Simulation of the random scission of C–C bonds in the initial stage of the thermal degradation of polyethylene

We performed molecular dynamics simulations to analyze the initial stage of the thermal degradation of polyethylene, which is dominated by the random scission reaction. The simulations were initiated from structures that were taken from previously equilibrated snapshots of the amorphous polymer and of a free-standing thin film. Isolated chains were also used for comparison. Our systems were coupled to a thermal heat bath, and the effect of different coupling constants was studied. Rate of random scission increases as the strength of the temperature coupling increases. Rates of reaction are almost similar in thin films and the bulk, whereas the rates are much faster in isolated chains. Expansion of the free-standing thin film accompanies degradation, producing fragments of various sizes. Chains of higher molecular weights than the initial chains can be produced due to recombination of fragments during the expansion of thin films. The polydispersity index of the resulting fragments is higher in thin films compared to the bulk. The bonds at the low density portion of the thin films have a higher probability of being broken.     

A CAS SCF CI study of the 1Σ+g and 3IIu states of the C2 molecule and the 4Σ−g and 2IIu states of the C+2 ion

Ab initio calculations of the X ¹Σ⁺_g and a ³II_u states of C₂ and the X⁴Σ⁻_g and a²II_u states of the C⁻₂ molecular ion are performed to determine the corresponding potential curves around the potential minima and at the dissociation limits. A large Gaussian basis set augmented by three d-type polarization functions on each carbon center is used to approximate the molecular orbits. The calculations are done at the complete-active-space SCF and multi-reference configuration interactions level. Spectroscopic constants and rotation—vibration energies are derived from the ab initio calculated potentials. Good agreement between theory and experiment is obtained for the X¹Σ⁺_g and a ³II_u states of C₂. In the earlier tentative assignment of the observed electronic transition around 2490 Å to the ²Σ⁻_g ← ²II_u system in C⁺₂, the lower state is confirmed by the present calculations to be C⁺ ₂ (²II_u).     

Anodic initiation of the hydrogen sulfide reactions with the hydrocarbons of C n H2n?2 series

We examined the interaction of alkynes and alkadienes with hydrogen sulfide under the conditions of electrochemical initiation of the reaction. One-electron oxidation of hydrogen sulfide leads to the generation of reactive particles (thiyl radical and proton), which react with unsaturated hydrocarbons by two different routes. Hydrogen sulfide acts as a bifunctional reagent forming new C-S bonds, but also exhibits the properties of hydrogenating agent. Varying the experiment duration, reactants ratio, and solvent, it is possible to influence the composition and amount of the formed organic sulfur compounds.     

Study of the system Pr2O3-SeO2-H2O at 100°C and the properties of the compounds obtained

The solubility of the system Pr₂O₃-SeO₂-H₂O at 100°C was studied. The fields of crystallization of Pr₂(SeO₃)₃·H₂O, PrH(SeO₃)₂, Pr(HSeO₃)₃ were established. The compounds obtained were identified by means of chemical, X-ray and thermal analysis. The thermal decomposition of Pr₂(SeO₃)₃·H₂O, PrH(SeO₃)₂, and Pr(HSeO₃)₃ was determined.This revised version was published online in November 2005 with corrections to the Cover Date.     

Kinetics and Mechanism of the Insertion Reaction of Arylisocyanates into C‒C Bond of Phosphabetaine Obtained ont the Basis of Triisopropylphosphine and 2-Cyanoethylacrylate

Abstract

On the basis of kinetic investigation o f previously discovered [I] insertion reaction of arylisocyanates into C-C bond of nvitter-ions, obtained on the basis of trialkylphoshines and Z-cyanoacrylates, the mechanism of this unusual reaction has been suggested. According to this mechanism the nucleophilic attack of nvitter-ion anionic center on the carbon atom of isocyanate group and nucleophilic attack of nitrogen atom on carbethoxylic group, leading to breakup of C-C bond, carry out relatively synchronously within one transition state on the concerted mechanism.     

Molecular Modeling of Interactions of the Benzolactam-V8 Modulators with the Cys2 Domain of Protein Kinase Cδ

Molecular modeling of interactions of four 7- or 8-substituted benzolactam-V8 （BLV） molecules with the cys2 activator-binding domain of protein kinase C （PKCδ） was carried out using molecular docking program Autodock. The docked models reveal that the hydroxymethyl group at the C（5） atom of the eight-membered ring of each BLV is bound at the bottom of the binding groove of the cys2 domain of PKCδ The BLV molecules make hydrogen bonds and hydrophobic interactions with PKCδ, which are similar to those in the crystal structure of the cys2 domain of PKCδ in complex with phorbol 13-acetate. BLV-1 does not contain a long side chain that is hydrophobic and necessary for membrane insertion, so that it would not be a potent modulator of PKCδ. The other three BLV molecules have long side chains substituted at C（7） or C（8） atoms, and it was predicted, based on the docking results, that they had the PKCδ-binding affinity in the order of BLV-2〉BLV-4〉BLV-3, and BLV-2 would be a potent activator of PKCδ.     

