Über Oxostannate(II). III. K2Sn2O3, Rb2Sn2O3 und Cs2Sn2O3 – ein Vergleich

On Oxostannates(II). III. K₂Sn₂0₃, Rb₂Sn₂0₃, and Cs₂Sn₂O₃ – a Comparison Hitherto unknown Rb₂Sn₂O₃ has been obtained by heating of mixtures of binary oxides [RbO_0.48 + SnO, Rb:Sn = 1.1:1, Al₂O₃?cylinders, Ar] as deep yellow powder or deep yellow single crystals. It is isotypic to K₂Sn₂O₃, R3 m-D with a = 6.086 Å, c = 15.101 Å, Z = 3, d_calc = 4.69, d_obs = 4.64 g X cm^?3. For 260 hkl it is R = 5.2₇% and R_w = 5.0₉% (MoKα, 4-circle diffractometer data). The structure of K₂Sn₂O₃ and Rb₂Sn₂O₃ is compared with that of Cs₂Sn₂O₃. For both types Effektive Coordination Numbers, ECoN, and the Madelung Part of Lattice Energy, MAPLE, have been calculated.     

Error analysis for complex kinetics systems methyl-vinyl cross-radical reactions

Many rate constants, particularly for radical-radical reactions, are determined in systems exhibiting complex kinetics. Parameters often comprise several species concentrations, absorption cross sections, and rate constants. They cannot be obtained from an exact analytic expression, and therefore, must be solved iteratively through an efficient numerical differential equation solver. We examine here the reaction kinetics of the CH₃ and the C₂H₃ mixed-radical system, generated by excimer laser photolysis of methyl vinyl ketone (MVK) at 193.3 nm. As a model, a detailed error analysis for determination of the cross-radical reaction rate constant, the initial CH₃ and C₂H₃ radical concentrations are performed using a least-squares modeling program (Acufit). The least-squares program, an assumed mechanism, a detailed set of signal-averaged data, are viewed as the measuring “instrument.” The output of the “instrument” consists of a “signal” that is fit to the data using the least-squares criterion. This gives estimated parameters that are examined with respect to random as well as systematic errors. The biases due to uncertainties of each known parameter and combined uncertainties on the measured parameters are assessed. © 1994 John Wiley & Sons, Inc.

1 This article is a US Government work and, as such, is in the public domain in the United States of America.

     

Flash photolysis resonance fluorescence study of the reaction Cl + O3 → ClO + O2 over the temperature range 213–298 K

Rate constants for the removal of Cl atoms in the reaction Cl + O₃ → ClO + O₂ were measured by the flash photolysis resonance fluorescence technique over the temperature range 213–298 K. The rate constant is given by the Arrhenius expression (2.94 ± 0.49) × 10^?11 exp[?(298 ± 39)/T] in units of cm³ molecule^?1 s^?1. Comparison with recent results from other laboratories are presented.     

Mechanisms of decomposition of mixtures of ethyl acetate and isopropyl bromide subjected to pulsed infrared laser irradiation

The infrared laser induced decomposition of mixtures of ethyl acetate and isopropyl bromide has been studied. The ratio of the yields of products ethylene and propylene, arising from the unimolecular decomposition reactions: ethyl acetate → ethylene + acetic acid, and isopropyl bromide → propylene + hydrogen bromide, were measured as a function of the ratio of ethyl acetate to isopropyl bromide and pressure of added helium. The results indicate clearly that in these systems non-equilibrium behavior is found up to the highest pressures used (about one atmosphere). A two level kinetic model is suggested which qualitatively explains the observations.     

E-1,2-Dichlorovinyl ethers as irreversible protease inhibitors

The synthesis of a novel motif for threonine protease inhibition is described. The desired E-1,2-dichlorovinyl ethers are obtained from alcohols and trichloroethylene as single diastereomers. Aqueous treatment at pH 11 unmasks the hidden α-chloroacetate, which is required for the reaction with the active site of the protease.     

