WAXS study on orthorhombic crystalline phases in hot-drawn high density polyethylene: POLYMER CHARACTERISATION

The temperature dependence in the range 23–135°C of hot-drawn HDPE plates maintained free from contraction was investigated by WAXS and DSC. A specially designed cylindrical chamber was used for the WAXS patterns. The results confirm the coexistence of two populations of orthorhombic crystals which have different lattice and thermal behaviour. The crystal lattice parameter a shows expansion, while b exhibits expansion up to 90°C, followed by contraction as the temperature is raised to melting. On the other hand, the c parameter is practically constant.     

Kinetics and mechanism of selenium-catalyzed isomerization of some cis-diarylacrylonitriles

The kinetics of the cis-trans isomerization of some ,β-diarylacrylonitriles has been investigated in decahydronaphthalene at 150–190° with selenium as a catalyst. The reaction is first order with respect to olefin and 1/2 apparent order with respect to catalyst. Isomerization rate is increased by electron donating groups in the -phenyl and by electron withdrawing ones in the β-phenyl. Satisfactory correlations between log k and substituent σ⁺ values are observed. Evidence is obtained for a free radical mechanism. The results are consistent with a reaction path involving the formation of a σ-complex between the -C atom and selenium diradical, followed by a free rotation around the single bond and removal of the catalyst. The effects of heteroaromatic nuclei linked to -C atom on the isomerization rate (phenyl>2-furyl>2-thienyl) indicate the lack of conjugation from heteroatom to the side chain in the rate determining step.     

Solvent effect on K to Na ion mutation: a Monte Carlo simulation study

Monte Carlo simulation studies of statistical perturbation theory (SPT) have been carried out to investigate the solvent effects on the relative free energies of solvation and the difference in partition coefficients (log P) for K⁺ to Na⁺ ion mutation in the several solvents. We compared the relative free energies for interconversion of K⁺ to Na⁺, in H₂O (TIP4P) in this study with those published works, that in H₂O (TIP4P) is −16.55 kcal/mol in this study, those of the published works are −17.6, −17.3 and −17.31 kcal/mol and that of the experiment is −17.6 kcal/mol, respectively. Comparing the relative free energies for interconversion of K⁺ to Na⁺, in CH₃OH in this study with those published works, that in CH₃OH is −18.08±0.28 kcal/mol in this study, that of molecular dynamic simulation is −19.6±0.4 kcal/mol and that of the experimental work is −17.3 kcal/mol, respectively. There is good agreement among the several studies if we consider both methods of obtaining the solvation (or hydration) free energies and the standard deviations. For the present K⁺ and Na⁺ ions, the relative free energies of solvation vs Born's function of solvents are decreased with increasing Born's function of solvent except for CH₃OH, THF and MEOME. There is also good agreement between the calculated structural properties in this study and the computer simulation, ab initio and experimental works.     

The nature of the structure difference between light and heavy water and the origin of the solvent isotope effect—I

The observed differences in heat content, entropy and free energy of light and heavy liquid water can be calculated within experimental error from measured infrared frequencies, using a model for water with tetrahedral hydrogen bonding and treating the hindered rotations (librations) of the water molecules as three-dimensional isotropic harmonic oscillators. The origin of the solvent isotope effect for monatomic ions in light and heavy water is their effect on the structure difference between light and heavy water. The quantity Δθ = h Δν/k provides a measure of this structure difference, where Δν is ν−ν′, the difference in average values of the librational frequencies of H₂O and D₂O, and ν′=ν/1·3815. Differences in heat content, entropy and free energy for solution of monatomic ions in light and heavy water and individual ionic activity coefficients can be calculated from Δθ. Hydronium and hydroxide ions are also discussed.     

