Lifetimes of triplet state alkylbenzenes in the vapor phase

Rate constants for triplet state decay of C₆H₆, C₆D₆ and 15 alkylbenzenes in the vapor phase have been found, using a flash-sensitized biacetyl phosphorescence technique, to range between 800 s^?1 and ? 18 000 s^?1. Only benzene has a significant positive activation energy for decay. Kinetic and spectroscopic evidence supports a photoisomerization decay channel in t-butylbenzene. Comparison of lifetimes with molecular size shows that increased density of rotational levels does not account for rapid decay of the triplet state. This contradicts a recent suggestion to explain short lifetimes of triplet state aromatic hydrocarbons in the vapor phase, relative to the long lifetimes of the same molecules in low temperat matrices. Evidence suggests that the dominant decay paths for triplet state alkylbenzenes are different in room temperature vapors and low temperature matrices.     

Intermediate case radiationless decay: the excited state dynamics of pyrazine

This paper reports new measurements of the non-exponential fluorescence decay of pyrazine, covering the time range 0.1-200 ns. The restits of our measurements remove the inconsistencies between experiment and the Frad-Lahmani-Tramer-Tric model of the radiationless decay in this molecule. In particular, our data show that the triplet component of the mixed singlet-triplet levels does increase with increasing triplet density of states. The effective density of triplet levels determined from our experimental data exceeds the theoretical density of vibrational levels. We propose that at the excitation levels achieved in the triplet manifold there is extensive scrambling of rotational states, but that conservation of nuclear spin states permits a level with total angular momentum quantum number f to couple to only (2J + 1/4) of the 2J+1 rotational levels built on one vibration. The appropriate theoretical density of states to be compared with experiment is then obtained by multiplying the vibrational density of states by J2. Good agreement is found between experimentally determined and calculated densities of states.     

Bonding studies in group IV substituted anilines : IV. Excited state interactions in the trimethyl derivatives

The solution ultraviolet spectra for N,N-dimethylaniline, p-t-butyl-, and p-trimethylsilyl-, p-trimethylgermyl-, and m-trimethylsilyl-N,N-dimethylaniline in acetonitrile and pentane are reported. The Group IV substituents perturb the excited states to a much larger extent than the ground states. Both the symmetric and antisymmetric π^* levels are affected by π interaction with the trimethylsilyl and trimethylgermyl substituents. The magnitude of the effects are proportional to the π density at the point of substitution. Simple Hückel calculations correlate well with experimental transition energies. The model appears to exaggerate π interactions between silicon or germanium and the π^* molecular orbitals.     

A new form of the equation of state for pure substances and its application to oxygen

Schmidt, R. and Wagner, W., 1985. A new form of the equation of state for pure substances and its application to oxygen. Fluid Phase Equilibria, 19: 175–200.A new wide range equation of state is presented and expressed analytically in the form of the free energy as a function of density and temperature. This fundamental equation contains, in addition to pure polynomial and “BWR”-terms, new exponential functions especially convenient for the critical region. To guarantee an effective structure, the combination of the terms of the equation was found by using an optimization method recently developed. As a result, the optimized function for the free energy is capable of representing the thermodynamic surface of oxygen in the range 54 ≤ T ≤ 300 K, 0 < p ≤ 818 bar and 0 < ρ ≤ 41 mol dm^?3 within the experimental uncertainty of the data available. With the exception of very few items of data, this statement is also valid for the whole coexistence curve and the critical region. Extrapolations of this new equation beyond the range of data yield physically meaningful results.     

A reaction-rate method for the determination of thiocyanate,based on its inhibitory effect on the iodate-hypophosphite reaction

An automatic spectrophotometric reaction-rate method is described for the determination of thiocyanate in aqueous solutions, based on its inhibitory effect on the iodate-hypophosphite reaction. The time required for the reaction to proceed between two pre-determined levels is measured automatically and related directly to the thiocyanate concentration. The recommended reaction-rate method is fast, simple, sensitive, accurate (to 2%) and precise (1.7%, relative standard deviation). The useful analytical range of concentrations of thiocyanate is 4 × 10^?5 to 5 × 10^?4M. Maximum tolerable amounts of interfering ions are also presented.     

