Radiative lifetime and quenching rates of seo2 vapor

A Nd: YAG pumped dye laser system was used 10 excite SeO₂ vapor at 2888 A (¹B₂-X?¹A₁) and the radiative lifetime was measured under collision free conditions by time gated photon counting Rate coefficients for quenching by Ar, N₂ and CO₂ have also been determined. The lifetime measurements covered a 90 μs range and exhibited exponential decay with τ_r = 36.1 = 2.5 μs.     

Synthetic studies on boron containing antibiotics

The preparation of two fragments containing the backbone of the antibiotic boromycin is described.     

Radiative lifetime for v = 1 and v = 2 ground state NO+ ions

Radiative lifetimes of vibrationally excited ions in their electronic ground state have been measured for the first time. A nonoptical technique has been developed, involving ion cyclotron resonance trapping in conjunction with chemical monitoring of the energy content of the ions. For NO⁺(X ¹Σ⁺) ions, the measured lifetimes of 95 ± 15 ms for v = 1 and 46 ± 10 ms for v = 2 are in excellent agreement with the values obtained from the ab initio calculations of Werner and Rosmus.     

First principles studies on boron sites in zeolites

A systematic computational investigation on protonated and nonprotonated boron-containing zeolites (boralites), performed by using different periodic density functional theory approximations, is presented. Both minimum energy structures and finite temperature behavior of model boron sodalites were analyzed. All of the adopted computational schemes agree in predicting an acid site composed of a silanol Si-OH group loosely linked to a planar BO(3) structure in the protonated system and a BO(4) tetrahedral site in the sodium-containing zeolite. Calculated structural and vibrational properties are in line with experimental data. Comparisons of the protonated boralite site with Al and Ga zeolitic acid sites are discussed as well. Results indicate that this class of mild acid catalysts is characterized by significant framework flexibility and pronounced thermal effects due to the loosely bound acid site.     

Micro-homogeneity studies of boron carbide powders

We have studied the micro-homogeneity of boron carbide powders by inductively coupled plasma optical emission spectrometry (ICP-OES) and total reflection X-ray fluorescence spectrometry (TXRF) using slurry sampling. To get information on the particle size distributions of the powders, the stabilized slurries of boron carbide powders were nebulized, the aerosols were transported into a Batelle impactor and the droplets were collected on the impactor stages bearing TXRF sample holders. In a first series of measurements, parameters of the impaction like the duration of the impaction and the use of glutinous substance on the sample holders were optimized. The different mass size fractions for industrial boron carbide powders were determined by weight measurements of the fractions collected on the different stages. The established particle size distributions were in the range of 0.5 to >16 μm and found similar to those determined by laser diffraction reported elsewhere. Analyses of the mass fractions by slurry sampling TXRF showed that Ca, Ti, Cr, Mn, Fe, Ni and Cu within the measurements errors were homogeneously distributed over the mass fractions between 0.5 and 4 μm and that their concentrations agreed with the bulk composition, as determined with ICP-OES subsequent to digestion. However, light underestimates were found at the 5 (Mn) up to 150 μg g^?1 (Fe) level. Finally, boron carbide powders were washed out with nitric acid with different concentrations and leaching solutions and the residues were analyzed by ICP-OES and TXRF respectively. It is shown that up to 60% of the residual trace impurities in the powder studied can be removed by leaching with 34% (v/v) of nitric acid.

Synthesis and thermal studies of heterocyclic siloxanes containing boron

Eight- and six-membered cycloborasiloxanes containing two and one boron atoms respectively are strained ring compounds which undergo ring opening polymerization and ring–ring transformation reactions on thermolysis. Prolonged heating at 200°C results in volatilization of the cyclic boroxine (PhBO)₃, whereas rapid heating of either compound to 1500°C in an inert atmosphere does not result in loss of boron, but affords instead an amorphous residue containing silicon carbide, boron oxide and carbon. Upon further pyrolysis at 1700°C the final product consists of microcrystalline silicon carbide in which are embedded large crystals of boron carbide.     

Syntheses and spectroscopic studies of diphenylphosphinato derivatives of boron

Summary 2-Diphenylphosphinato-1,3,2-dioxaborolanes and -borinanes of the type (whereG = -CH₂CHMe-, -CH₂CH₂CH₂-, -CH₂CH₂CHMe-,-CMe₂CMe₂-, -CMe₂CH₂CHMe-,-CH₂CMe₂CH₂-, -CH₂CEt₂CH₂-, and -C₆H₄-) are obtained by the reaction of diphenylphosphinic acid with the corresponding 2,2-oxo-bis-1,3,2-dioxaborolanes and-borinanes. The products are white crystalline solids, which have sharp melting points and are hydrolytically stable. They have been characterized by elemental analysis, IR and multinuclear NMR (¹¹B,³¹P, and¹¹⁹Sn) studies. The data suggest structures with monodentate phosphinato moieties and 3-coordinated boron atoms.

