (110) silver surfaces

Ab initio calculations have been carried out for the interactions of atomic hydrogen with Ag₅ cluster models, simulating the adsorption on the one-fold and four-fold sites of the (100) and (110) silver surfaces. It is found that one-layer cluster models lead to unreasonable results as a consequence of their high instability. Chemisorption on the one-fold site is found to be the preferred one. This results is easily interpreted on the basis of symmetry properties of the higher occupied molecular orbitals.     

Effect of mechanical pretreatment on thermal decomposition of silver oxalate under nonisothermal conditions

The effect of mechanical pretreatment on the thermal decomposition of silver oxalate was investigated using mass-spectrometric thermoanalysis. Starting at different levels of predecomposition, kinetic data were obtained by analysis of the kinetics at the initial rise of decomposition. The observed mechanically induced increase in reactivity of the silver oxalate used is due to three effects: (i) A thermally more stable “phase” at the surface of the grains is rendered ineffective by the creation of new surfaces. (ii) Disintegration processes increase the specific surface by several orders of magnitude. (iii) The number of potential nuclei in the surface is considerably increased by the generation of lattice defects.     

Partial phase diagrams for the water—cadmium nitrate, water—silver nitrate and water—cadmium nitrate—silver nitrate systems

Sub-ambient differential scanning calorimetry has been used to determine partial binary phase diagrams for the water—silver nitrate and water—cadmium nitrate systems. Both systems are shown to be eutectic with eutectic temperatures of 263 ± 0.5 K and 256.5 ± 0.5 K for the water—silver nitrate and water—cadmium nitrate systems, respectively. The eutectic compositions are at approximately 40.5 wt.% nitrate for each system. The freezing of ternary solutions containing a fixed silver nitrate/cadmium nitrate ratio but increasing concentrations of total nitrates are explained in terms of a partial ternary system containing two binary eutectic systems and a ternary eutectic reaction. The ternary eutectic temperature is shown to be 254 ± 0.5 K.     

Surface enhanced Raman spectra of nucleic acid components adsorbed at a silver electrode

High-resolution vibrational spectra of nucleic acid components adsorbed on a silver electrode were obtained using a spectroelectrochemical method based on the large-intensity enhancement for Raman scattering at electrode surfaces.The laser surface Raman spectra of purine, adenine, adenosine, deoxyadenosine, adenine mononucleotides, adenylyl-3′, 5′-adenosine and polyriboadenylic acid were recorded in the range of 150–3500 cm^?1. The intensities of the vibrational bands were highly dependent upon the electrochemical preparation of the electrode, the applied potential and the nature of the adsorbate species. High-intensity spectra in rather dilute bulk solutions were obtained.The phosphate derivatives of adenosine exhibited strongly enhanced Raman scattering. Spectral band frequencies corresponded closely with normal Raman spectra of these molecules in solution. The adenine ring breathing mode at 740 cm^?1 and the adenine ring skeletal vibration at 1335 cm^?1 produced prominent Raman scattering. A strong band at about 240 cm^?1 for the adenine mononucleotides was attributed to silver/adsorbed phosphate group vibrations.     

Calorimetric determination of the thermodynamic properties involved in the complex formation of silver(I) with some sulphur-containing pyridines

The enthalpy changes involved in the complex formation of Ag⁺ with several sulphur-containing pyridines of general formula

where R = Ch₃, C₂H₅ and CH₂CH₂OH and x=1,2 have been determined by direct calorimetric measurements at 25°C in 0.5 M KNO₃ solution.From the ΔH values obtained and from the free energy changes reported in an earlier publication, the corresponding entropy changes ΔS have been calculated. Changes in ΔH and ΔS are discussed in terms of inductive and steric effects. It is shown that enthalpy values are linearly correlated with the Taft σ^* parameters of the substituents on the ligand.     

Kinetics of simple inner- and outer-sphere electrochemical reactions at rotating silver electrodes as examined using surface-enhanced raman spectroscopy

An application of surface-enhanced Raman spectroscopy (SERS) to evaluating the kinetics of electrochemical reactions is outlined. This involves monitoring the potential-dependent SERS intensities of the adsorbed reactant at a rotating disk electrode in a potential region where the kinetics are under mixed mass transfer-heterogeneous electron transfer control. Comparisons are made between the kinetic behavior extracted from the SERS intensity-potential dependence and that obtained from the faradaic current flow. For Co(NH₃)₆³⁺ reduction at silver, where the reactant is electrostatically attracted but not bound to the surface, the SERS and electrochemical rate behavior is closely compatible. However, for Cr(NH₃)₅Br²⁺ and Cr(NH₃)₅NCS²⁺ reduction, where the reactants bind directly to the surface prior to electron transfer, the reactivities at the SERS-active sites are substantially higher than for the prevalent adsorbate as obtained electrochemically. These findings suggest that surface sites displaying efficient Raman scattering may also provide centers of catalytic activity.     