Five-membered OsC3N heterocycles from osmium azavinylidenes as precursors

The azavinylidene osmium complex [(mes)Os(=N=CPh₂(PⁱPr₃)]PF₆ (3; MES = 1.3.5-C₆H₃Me₃), which was prepared in two steps from [(mes)OsCl₂(PⁱPr₃], CH₃CO₂Na/KPF₆ and HN=CPh₂, has been shown to react with trifluoracetic acid to give the isomeric heterocycle [(mes) ₆H₄)(Pⁱ Pr₃)]PF₆ (4) in almost quantitative yield. With CF₃CO₂D, the monodeuterated compound 4-d₄,containing a N---D unit in the five-membered ring, is obtained. An analogue of 4 with the composition [(mes) ₆H₄)(PMe₃)]X(X = PF₆ 14a, SbF₆ 14b) has been made both from [(mes)Os(=N=CPh₂)(PMe₃)]PF₆ 12 and CF₃CO₂H and from [(mes)OsCl(NH= CPh₂(PMe3)]SbF6 (13) and CF₃CO₂Ag. The reaction of 4 with NaH or KO^tBu gives the uncharged heterocycle [(mes) ₆ H₄)(PⁱPr₃)] (15), while treatment of [(mes)OsCl ₂(NH=CPh2)] (19) with NaS^tBu gives the related complex [(mes) ₆H₄)(S^t Bu)] (20). The crystal structure of 4 has been determined. kw]Osmium; Azavinylidene; Cyclometallation; Crystal structure; Arene complexes     

The quenching of CdSe quantum dots photoluminescence by gold nanoparticles in solution

The photoluminescence (PL) of CdSe quantum dots (QD) in aqueous media has been studied in the presence of gold nanoparticles (NP) with different shapes. The steady state PL intensity of CdSe QD (1.5-2 nm in size) is quenched in the presence of gold NP. Picosecond bleach recovery and nanosecond time-resolved luminescence measurements show a faster bleach recovery and decrease in the lifetime of the emitting states of CdSe QD in the presence of quenchers. Surfactant-capped gold nanorods (NR) with aspect ratio of 3 and surfactant-capped and citrate-capped nanospheres (NS) of 12 nm diameter were used as quenchers in order to study the effect of shape and surface charge on the quenching rates. The Stern-Volmer kinetics model is used to examine the observed quenching behavior as a function of the quencher concentration. It was found that the quenching rate of NR is more than 1000 times stronger than that of NS with the same capping material. We also found that the quenching rate decreases as the length of the NR decreases, although the overlap between the CdSe emission and the NR absorption increases. This suggests that the quenching is a result of electron transfer rather than long-range (Forster-type) energy transfer processes. The quenching was attributed to the transfer of electron with energies below the Fermi level of gold to the trap holes of CdSe QD. The observed large difference between NR and NS quenching efficiencies was attributed to the presence of the [110] facets only in the NR, which have higher surface energy.     

Effect of the Media on the Quantum Yield of Singlet Oxygen (O2(1Δg)) Production by 9H‐Fluoren‐9‐one: Solvents and Solvent Mixtures

We have investigated the effect of a series of 18 solvents and mixtures of solvents on the production of singlet molecular oxygen (O₂(¹Δ_g), denoted as ¹O₂) by 9H‐fluoren‐9‐one (FLU). The normalized empirical parameter E derived from E_T(30) has been chosen as a measure of solvent polarity using Reichardt's betaine dyes. Quantum yields of ¹O₂ production (Φ_Δ) decrease with increasing solvent polarity and protic character as a consequence of the decrease of the quantum yield of intersystem crossing (Φ_ISC). Values of Φ_Δ of unity have been found in alkanes. In nonprotic solvents of increasing polarity, Φ_ISC and, therefore, Φ_Δ decrease due to solvent‐induced changes in the energy levels of singlet and triplet excited states of FLU. This compound is a poor ¹O₂ sensitizer in protic solvents, because hydrogen bonding considerably increases the rate of internal conversion from the singlet excited state, thus diminishing Φ_Δ to values much lower than those in nonprotic solvents of similar polarity. In mixtures of cyclohexane and alcohols, preferential solvation of FLU by the protic solvent leads to a fast decrease of Φ_Δ upon addition of increasing amounts of the latter.     

Core excitations of naphthalene: vibrational structure versus chemical shifts

High-resolution x-ray photoelectron emission (XPS) and near-edge x-ray absorption fine structure (NEXAFS) spectra of naphthalene are analyzed in terms of the initial state chemical shifts and the vibrational fine structure of the excitations. Carbon atoms located at peripheral sites experience only a small chemical shift and exhibit rather similar charge-vibrational coupling, while the atoms in the bridging positions differ substantially. In the XPS spectra, C-H stretching modes provide important contributions to the overall shape of the spectrum. In contrast, the NEXAFS spectrum contains only vibrational progressions from particular C-C stretching modes. The accuracy of ab initio calculations of absolute electronic transition energies is discussed in the context of minute chemical shifts, the vibrational fine structure, and the state multiplicity.