Investigations of changes in the oxidation state of the catalyst during the OsO4-catalysed decomposition of hydrogen peroxide

By using different methods, such as spectrophotometry, potentiometric titration, polarography and extraction, it was found that at pH 8.5 osmium (VIII) is reduced by hydrogen peroxide to osmium(VI) to various extents. At pH 10.6, where the rate of the OsO₄-catalysed decomposition of hydrogen peroxide reaches its maximum, the concentration ratio of osmium (VIII) and osmium (VI) was found to be nearly one. This favours the explanation that the maximum rate of hydrogen peroxide decomposition is found at the pH where the rate of reduction of osmium (VIII) by hydrogen peroxide just becomes equal to the rate of oxidation of osmium (VI) by hydrogen peroxide.     

Thermal reaction of aquaammineruthenium(III) complex with amino acid or imidazole derivative in the solid state

Intermolecular thermal-substitution reaction between aquaammineruthenium(III) complex and amino acid or imidazole derivative has been investigated in the solid state by the TG-DTA method. Pentaammineruthenium(III) complexes containing amino acid or imidazole derivative have been obtained directly by the thermal reactions. Glycine, β-alanine, and γ-aminobutyric acid coordinate to Ru(III) through their carbonyl oxygen, and imidazole does through its N(3) atom. Distinct coordination site is provided in the complex with histidine and/or adenine: the bonding site depends on the outer-sphere anion of aquaammine complex. The N(3) atom of the histidine and N(7) atom of the adenine coordinate to Ru(III) taking the paratoluenesulphonate salt of aquaammineruthenium(III) into the reaction. When the methanesulphonate salt is used, the nitrogen atom in the side-chain amino-group participates in complexation. Direct chelation of the glycine, histidine, and adenine to the deaquated cis-diaquatetraammineruthenium(III) complex has been confirmed.     

Critical evaluation of methane-hydrocarbon high-pressure experimental vapour-liquid equilibrium data using equations of state

Legret, D., Richon, D. and Renon, H., 1984. Critical evaluation of methane-hydrocarbon high-pressure experimental vapour-liquid equilibrium data using equations of state. Fluid Phase Equilibria, 17: 323–350.An evaluation of literature vapour-liquid equilibrium data for methane-hydrocarbon systems is presented, based on the application of cubic equations of state. Lists of interaction parameters and data sets are presented.     

Bubble pressures and saturated liquid molar volumes of trichlorofluoromethane—chlorodifluoromethane mixtures. Representation of refrigerant-mixtures vapor—liquid equilibrium data by a modified form of the Peng—Robinson equation of state

Experimental vapor—liquid equilibrium data and saturated liquid molar volumes of chlorodifluoromethane—trichlorofluoromethane binary mixtures have been obtained at four temperatures (298.15, 323.15, 348.15 and 373.15 K) using apparatus described previously.The experimental vapor—liquid equilibria are represented well by a modified form of the Peng—Robinson equation of state with one interaction parameter, but the mean deviation between the calculated and experimental densities is 5%.Vapor—liquid data for binary refrigerant mixtures from the literature are treated using the modified form of the Peng—Robinson equation of state with one adjusted interaction parameter in the mixing rule for a. The representation is fair and is not improved by introducing an additional parameter in the mixing rule for b.     

Solid state properties and catalytic behavior of the α- and β-vanadium bronzes

Decomposition of isopropanol (IPA) on V₂O₅, Li_0.02V₂O₅, Na_0.02V₂O₅, Na_0.06V₂O₅, Li_0.33V₂O₅, and Na_0.33V₂O₅ has been studied in the temperature range 186–300°C. The first four catalysts (α-phase) show predominately dehydration, whereas the last two (β-phase) have comparable dehydration and dehydrogenation activity. Dehydration activity increases with alkali metal concentration within the α-phase, but falls sharply on the β-phase catalysts. This difference is attributed to the different rate determining steps for the reaction on the α- and β-phase catalysts. X-ray and ir spectral data show that the β-phase catalysts are much more stable than the α-phase. A mechanism for the dehydration of IPA based on the electrical resistivity, ESR spectra, and kinetic data has been proposed.     