Chemical reactions of chromium and vanadium in graphite furnace atomic absorption spectrometry

The chemical reactions of V and Cr in the graphite furnace are determined by absorption measurements, X-ray diffraction and electron microscopy techniques. It is shown that both elements form carbides, VC and Cr₃C₂, respectively. An increase in absorption is obtained in tubes coated with pyrolytic graphite. Interferences of carbideforming elements on the absorption signal are studied. Reaction paths are proposed for both elements during their heat treatment in graphite furnace AAS.     

Polymerization of acrylonitrile initiated by the redox system peroxydisulphate-isoamyl alcohol catalyzed by silver ion

The polymerization of acrylonitrile in aqueous media initiated by peroxydisulphate-isoamyl alcohol(IAA) redox pair catalyzed by silver ion was studied at low conversion. The polymerization had kinetic orders 1.5 with respect to monomer, 0.6 (peroxydisulphate) and 0.5 (silver ion). The reactions were carried out at various temperatures and the overall activation energy was found to be 5.5 kcal/mol. The effect of certain water-miscible organic solvents was also investigated. A mechanism of initiation is tentatively proposed.     

Influence of sulfide and cyanide ions on the electrochemical behavior of the Fe(II)/Fe(Hg) system in thiocyanate solutions at mercury electrodes

The reduction and reoxidation processes of the Fe(II)/Fe(Hg) system in thiocyanate solutions at stationary mercury electrodes have been investigated by cyclic voltammetric, anodic stripping and controlled potential electrolysis methods. In 0.1 M NaSCN and 0.4 M NaClO₄ solution containing 1×10^?3M Fe(II), the voltammogram on the first cycle at. 0.05 V s^?1 gives two consecutive cathodic peaks near ?1.2 and ?1.39 V with a hysteresis on the reversal, and an anodic wave with two large peaks near ?0.58 and ?0.05 V and two small peaks near ?0.52 and ?0.43 V, respectively. The multicyclic voltammogram under the same conditions in the potential region between 0.00 and ?1.50 V gives a cathodic wave with a principal peak near ?1.02 V and two small peaks near ?0.02 and ?0.53 V, respectively, and an anodic wave with a principal peak near ?0.72 V, three small peaks near ?0.64, ?0.52 and ?0.40 V, and with a shoulder near ?0.05 V, respectively. The variation of the shape of the voltammogram on the second and subsequent runs is due to the formation of S^2? and CN^? during the process of electroreduction of Fe(II). A mechanism is proposed which involves an initial reduction of Fe(II)?SCN^? produced in an activation step at a mercury electrode, followed by the chemical redox reaction of a part of Fe(0)?SCN^? in the species giving FeS and CN^?, and takes into account the influence of FeS and CN^? on the further reduction and reoxidation of iron. Both FeS and CN^? stimulate further reduction, and reoxidation of iron. The hysteresis of the cathodic wave on the first cycle arises from the fact that Fe(II) is reduced more easily at the mercury electrode covered with FeS than at a pure mercury electrode.     

Electron-transfer rate constants for redox systems of Fe(III)/Fe(II) complexes with 2,2′-bipyridine and/or cyanide ion as measured by the galvanostatic double pulse method

The formal standard rate constants for the redox systems, Fe(bipy)₃³⁺/Fe(bipy)₃²⁺,Fe(bipy)₂(CN)₂^?/Fe(bipy)₂(CN)₂, Fe(bipy)(CN)₄^?/Fe(bipy)(CN)₄^2? and Fe(CN)₆^3?/Fe(CN)₆^4? (bipy=2,2′-bipyridine), in aqueous solution and N,N-dimethylformamide solution are measured with the aid of the galvanostatic double pulse method. The standard rate constant decreases as the number of the coordinated 2,2′-bipyridine decreases. It is in accordance with the trend in the homogeneous rate constants for these systems and is interpreted on the basis of the extension of ligand π-orbitals. This finding may be evidence for the mechanistic similarity of the electrochemical electron-transfer reaction of a redox system to the corresponding homonuclear electron-exchange reaction occurring in solution phase. An empirical relation between rate constants for both kinds of reactions is discussed. It is noted that the maximum electrochemical rate constant is limited at a value much smaller than the one theoretically allowed.     

Thermal studies on purine complexes. VII. 8-theophylline derivatives and their silver(I) complexes

Four theophylline derivatives: 8-ethyltheophylline (ETH), 8-isopropyltheophylline (IPH), 8-propyltheophylline (PRH), and 8-phenyltheophylline (FTH) have been synthesized and characterized by spectroscopic techniques and thermal analysis. The melting points of these theophylline derivatives increase with the molecular weight, from 263.8°C (ETH) to 384.7°C (FTH). The values of the fusion enthalpy increase in the same order, from 28.4 kJ mol⁻¹ (ETH) to 36.5 kJ mol⁻¹ (FTH).

The interaction of these theophylline derivatives with Ag(I) has been studied by means of spectroscopic techniques, TG, DTG and DSC. In all cases, the pyrolysis of these compounds gave metallic silver.