Potential function of internal rotation for isoprene from ab initio calculations and experimental data

Ab initio computations of the potential energy curve of internal rotation around the central single C---C bond of isoprene have been performed at the Hartree—Fock level with a 3·21G basis set. The similarity of the slope of the curve obtained and the potential energy curve calculated for a more complete basis set (7s3p/4s2p) [Kavana-S2ebø, J. Mol. Struct. 106 (1984) 259] is discussed. The values of the Pitzer function F(φ), its Fourier expansion coefficients, and coefficients of the potential energy expansion were calculated from data given in the above reference. The correction of the potential energy expansion coefficients was carried out from frequencies of torsional “hot” bands of isoprene and torsional overtone of its second rotational isometric form. It was shown that the isoprene second isomer is realized as a gauche-form. The potential energy expansion coefficients were obtained as follows: V₁ = 399.9, V₂ = 1330.22, V₃ = 781.8 and V₄ = −175.8 cm⁻¹.     

Pt-induced faceting of W field emitter tips

Changes in surface morphology that accompany the adsorption of Pt on W are studied using field emission microscopy. During the adsorption of Pt at coverages of >1 monolayer (ML) at 800–1400 K, the shape of a W microcrystal tip is changed from nearly hemispherical to a more polyhedral form. This transition is connected with faceting of the substrate, which is manifested by changes in size and in electron emission of planes mainly along the 1 1 1 zones, e.g., {2 1 1}, {3 2 1}, {8 7 1}. The sizes of (  1 2) and (  2 3) facets are determined along and perpendicular to the [1   1] directions during adsorption up to an average Pt coverage 3.5 ML. The biggest increase of surface area is observed for {2 1 1} facets at 1400 K, which implies that the surface free energy γ₁₁₂ is considerably reduced relative to the average surface free energy. The results are compared with data for other bimetallic systems, and are discussed in terms of the anisotropy of surface free energy, the equilibrium crystal shape (ECS) and the steady-state tip shape (SSTS). There are quantitative differences found between the SSTS and the ECS for Pt/W.     

Foams and surface rheological properties of β-casein, gliadin and glycinin

Interfacial rheological properties and their suitability for foam production and stability of two vegetable proteins were studied and compared to β-casein. Proteins used ranged from flexible to rigid/globular in the order of β-casein, gliadin and soy glycinin. Experiments were performed at pH 6.7. Network forming properties were characterised by the surface dilational modulus (determined with the ring trough) and the critical falling film length (L_still) at which a stagnant protein film will break. Gliadin had the highest dilational modulus, followed by glycinin and β-casein, whereas glycinin formed the strongest film against fracture in the overflowing cylinder. The rate of decrease in the surface tension was studied at the air–water (Wilhelmy plate method) and the oil–water interface (bursting membrane) and the dynamic surface tension during compression and expansion in the caterpillar. Gliadin had the lowest equilibrium interfacial tensions and β-casein the lowest dynamic surface tension during expansion. Hardly any foam could be formed at a concentration of 0.1 g/l by shaking. At a concentration of 1.4 g/l most foam was formed by β-casein, followed by gliadin and glycinin. It seems that in the first place the rate of adsorption is important for foam formation. For the vegetable proteins, adsorption was slow. This resulted in lower foamability, especially for glycinin.     

Structure cristalline et conformation en solution aqueuse de la carnosine (β-alanyl-L-histidine)

Carnosine (β-alanyl-L-histidine) is a biologically active molecule involved in muscular metabolism. It crystallises in the C; space group with a = 24.725 Å b = 5,427 Å c = 8,004 Å β = 100,2° (Z = 4)

In the crystal, acid and basic groups are engaged in hydrogen bonds whose strength is evaluated through IR frequencies. Molecular conformation in the solid state is defined by τ₁ = /t-177° τ₂ = −38° φ = −96° ψ = +131° χ₁ = 181° χ₂₁ = 62°

NMR study of carnosine in aqueous solution indicates that rotation about CH₂-CH₂ is free and that the other angles take the following values: Ø −150° or −90° and X₁ = 165° or 315°. Infrared and Raman spectra suggest that τ₂ undergoes small changes when going from crystal to solution while ψ is close to +150°.     

