Influence of molecular weight on the thermal decomposition of hydroxyl terminated polybutadiene

Thermogravimetric analysis of hydroxyl terminated polybutadiene (HTPB) and its fractions of different molecular weights separated by preparative GPC shows two major stages of weight loss of different nature in a nitrogen atmosphere. The first stage is primarily depolymerisation, cyclisation and crosslinking of molecules and the second stage is mainly the decomposition of the residue from the first stage. The kinetic parameters, viz. activation energy E and pre-exponential factor A using four different non-isothermal integral equations show a systematic increase with increase in molecular weight for the first stage, whereas for the second stage, the effect of molecular weight on E and A values is not prominent. The increase in E and A values for the first stage is attributed to the formation of greater number of cyclised and crosslinked products from molecules of higher dimensions. Quantitative correlations between the kinetic constants and the molecular weight parameters were derived for the first stage as a quadratic curve following the equation: E or ln A = K₁ − K₂/M (where K₁ and K₂ are empirical constants whose values are different for the different molecular weight averages, viz. M_n, M_w and M_z and for the different equations).     

Bethe surface and compton profile for CO2 obtained by use of 35 keV incident electrons

Electron impact spectra for CO₂ have been obtained at 25 different scattering angles ranging from 1.12° to 14.06°. The measured intensities were converted to generalized oscillator strengths and normalized by use fo the Bethe sum rule, leading to the mapping of the Bethe surface over large momentum transfer K and energy loss E ranges. Substantial deviations from the binary encounter theory were observed for K values smaller than 3 au. A discussion is given on the possibility of extracting partial Compton profiles from the data. The total Compton profile was obtained at large K values and found to be in good agreement with recent calculations, within the experimental uncertainties.     

Evaluation of carrier gases for use in high-field asymmetric waveform ion mobility spectrometry

Effects of carrier gas type (N₂, O₂, CO₂, N₂O, and SF₆) on changes in the ratio of high- to low-field ion mobility, K_h/K, of cesium, gramicidin S, tetrahexylammonium, heptadecanoic acid, and aspartic acid in fields of up to 67 Td are presented. The theory of the mobility of ions at high E/N in different gases is discussed. Plots of K_h/K as a function of the ionic energy parameter, E/N, for the five ions in each of the gases were derived from experimental data collected using a high-field asymmetric waveform ion mobility spectrometer. The change in the ratio of high- to low-field ion mobility of cesium in carrier gases of O₂ and N₂ showed excellent agreement with literature values. The behavior of cesium in O₂ and N₂ is used to illustrate that the ratio K_h/K as a function of effective temperature is invariant with gas type as long as the well depth of the interaction potential significantly exceeds thermal energy. From these results, it appears that the well depth of the interaction potential of the heavier ions studied here, including gramicidin S, tetrahexylammonium, and heptadecanoic acid, with bath gases such as N₂ and O₂, is shallow relative to thermal energy.     

Transport phenomenon as a function of counter and co-ions in solution: chronopotentiometric behavior of anion exchange membrane in different aqueous electrolyte solutions

Anion exchange membrane has been investigated in different electrolyte solutions by chronopotentiometry to explore the influence of co-ion and counterion of the exchange group of the membrane, on the transport phenomena. Chloride, nitrate, sulfate and acetate in sodium salts were used as counterions and sodium, potassium, calcium and ammonium in chloride salts were used as co-ions. The membrane showed a potential drop (E₀) in all these electrolytes when a constant current was applied across it, which remained constant for a period less than τ, called the transition time and rose gradually to a maximum (E_max) value. The parameters such as τ, E₀ and E_max and the potential jump (ΔE) and τ and the inflection zone (Δt) along the time axis have been measured and compared at an applied current density (I) of 10 mA cm⁻² in 10 mM solutions. The values of τ^1/2/z_A[A₀] or τ^1/2/z_C[C₀], with or , E₀ and ΔE with or (where r_A and r_C are the ionic radii of counter and co-ions, respectively) have been correlated. Permselectivity (P) and transference number of the membrane with respect to each one of the above electrolytes have been evaluated and discussed.     