---

Synthesen und spektroskopische Untersuchungen von Diphenylphosphinatoderivaten von Bor

Zusammenfassung 2-Diphenylphosphinato-1,3,2-dioxaborolane und -borinane des Typs (mitG = -CH₂CHMe-, -CH₂CH₂CH₂-,-CH₂CH₂CHMe-, -CMe₂CMe₂-,-CMe₂CH₂CHMe-, -CH₂CMe₂CH₂-, -CH₂CEt₂CH₂- und C₆H₄) erhält man durch Reaktion von Diphenylphosphinsäure mit den entsprechenden 2,2-Oxo-bis-1,3,2-dioxaborolanen und -borinanen. Die Produkte sind weiße, kristalline, hydrolyseunempfindliche Festkörper. Sie wurden mittels Elementaranalyse, IR-Spektroskopie und multinuklearer NMR-Spektroskopie (¹¹B,³¹P und¹¹⁹Sn) charakterisiert. Die Resultate legen Strukturen mit monodentaten Phosphinatoeinheiten und dreifach koordinierten Boratomen nahe.

     

An expeditious synthesis of DPD and boron binding studies

A practical synthesis has been developed for DPD (4,5-dihydroxypentane-2,3-dione), an unstable small molecule that is proposed to be the source of universal signaling agents for quorum sensing in bacteria. The synthesis allows preparation of isotopically labeled DPD and ent-DPD as well as detailed studies of spontaneous binding to borate to give the unusual borate complex 6, the signal for marine bacteria such as Vibrio harveyi. [reaction: see text]     

Positron annihilation lifetime studies of free volume in gelatin

The variations of size, intensity, and size distribution of free volumes generated in the network of molecular chains of gelatin at the sol-gel transition were studied by means of the positron annihilation lifetime technique. Although variation in average free-volume radius was not recognized, a variation of free-volume content was observed at the sol-gel transition point of gelatin with an addition of saccharose.     

γ-Alumina-supported boron trifluoride: Catalysis, radiotracer studies and computations

The irreversible adsorption of boron trifluoride on calcined γ-alumina and amorphous chromia, in both cases at room temperature, has been studied using [¹⁸F]-labelled BF₃. Although the resulting γ-alumina surface has some catalytic activity for the room temperature fluorination by anhydrous HF of CH₃CCl₃ under static conditions, its activity is far lower than that of γ-alumina, which has been fluorinated with SF₄, nominally at room temperature. A possible explanation for the observed behaviour is given.     

Kinetic and mechanistic studies of vanadium-based, extended catalytic lifetime catechol dioxygenases

Recently we showed that V-containing polyoxometalates such as (n-Bu4N)7SiW9V3O40 or (n-Bu4N)9P2W15V3O62, as well as eight other V-containing precatalysts tested, evolve to high-activity, long catalytic lifetime (> or = 30,000-100,000 total turnovers) 3,5-di-tert-butylcatechol (DTBC) dioxygenases in which Pierpont's complex [VO(DBSQ)(DTBC)]2 is apparently a common catalyst resting state [Yin, C.-X.; Finke, R. G. J. Am. Chem. Soc. 2005, 107, 9003-9013]. In a separate paper, autoxidation of DTBC to the corresponding benzoquinone and H2O2 was shown to be a key to the catalyst evolution process: the H2O2, DTBC, and O2 plus virtually any V-based precatalyst tested form [VO(DBSQ)(DTBC)]2 under the catalytic conditions, that catalyst formation process being autocatalytic in H2O2. The resulting novel concept is that of an autoxidation-product-initiated dioxygenase [Yin, C.-X.; Sasaki, Y.; Finke, R. G. Inorg Chem. 2005, in press]. Herein the following questions about this record catalytic lifetime 3,5-di-tert-butylcatechol dioxygenase catalyst are explored: (i) What is the rate law for 3,5-di-tert-butylcatechol dioxygenation when one begins with Pierpont's [VO(DBSQ)(DTBC)]2? (ii) Does it support the hypothesis that this complex is a catalyst resting state or, perhaps, even the true catalyst? (iii) Can a mechanism be written from that information and from the knowledge in the dioxygenase literature? The results answer each of these questions and provide considerable mechanistic insight into the most catalytically active and long-lived DTBC dioxygenase catalyst presently known.     

Positron annihilation lifetime studies of free volume in ovalbumin gels

The dependence upon concentration of sodium chloride of free volume in ovalbumin gels, a main component of an egg white, is studied by the positron annihilation lifetime technique. The average free-volume radius in ovalbumin gels was about 0.27 nm at 298 K, smaller than those of organic polymers such as low-density polyethylene (0.34 nm at 300 K) and polystyrene (0.29 nm at 300 K). These differences suggest that the positronium annihilates in free volumes located close to hydrogen bonds, thus decreasing average free-volume radius. Free-volume content decreased with the increase of NaCl concentration up to 30 mM. At lower concentrations of NaCl, it seems that a correlation exists between microscopic free volume and macroscopic hardness of the ovalbumin.     