聚α烯烃光催化降解的机理研究

采用光催化微反应器对铜箔表面附着的聚α烯烃进行了光催化降解处理, 利用X射线光电子能谱(XPS)、 电子自旋共振谱(ESR)和傅里叶变换红外光谱(FTIR)等对光催化降解前后聚α烯烃的元素化学状态、 自由基和官能团进行了检测, 并探讨了光催化降解机理. 结果表明, 聚α烯烃光催化降解过程中发生了含氧基团的引入和CO₂脱附; 参加降解反应的自由基主要为羟基自由基·OH, 降解过程中—OH逐渐增多、 C—H键逐渐减少; 聚α烯烃光催化降解过程中, ·OH进攻聚α烯烃是从取代反应开始的, 首先取代的是末端的氢原子, 然后是末端羟基向醛基和羧基的转变, 最后是碳链断裂形成小分子有机物, 这一过程持续进行, 最终生成CO₂和H₂O等无机物.     

Gradient expansion of the kinetic energy density functional: Local behavior of the kinetic energy density

Calculations with Hartree—Fock electron densities for the rare gas atoms He through Xe show that the gradient expansion for the kinetic energy functional, T[] = T₀[] + T₂[] + T₄[] + … = ∫t() dτ, approximates the kinetic energy by averaging over the shell structure present in the true local kinetic energy density, t(), and that the accuracy of the gradient expansion improves with increasing atomic number. Components of t(), t₀(), t₂() and t₄(), are exhibited and discussed. The defined function t() is everywhere positive.     

Dynamic, electrooptical and energetic nonequivalency of NH bonds in 1:1 and 1:2 complexes of aminopyridines with proton acceptors

The influence of the –NH₂ group position in the pyridine ring on the proton donor ability of N–H groups in hydrogen bonding as well as on the spectral behaviour of stretching and bending vibrations of aminopyridines has been studied. The proton donor ability was shown to increase in the row: meta-, ortho-, and para-aminopyridines. It was established tha N–H bonds in ortho-aminopyridine were not equivalent, and the evaluation of their dynamic nonequivalence was made.

The influence of temperature on the spectral characteristics of the absorption bands of the stretching vibrations of amine groups in the free and hydrogen bonded molecules in CCl₄ has been studied (in temperature range 290–330 K), the formation constants of the complexes have been determined, enthalpy of the 1:1 complexes formation (−ΔH₁) between ortho- and meta-aminopyridines with dimethylformamide, dimethylsulphoxide and hexamethylphosphoramide has been calculated in temperature range 290–330 K. The 1:2 complexes of ortho-, meta- and para-aminopyridines with acetonitrile, tetrahydrofurane, dimethylsulphoxide, hexamethylphosphoramide were studied at the indoor temperature. Enthalpy of the 1:2 complex (−ΔH₂) was estimated on the basis of ‘intensity rule’; −ΔH₁=ΔB^1/2 assuming that parameter does not depend on the composition of a complex.

The vibrational and electrooptical tasks were solved for the free and H-bounded molecules of aminopyridines as well as its complexes of the 1:1 and 1:2 compositions. Dynamic, electrooptical and energetic nonequivalency of NH bonds of aminogroups in aminopyridines was studied quantitatively. The independent calculations of dynamic constants proved mentioned above nonequivalency of NH bonds.

Correlations between spectral characteristics of the absorption bands, geometric, dynamic and electrooptical parameters of –NH₂ group in aminopyridines in the free and hydrogen bonded molecules have been established. Those correlations allow to determine the most important molecular characteristics obtained on the basis of spectral measurements in the range of the absorption bands of the stretching vibrations of aminogroup.     