Multiparametric curve fitting-XIII Reliability of formation constants determined by analysis of potentiometric titration data

The formation (protonation) constants log K_i, of the acid H_jL are determined by regression analysis of potentiometric titration data when common parameters (log K_i, i = 1,…, j) and group parameters (E⁰′, L₀, H_T) are refined. The influence of three kinds of error on the protonation constants has been investigated: error from the strategy of minimization, random error, and error from uncertain estimates of group parameters. An analysis of variance of the log K_i, matrix was made for 7 identical titrations and 8 computational strategies, or of 7 identical titrations and 8 different options of group parameters to be refined. The influence of the standard potential E⁰ of the glass-electrode cell on the systematic error in log K is greater than that of the acid concentration (L₀) or the concentration of titrant used (H_T). The ill-conditioned group parameters should be refined together with the common parameters (K_i), otherwise the estimates of log K_i, are not accurate enough. Two ways of calibrating the glass electrode cell were compared. Internal calibration (performed during titration) was more accurate than external calibration done separately. Of the programs tested ESAB and ACBA are the most powerful because they permit refinement of group parameters and internal calibration. Citric acid was chosen as model substance.     

Adriblastina-single stranded DNA interaction with statistical analysis

The electrochemical behavior of adriblastina has been studied at in situ mercury film electrode (in situ MFE) and platinum electrode (PtE) in the presence of phosphoric acid as supporting electrolyte using Osteryoung square-wave stripping voltammetry (OSWSV) and cyclic voltammetry (CV). Optimal experimental and operational parameters have been selected for the drug accumulation and determination in aqueous medium. The interaction of the drug with single stranded DNA (ssDNA) has been studied and validated by using classical least square and partial least square with propagation of error. The formal potentials E° and E°′ and the equilibrium constants K₁ and K₂ have been calculated. It was found that K₂ for the oxidized form of adriblastina is 63 times than K₁ for the reduced form. Among several possible interfering metal ions, a complex formation reaction was observed between adriblastina and Cu(II) ions at in situ MFE. Cu(II) ions formed 1:2 metal:drug complex which is more stable than ssDNA–drug interaction and consequently it inhibits drug biochemical damage effects. The copper complex offers sub-nanograms determination of adriblastina in that 5.80 and 180 pg/ml could be easily detected in aqueous and urine media, respectively, with R.S.D. less than 4%. F-test and t-test have been applied in urine media giving good results that indicated the high accuracy of the proposed method.     

The Ericksen number and Deborah number cascades in sheared polymeric nematics

Samples of liquid crystalline poly(γ-benzyl-glutamate) solutions are sheared between glass surfaces with gaps, d = 10-500 μ, and shearing velocities, V = 0·05-10 000 μs^-1 so that the Ericksen number E ≡ Vdγ₁/K is varied over a large range, E ≈ 1-10⁷. Here γ₁ is the rotational viscosity and K₁ is the Frank splay constant, with γ₁/K₁ estimated to be approximately 1 s μ^-2 for our samples. We observe by polarizing microscopy a sequence of transitions with increasing Ericksen number analogous to that observed in small molecule tumbling nematics: namely rotation of the director out of the shearing plane and into the vorticity direction at Vd ≈ 25 μ² s^-1, and formation of roll cells at Vd ≈ 50 μ² s^-1. The roll cells become finer with increased Vd in accord with predictions of linear stability theory using the Leslie-Ericksen equations, and at Vd ≳ 500 μ² s^-1, the cells become very irregular, producing director turbulence. The turbulence becomes finer in scale as Vd increases, reaching sub-micron, and possibly molecular scales when Vd ≧ 10⁵ μ² s^-1. At the highest velocities, transitions in orientation and texture are controlled by the Deborah number De≡λV/d, where λ is the molecular relaxation time, and uniform texture-free samples are obtained when De ≳ 5.     

1,1,2,2-tetrabromodisilane: gas-phase molecular structure and conformational composition as determined by electron diffraction

The molecular structure of 1,1,2,2-tetrabromodisilane has been investigated using gas-phase electron diffraction data obtained at 110°C. At this temperature the molecules exist as a mixture of about equal parts (X = 0.5 ±0.2) of the two conformers with the H---Si---Si---H torsion angle equal to 180° (anti) or 60° (gauche). Assuming that the two conformers differ in their geometries only in the torsion angle φ, some of the important distance (r_a) and angle () parameters are: r(Si---Si) = 2.349(19) Å, r(Si---Br) = 2.205(5) Å, r(Si---H) = 1.485 Å (assumed), Br---Si---Br = 110.1(1.6)°, Si---Si---Br = 107.1(1.2)° Si---Si---H = 108.6° (assumed). The error limits are 2σ. The observed conformational composition (X_anti = 0.5(0.2)) corresponds to an energy difference between the conformers of ΔE = E(gauche) — E(anti) = 0.5 ± 0.6 kcal mol⁻¹, assuming ΔS = Rln2.     