A new graphical method based on stoichiometric dilution for the classification of binary complexes with mole ratio 1:1

A simple approach to the treatment of spectrophotometric data obtained by stoichiometric dilution is presented for complexes of the type M(m)B(n) and m/n = 1. The values of m and n are obtained by matching the slope of a straight line with a mathematically predicted value. The method can be applied to moderately strong complexes, the concentration of which can be determined by absorbance measurements, and allows calculation of the formation constant as well.     

Valence state of the Fe ions in Sr1−yLayFeO3

The Sr²⁺_1?yLa³⁺_yFeO₃ system with 0.1 ≦ y ≦ 0.6 has been studied mainly by the Mössbauer effect. The results are discussed referring to the Ca_1?xSr_xFeO₃ system. The following four kinds of electronic phases have been observed: the paramagnetic and the antiferromagnetic average valence phases and the corresponding mixed valence phases. Two kinds of Fe ions coexist, in general, in the mixed valence phases. In the antiferromagnetic mixed valence phase, typically at 4 K, the magnetic hyperfine field and the center shift each takes a wide range of value depending on the composition, while a beautiful correlation is kept between them. The extreme values are close to those expected for Fe³⁺ and Fe⁵⁺. The appropriate chemical formulas are, therefore, Ca_1?xSr_xFe^(3+Δ)+_0.5Fe^(5?Δ)+_0.5O₃ and $\text{Sr}{}_{\mathrm{1?y}}\text{La}{}_{\mathrm{y}}\text{Fe}{}^{\mathrm{(3+\delta )+}}{}_{\mathrm{<mtext>(1+y)</mtext><mtext>2</mtext>}}\text{Fe}{}^{\mathrm{(5?\delta )+}}{}_{\mathrm{<mtext>(1?y)</mtext><mtext>2</mtext>}}\text{O}{}_3$.     

Calculation of high pressure vapour—liquid equilibria for systems containing polar substances with the use of an augmented van der Waals equation of state

An augmented van der Waals equation of state based on a perturbation theory has been applied to the calculation of high pressure vapour—liquid equilibria for systems containing polar substances. The equation of state comprises four terms, which imply the contributions from repulsion, symmetric, non-polar asymmetric, and polar asymmetric interactions. The characteristic parameters of each pure substance have been determined by three methods with the use of vapour pressures and saturated liquid densities. Mixing models for the terms of the repulsion, symmetric, and non-polar asymmetric interactions are the same as used previously. Two types of mixing models based on a three-fluid model and/or a one-fluid model are developed for the polar asymmetric term. The polar asymmetric term has a large effect on the prediction of the vapour—liquid equilibrium. With the introduction of a binary interaction parameter, the equation is found to be useful in correlating the vapour—liquid equilibria for a system containing a polar substance except near a critical region.     

Atomic absorption spectrometric determination of Cd,Pb, Zn,Cu, Co and Fe in oyster tissue by direct atomization from the solid state using the graphite furnace platform technique

This paper describes a simple and rapid method for direct determination of traces of Cu, Zn, Pb, Co, Fe and Cd in the NBS oyster tissue, SRM No. 1566, by graphite furnace atomic absorption spectrometry. The solid sampling technique has been used and this results in much higher relative sensitivity by avoiding large dilution factors involved in the sample dissolution technique. The solid sampling technique also greatly reduces or eliminates serious risk of introducing contamination and/or loss of analytes involved in sample dissolution. The organic matrix of the oyster tissue is burnt off and removed by using a well-defined selective volatilization technique. Loss of Cd during charring (pyrolysis) stage is prevented by forming a relatively thermally stable compound of cadmium by adding (NH₄₂SO₄ solution to the oyster tissue sample. Also, quantitative data are presented on the effectiveness of the platform technique in removing matrix interferences. The platform technique uses an anisotropic pyrolytic graphite platform which is inserted into a commercial graphite tube.     