DFT studies on tautomerism of C5-substituted 1,2,4-triazoles

DFT (B3LYP/6-311++G**, B3PW91/6-311++G**) Gibbs free energy and single point CCSD(T)/6-311++G**//DFT total energy calculations were performed to investigate stability and tautomerism of C5-substituted 1,2,4-triazoles. Three different tautomers are possible for the substituted 1,2,4-triazoles: N1–H, N2–H, and N4–H. Unlike for the 1,2,3-triazoles, where the most stable is the N2–H tautomer regardless of substituent applied, for the 1,2,4-triazoles, the electron donating substituents (–OH, –F, –CN, –NH₂, and –Cl) and the C5-cation stablize the N2–H tautomer, whereas the electron withdrawing substituents (–CONH₂, –COOH, –CHO, –BH₂, and –CFO) and the C5-anion stablize the N1–H tautomer. Except for the C5-anion and C5-cation, the N4–H form is the least stable tautomer. The relative stability of the C5-substituted 1,2,4-triazole tautomers is strongly influenced by attractive and/or repulsive intramolecular interactions between substituent and electron donor or electron acceptor centres of the triazole ring.     

(QCO) as the model for bonding in non-classical carbonyls: a force approach study

As a model for non-classical metal carbonyls, we investigate CO in presence of a unit positive charge placed at different distances along the bond axis. We use the force approach, to look into the nature of the individual molecular orbitals. We find that in free CO the HOMO (σ(3)) is antibinding. As the positive charge approaches form carbon side, σ(1) and degenerate π orbitals become more binding, while σ(2) and σ(3) become more antibinding. The overall effect is more binding resulting in a shorter C–O bond. If the charge approaches from oxygen side, then σ(1), σ(3) and degenerate π orbitals become less binding, while σ(2) becomes slightly more binding, resulting in a lengthening of C–O bond.     

Colorimetric method for determination of chlorine with 3,3',5,5'-tetramethylbenzidine

A new method is developed for the calorimetric determination in water of chlorine (free and combined) with 3,3′,5,5′-tetramethylbenzidine. The procedure proposed achieves a detection limit of 2 ng/ml and its sensitivity is greater than that of the methods used at present.     

Quantum mechanical study of tautomerism of methimazole and the stability of methimazole–I2 complexes

DFT and ab initio theoretical methods were used to calculate the relative stability of tautomers in the methimazole (MMI). The calculations show that the thione form of MMI 1 is more stable than the thiol tautomer in good agreement with the experimental results. The DFT and ab initio calculations were also used to determine the stability of MMI–I₂ complexes. All methods suggest that the methimazole in the MMI–I₂ complex exists almost exclusively as the thione tautomer. The Gibbs free energy difference between planar and perpendicular forms of thione tautomer of MMI–I₂ complex indicates that the planar form is the predominant complex. The counterpoise corrected Gibbs free energy also shows that the MMI–I₂(plan.) complex is more stable than the MMI–I₂(perp.) complex. These predictions are in good agreement with the experimental results. By using the natural bond orbital (NBO) approach, the effects of charge transfer interactions on the stability of MMI–I₂ complexes were investigated. The LP₃(S)→σ*(I–I) and LP₃(I)→σ*(N–H) charge transfer interactions may be very important in the stability of the planar form. The results show that the LP₃(S)→σ*(I–I) charge transfer interaction causes a greater increase in the σ*(I–I) antibond occupation number, and concomitantly, a greater increase in the corresponding I–I bond length in the planar complex with respect to the perpendicular complex. The LP₃(S)→σ*(I–I) charge transfer interaction is assisted by NHI intermolecular hydrogen bonding. The atom in molecule (AIM) analysis shows that the charge density and its Laplacian at the SI bond critical point of the planar complex is greater than the perpendicular complex.     

Adsorption of oligooxypropylenated amines at the aqueous solution—air interface: Homologous series of piperidine and morpholine derivatives

The surface tension isotherms for pure oligooxypropylenated piperidine and morpholine at the aqueous solution—air interface were determined and interpreted. The surface excess concentration, Γ, the surface area per molecule, A, and the standard free energy of adsorption, ΔG°, were calculated according to a new empirical adsorption equation. The standard free energy contribution for the oxypropylene group (PO) in morpholine derivatives,ΔG° (PO) = −3.34 kJ mol⁻¹, is substantially lower than that for the PO group located in the piperidine derivatives, i.e. ΔG° (PO)= −3.12 kJ mol⁻¹.     