The optical properties of polymethylmethacrylate polymer dispersed liquid crystals

The effect of the addition of polymer liquid crystals as dispersed molecules to polymethylmethacrylate (PMMA) on the optical properties in the UV-visible and near infrared regions is investigated. From transmission, absorption and reflection spectra the absorption coefficient (ω) and refractive index (n) at angular frequency of radiation (ω) have been calculated at room temperature. The values of the optical band gap (E_opt) have been obtained from the direct allowed transitions in k-space. The width of the tails of localized states in the band gap (ΔE) was evaluated from Urbach edges. Both the parameters (E_opt) and (ΔE) vary with the mixing ratio of dispersed liquid crystals.     

Temperature dependence of the rate constant for reactions of hydrated electrons with H, OH and H2O2

The temperature dependence of the rate constants, for the reactions of hydrated electrons with H atoms, OH radicals and H₂O₂ has been determined. The reaction with H atoms, studied in the temperature range 20–250°C gives k(20°C) = 2.4 × 10¹⁰M^-1s¹ and the activation energy E_A = 14.0 kJ mol^-1 (3.3 kcal mol^-1). For reaction with OH radicals the corresponding values are, k(20°C) = 3.1 × 10¹⁰M^-1s^-1 and E_A = 14.7 kJ mol^-1 (3.5 kcal mol^-1) determined in the temperature range 5–175°C. For reaction with H₂O₂ the values are, k(20°C) = 1.2 × 10¹⁰M^-1s^-1 and E_A = 15.6 kJ mol^-1 (3.7 kcal mol^-1) measured from 5–150°C. Thus, the activation energy for all three fast reactions is close to that expected for diffusion controlled reactions. As phosphates were used as buffer system, the rate constant and activation energy for the reaction of hydrated electron with H₂PO₄^- was determined to k(20°C) = 1.5 × 10⁷M^-1s^-1 and E_A = 7.4 kJ mol^-1 (1.8 kcal mol^-1) in the temperature range 20–200°C.     

单壁AlAs(111)纳米管结构和电子性质的密度泛函理论研究

通过卷曲立方AlAs(111)单层片(sheets)构造了一系列(n,0)和(n,m)一维单壁纳米管。用周期性密度泛函理论(DFT)计算并比较了不同类型AlAs纳米管在几何结构、能量及电子性质等方面的差别。计算结果表明锯齿型和椅型纳米管应变能均为负值,并随着管径变大而逐渐变小。然而,它们的带隙相当不同:椅型纳米管为间接带隙,随着管径的增大而带隙减小;锯齿型纳米管为直接带隙,管径为1.87 nm时存在着一个极大带隙值(2.11 eV)。这种不同主要源于锯齿型纳米管铝原子间3p轨道的耦合贡献。     

Photodecomposition of molybdenum(II) and tungsten(II) carbonyl complexes with triazole, benz-imidazole, and oxadiazole acetylinic derivatives

Photodecomposition of 10 different molybdenum and tungsten mixed carbonyl complexes, [M(CO)₃(B)(A)]I₂ where B=o-phenanthroline or bipyridyl, A=3-(2-propynyl)thio-4,5-diphenyl-4H-1,2,4-triazole (TRZA) or S-propynyl-2-thio benz-imidazole (BIMDA) and 2(2-propynyl-thio(5-phenyl)-1,3,4-oxadiazole (OXA). M(CO)₃(TRZA)I₂, [M(CO)₂(PPh₃)_X(TRZA)I_Y]I_Z where M=Mo, X, Y and Z=1 and M=W, X and Z=2, Y=0, have been performed at 365 nm in oxygen saturated chloroform at 25 °C. The absorbance spectrum of these complexes have been recorded with the time of irradiation in order to examine the kinetics of photodecay.

The apparent rate constant (K_d) for the first-order reaction have been calculated and found to be (3.32–4.79)×10⁻⁵ s⁻¹. The primary quantum yields (Φ) has also been calculated and were in the range (8.33–12.1)×10⁻⁴. The mechanism of the photodecomposition has been suggested according to the kinetic, and photoproduct analysis data, and is similar to reaction of photo-oxidative degradation of polluted molecules in the water.     