Mössbauer and infrared investigation of reactions of laser-evaporated iron atoms with ethylene

Reactions of laser-evaporated iron atoms with ethylene molecules were investigated. The reaction products were trapped in a low-temperature matrix observed by Mössbauer and infrared spectroscopies. Fe(C₂H₄) was produced at low ethylene concentration, and Fe(C₂H₄)₂ was obtained as the major product at higher ethylene concentration. The assignments were confirmed through molecular orbital calculations.     

Radiative lifetime determination for the 3s4p-1P1 state of magnesium using pulsed level-crossing spectroscopy

The natural radiative lifetime of the 3s4p ¹ P ₁ level of neutral magnesium was measured by pulsed level-crossing spectroscopy at the short excitation wavelength of 202.6 nm. We obtained =13.4(5) ns for this state, strongly deviating from a previous beam-foil result, but in good agreement with a multi-configuration Hartree-Fock calculation. A discussion of the applicability of the pulsed Hanle effect method is given.     

Further studies on polyhalide formation in polar solvents using the positron annihilation lifetime spectroscopy

The rate constants for the reactions of positronium with I ₃ ⁻ , Br⁻, and I⁻ in dimethylsulphoxide (DMSO) and for I ₃ ⁻ in water have been determined, and the orthopositronium lifetime variations have been used for studying polyhalide formation in the (I₂+Br⁻)/DMSO system. It is found that the formation of more than one polyhalide is needed to account for the data. On basis of this new evidence, previous results in methanol and in DMSO are re-examined. Besides the primary I₂X⁻ (X⁻: halide), I₂X ₂ ²⁻ complexes have to be invoked. Trial absorption measurements confirm the presence of solvolysis equilibria for I₂ in DMSO. Taking these facts into account, much higher, albeit poorly defined, values than previously reported are found for the I₂X⁻ formation constants. The reliable K_c values deduced are 28M⁻¹, 0.57M⁻¹ and 2.5M⁻¹ respectively for I₂Cl⁻, I₂Cl ₂ ² and I₂Br ₂ ²⁻ in methanol, and 1.4M⁻¹ and 0.8M⁻¹, respectively for I₂Cl ₂ ²⁻ and I₂Br ₂ ²⁻ in DMSO.     

Positron annihilation lineshape studies as a supplement method to lifetime measurement

We studied positron annihilation lineshapes by a Ge(Li) γ-ray spectrometer for CCl₄ solution in benzene and two monomer crystals which are polymerized by γ-ray irridation. From the lineshapes we evaluated the para-positronium intensity, and the results agree well with those of lifetime measurements in the cases of CCl₄ solutions in benzene and solid acrylamide. But in solid dimethylitaconate the results from the two different methods have not agreed.     

Positron annihilation lifetime studies of the volume phase transition of polyacrylamides

Discontinuous and continuous volume phase transitions of organic polymer hydrogels, such as polyacrylamide (PAAm) and poly(N-isopropylamide) (PNIPA) gels, uponpH and temperature were studied by the positron annihilation lifetime measurement, which allows the estimation of size, intensity and size distribution of the free volume. Microscopic changes of physical and chemical interactions between gel network and solvent molecules and among conjugated solvent molecules at volume phase transitions of polyacrylamide gels were discussed.     

Radiative lifetime of the Te2 B0+u and A0+u states excited by a pulsed dye laser

A pulsed dye laser has been used to measure the radiative lifetimes and quenching rates of transitions of the B0⁺_u and A0⁺_u states of Te₂. The observed zero pressure lifetimes vary from 55 to 730 ns. The quenching rates vary from 0.9 × 10⁶ to 40 × 10⁶ s⁻¹ Torr⁻¹.     

Radiative electron capture: a tool for structure studies of heavy few-electron ions

Radiative Electron Capture (REC) in collisions of hydrogenic germanium ions with hydrogen is measured for projectile energies between 4 and 12 MeV/u. Extrapolating the resulting centroid energies of the K-REC radiation to zero collision velocity theK-binding energy in helium-like germanium-ions is determined. The value compares well with the theoretical prediction. REC in atomic collisions is advertised as a spectroscopic tool for structure investigations of very heavy few-electron projectiles.     

UV photolysis of n-C3F7I. Radiative lifetime of metastable iodine atoms

The photolysis of n-C₃F₇I and C₂F₅I has been studied with a frequency doubled dye laser at fixed frequency in the UV by detection of time resolved resonance fluorescence of the metastable iodine formed. The rate coefficient for quenching of I(²P_{$\text{1}\text{2}$}) by n-C₃F₇I has been obtained as k = (4.6 ± 0.3) × 10^?17 cm³ molecule_?1s_?1. This value is by more than an order of magnitude lower than published values. Further proof is given for the value of the radiative lifetime of the metastable atoms which was obtained recently by this experimental method. This answers questions raised by Husain et al. on this subject in this journal.