Mössbauer studies of solid state photolysis of barium and strontium tris(oxalato) ferrates (III)

Solid state photolysis of strontium and barium tris(oxalato) ferrate(III) was done under a medium pressure lamp and investigated with Mössbauer spectroscopy. The product [Fe^II(C₂O₄) (H₂O)₂]^2? formed during photolysis is found to be quite stable and does not convert to ferric state on long standing in air.     

Solvent effects on the structure of benzo[b]selenophene oriented in liquid crystals

Proton magnetic resonance spectra of benzo[b]selenophene including ⁷⁷Se satellites have been investigated in thermotropic liquid crystal solvents. Relative proton-proton and proton-selenium distances have been derived from the dipolar couplings. A comparison of the results on benzo[b]-furan, -thiophene and -selenophene and the application of bond polarisation hypothesis indicate significant solvent effects in the liquid crystal formed by an eutectic ternary mixture of propylpentyl- and heptyl-bicyclohexylcarbonitrile.     

Cyclobutyl neighboring group effect on the stability of bicyclo-[2.2.2]octyl bridgehead carbenium ions

Solvolysis of the novel bridgehead cyclobutyl methyl chloride

proceeds without skeletal rearrangement. Its relative rate is the first unequivocal proof for a small, but significant ability of the cyclobutyl group to stabilize an adjacent positively charged center.     

Dependence of the closed-shell repulsive interaction on the overlap of the electron densities

The relationship between the closed-shell repulsive interactions and overlap of the electron densities is investigated. It is found that the repulsive interactions vary in almost direct proportion to the absolute size of the density overlaps.     

Kinetics of electron transfer reactions of metal complexes at impermeable redox active polymeric films on electrode surfaces and charge transport within the polymer film

At rotated Pt disk electrodes coated with thin films of the redox polymer poly-[Ru(vbpy)₃]²⁺, ruthenium and iron bipyridine complexes dissolved in acetonitrile can become oxidized by two pathways. The first is diffusion of the solute complex through the polymeric film to react at its normal potential E_sub⁰, at the Pt/polymer interface. The second is a mediated electron transfer cross-reaction between the solute complex and poly-[Ru(vbpy) ₃]²⁺ sites generated in the film at adequately positive potentials. The mediated reaction, as judged from the lack of variation of its rate k_crsΓ with the quantity of polymer mediator sites present in the multimolecular layer film, and from other evidence, is confined to the outer few (one?) monolayers of ruthenium polymer film sites. The mediated reaction becomes the dominant pathway for films with Γ_T ～2×10^?9 mol/cm² of ruthenium polymer sites, owing to the low permeability (measured independently) of the solutecomplexes into the film. The rate k_crsΓ could be measured when the solute complex oxidation potential is more positive than that of the redox film, and is too fast to measure when E_sub⁰, is more negative than the redox film E_cal?⁰, New theory is presented and evaluated to describe the rising portions of the voitammetric waves for the nine solute complexes studied. The rate of charge transport through the poly-[Ru(vbpy)₃]²⁺ film becomes controlling under certain conditions and can be thereby measured as well.     

Temperature dependence of the Mössbauer spectrum and magnetic susceptibility [Fe(uridine)Cl2]∞: Novel magnetic ordering of transition metal complexes based on nucleoside ligands

The zero-field Mössbauer spectra and magnetic susceptibility of a polycrystalline sample of the polymeric material [Fe(uridine)Cl₂]_∞ have been measured in the temperature range 1.7-300 K. The compound exhibits two magnetic transitions. The isomer shift and quadrupole splitting indicate a distorted pseudo-octahedral FeCl₄O₂ chromophore resulting from condensation to a network structure via chloro and uridine ligand bridging.