Guest and host deuterium effects of the large ZFS parameters D of dimethylbenzaldehydes in durene single crystals

The ZFS parameters D of 2,4-, 2,5- and 3,4-dimethylbenzaldehyde-1h₁ and -1d₁ guests in perhydrogenated and perdeuterated durene single crystals are determined by comparing the experimental and calculated resonance curves. It is found that the deuterium substitution of the guest aldehydic group in a given host leads to the decrease of the D values and to the increase of the energy gaps ΔE_T between the zero-point levels of the ³nπ* and ³ππ* states of the guests. On the other hand, the perdeuteration of the host results in the decrease of ΔE_T with a corresponding increase of the D value of a given guest. The D value of 1 cm⁻¹ determined for 2,5-dimethylbenzaldehyde-1h₁ in perdeuterated durene is the lategest ever found for an aromatic carbonyl compound. Correlations between D and ΔE_T indicate that the ZFS parameters D of the guests are determined by contributions from both spin-spin and spin-orbit interactions between the ³nπ* and ³ππ* states. The large guest and host deuterium effects observed on the D values are attributed to the changes of the gaps ΔE_T of the guests.     

NH2/ND2-vapour pressure isotope effect of cyclopropylamine

The NH₂/ND₂-vapour pressure isotope effect has been determined between 283 and 333 K for cyclopropylamine, an amine with a strong ring strain. The measurements are represented by the relation ln[P(C₃H₅N²H₂)/P(C₃H₅NH₂)] = −(8821.73 ± 68.949) (K/T)² + (23.379 ± 0.223)K/T and correspond to a normal (P_D/P_H < 1) effect. They suggest an association that is slightly weaker than that of propylamine and nearly agrees with that of isopropylamine. The differences are discussed in terms of acidities and steric factors.     

Fluorescence of CF2(B1, 040,K'' = 9) excited by the 253.7 nm Hg resonance line

Resonance fluorescence spectra of CF₂(¹B₁, 040, K' = 9) excited at 253 7 nm have been observed in the photolysis of C₂F₄ using a low-pressure mercury lamp. At low C₂F₄ pressures, the spectra with a series of doublet hands from 254 to 400 nm were consistent with the transitions from ¹B₁, 040, K'=9 to ¹A₁, 0v'₂0, K' = 8, 10, and also to ¹A₁, 1v'₂0, K' = 8, 10 The appearance of doublet bands (Δ K = ±1) provides clear evidence that the transition is of the perpendicular type.     

Studies in macrolide synthesis: A highly stereoselective synthesis of (+)-(9S)-dihydroerythronolide a using macrocyclic stereocontrol

(+)-(9S)-Dihydroerythronolide A, 1, is prepared in 8 steps from macrolide 3 by exploiting the conformational preferences of the (5E,11E)-diene intermediate 2. The stereocontrolled introduction of the hydroxyl groups at C₆, C₁₁, and C₁₂ is achieved by osmylation, 2 → 13 and 15 → 1, while that at C₅ is obtained by a Zn(BH₄)₂ reduction, 13 → 14.     

Magnetism down to 0.90 K, zero-field splitting, and the ligand field in chromocene

The magnetism of chromocene, Cr(cp)₂ where CP = C₅H⁻₅, has been measured as a function of temperature between 0.90 and 303.2 K. The results are reproduced by complete ligand field theory in slightly distorted C_∞v symmetry (Dt = 1153, Ds = 4212, Dq = 28, ζ = 67, B = 553 cm⁻¹, C/B = 4 and k = 0.37). The ground state ³E₂(a₁e³₂) shows a zero-field splitting, D = 15.1 cm⁻¹, E = 0.     

Thermal decomposition of poly(1,4-dioxan-2-one)

To evaluate the feasibility of poly(1,4-dioxan-2-one) (PPDO) as a feed stock recycling material, the pyrolysis kinetics of PPDO were investigated. The pyrolysis of PPDO exclusively resulted in the distillation of 1,4-dioxan-2-one (PDO). From thermogravimetric measurements conducted at different heating rates, the kinetic parameters of the pyrolysis: activation energy, E_a=127 kJ mol⁻¹; order of reaction, n=0; and pre-exponential factor, A=2.3×10⁹ s⁻¹, were estimated by plural analytical methods. The estimates show that the decomposition of PPDO proceeds by unzipping depolymerization as main reaction and random degradation process with lower E_a and A values. Equivalent isothermal degradation curves calculated from the thermogravimetric curves were supported by experimental isothermal degradation data. The calculation that PPDO is converted smoothly into PDO at 270°C agrees with the reported ceiling temperature of PPDO.