Freezing of water bound in lichen thallus as observed by H NMR. II. Freezing protection mechanisms in a cosmopolitan lichen Cladonia mitis and in Antarctic lichen species at different hydration levels

Proton NMR spectra at 300 MHz for dehydrated and hydrated thallus of Cladonia mitis Sandst. [=C. arbuscula (Wallr.) Flot ssp. Mitis (Sandst.) Ruoss], Himantormia lugubris (Hue) Lamb and Usnea aurantiaco-atra (Jacq.) Bory were recorded. The temperature was decreased from room temperature down to −45 °C. Pulse length was set to π/2=8.3 μs, which allowed the observation of tightly bound and loosely bound/or free water fraction, whereas the signal from solid matrix of thallus was reduced. In hydrated thallus a narrow Lorentzian line coming from loosely bound/free water fraction was recorded. For the temperature range between −5 and −20 °C a discontinuous increase in line halfwidths, accompanied by a decrease in area under the peak, was observed. This was attributed to the cooperative freezing of bulk water present in lichen thallus. In dehydrated thallus the NMR line consists of two components: a narrow, Lorentzian one (coming from loosely bound/free water fraction) and a broad line (from water tightly bound in lichen thallus). The overall area under peak remains unchanged down to −5 °C, and then between −5 and −20 °C it continuously decreases due to non-cooperative water immobilisation. As the temperature is decreased, for temperatures above −5 °C, the contribution made by the broad line component increases at the expense of the narrow line component. The mechanism of loosely-to-tightly bound water transfer is, at least partially, responsible for the freeze-protection of thallus in the lichen species investigated. No significant differences between the freeze protecting loosely-to-tightly bound water transfer mechanism of Antarctic lichens and that of cosmopolitan lichens was noticed.     

Path integrals and optical potentials for elastic and inelastic scattering

When relative degrees of freedom are taken to traverse a fixed trajectory R(τ), the properties of Feynman path integrals are used to derive a series expansion of the logarithm of the internal state elastic scattering amplitude in terms of the internal relative coupling potential. Likewise transition amplitudes for inelastic scattering are expressed in terms of an exponential of an infinite series of 2 x 2 matrices. These results represent a reordering of the usual perturbation series of time dependent quantum mechanics to obtain a series expansion in the exponential as well as truncation from the space of all channels to that of only one or two (or more, if necessary). When the relative degrees of freedom are to be treated quantum mechanically, R(τ) is now the integration variable in a Feynman path integral. The above noted exponentiated series then correspond to the exact optical potentials for elastic and inelastic scattering within the Feynman path integral formulation of quantum mechanics.     

Analysis on absorption spectra of Nd in poly(methyl methacrylate)

The absorption spectrum of neodymium (III) (Nd³⁺ doped in poly(methyl methacrylate) was measured. The Nephlauxetic effect was found in the spectrum compared to other spectra of Nd³⁺ doped in various matrices. The experimental data used, Slater-Condon parameters (F₂,F₄,F₆) and Lander parameter (ζ_4f) were calculated by the Taylor series expansion based on the assumption that the energy separation between J-levels of the 4fⁿ-configuration is a function of F₂,F₄,F₆ and ζ_4f. The Judd-Ofled intensity parameters (Ω₂,Ω₄,Ω₆) were also calculated. Analysis of the Nephlauxetic effect and the parameters variation on the host matrices was carried out.     

Theoretical studies of carbenes and carbenoids. 5. Intramolecular reactions of cyclopropylcarbene

Intramolecular reactions of cyclopropylcarbene in singlet ground state have been studied using HF/3-21G- gradient methods. Relative energies are estimated with 6–31G^**. The “super-conjugation” between the cyclopropyl and the p AO at the carbene site stabilizes the carbene and retards its exo-endo isomerization with a barrier of 13.7 kcal/mol. The ring expansion occurs preferably from the exo conformation with a barrier of 12.9 kcal/mol. Two terminals disrotate in different rotation directions before and after the transition state. The ring expansion from the endo conformation is a more energetic pathway. The 1,2 H shift is not competitive to the ring expansion for its much higher barrier.