Prediction of the solubility of HCl-MgCl2-H2O system at −5°C using the ion-interaction model

The component solubilities of the HCl-MgCl₂-H₂O system at −5°C were predicted by using the Pitzer ion interaction model and its extended Harvie and Weare model. The results indicated that the model supplies a good prediction of the above-mentioned system. The values of parameters of β⁽⁰⁾, β⁽¹⁾, and C ^φ of HCl and MgCl₂ were obtained by using the same optimization methods as in the literature, while those of θ_MgH and Ψ_MgHCl were derived by extrapolating the published data of 5, 15, 25, 35, and 45°C, as well as coupling the activity coefficients with the solubility data. The comparison between the calculated results and the experimental data indicated that the Pitzer ion interaction model and its extended Harvie and Weare model are quite reliable in the solubility prediction. The text was submitted by the authors in English     

Challenges in measuring the δ13C of the soil surface CO2 efflux

The δ13C of the soil surface efflux of carbon dioxide (δ13CRS) has emerged as a powerful tool enabling investigation of a wide range of soil processes from characterising entire ecosystem respiration to detailed compound-specific isotope studies. δ13CRS can be used to trace assimilated carbon transfer below ground and to partition the overall surface efflux into heterotrophic and autotrophic components. Despite this wide range of applications no consensus currently exists on the most appropriate method of sampling this surface efflux of CO2 in order to measure δ13CRS. Here we consider and compare the methods which have been used, and examine the pitfalls. We also consider a number of analysis options, isotope ratio mass spectrometry (IRMS), tuneable diode laser spectroscopy (TDLS) and cavity ring-down laser spectroscopy (CRDS). δ13CRS is typically measured using chamber systems, which fall into three types: closed, open and dynamic. All are imperfect. Closed chambers often rely on Keeling plots to estimate δ13CRS, which may not be appropriate without free turbulent air mixing. Open chambers have the advantage of being able to maintain steady-state conditions but analytical errors may become limiting with low efflux rates. Dynamic chambers like open chambers are complex, and controlling pressure fluctuations caused by air movement is a key concern. Both open and dynamic chambers in conjunction with field portable TDLS and CRDS analysis systems have opened up the possibility of measuring δ13CRS in real time permitting new research opportunities and are on balance the most suited to this type of measurement.     

Hygrometric Determination of the Water Activities of the Aqueous Solutions of the Mixed Electrolyte NaCl–CaCl2–H2O at 25°C

The mixture {yNaCl + (1 – y)CaCl₂}(aq) has been studied with the hygrometric method at the temperature 25°C. The water activities were measured at total molalities from 0.25 mol-kg^–1 to near saturation for different ionic-strength fractions y of NaCl with y = 0.33, 0.50, and 0.67. The obtained data allow the calculation of osmotic coefficients. The experimental results are compared with the predictions of the Zdanovskii–Stokes–Robinson (ZSR), Kusik and Meissner (KM), Robinson and Stokes (RS), Lietzke and Stoughton (LS II), Reilly–Wood–Robinson (RWR), and Pitzer models. From the measured osmotic coefficients, the Pitzer ionic mixing parameters _{Na Ca} and _{Na Ca} Cl are determined and are used to predict the solute activity coefficients in the mixtures. The excess Gibbs energy is also determined. These results are compared with those given in the literature.     

Theoretical study on the mechanism of C2Cl3 + NO2 reaction

The radical-molecule reaction of C₂Cl₃ with NO₂ is explored at the B3LYP/6-311G(d,p) and CCSD(T)/6-311+G(d,p) (single-point) levels. On the singlet potential energy surface (PES), the association between C₂Cl₃ and NO₂ is found to be carbon-to-nitrogen attack forming the adduct C₂Cl₃NO₂ (1) without any encounter barrier, followed by isomerization to C₂Cl₃ONO (2). Starting from 2, the most feasible pathway is the N–O1 bond cleavage which lead to P ₁ (C₂Cl₃O + NO). Much less competitively, 2 transforms to the three-membered ring isomer c-OCCl₂C–ClNO (4 ^a ) which can easily interconvert to c-OCCl₂C–ClNO 4 ^b . Then 4 (4 ^a , 4 ^b ) takes direct C1–C2 and C2–O1 bonds cleavage to give P ₂ (COCl₂ + ClCNO). The lesser competitive channel is the 4 ^a isomerizes to the four-membered ring intermediate O-c-CNClOCCl₂ (5) followed by dissociation to P₃ (CO + ClNOCCl₂). The concerted 1,2-Cl shift along with C1–O1 bond rupture of 4 ^b to form ONC(O)CCl₃ (6) followed by dissociation to P ₄ (ClNO + OCCCl₂) is even much less feasible. Moreover, some of P ₃ and P ₄ can further dissociate to P ₅ (ClNO + CO + CCl₂). Compared with the singlet pathways, the triplet pathways may have less contribution to the title reaction. Our results are in marked difference from previous theoretical studies which showed that two initial adducts C₂Cl₃–NO₂ and C₂Cl₃–ONO are obtained. Moreover, in the present paper we focus our main attentions on the cyclic isomers in view of only the chain-like isomers are considered by previous studies. The present study may be helpful for understanding the halogenated vinyl chemistry.     

Reinvestigate the C2Π-X2Π(0,0) Band of AgO

The \begin{document}$ C^2\Pi $\end{document}-\begin{document}$ X^2\Pi $\end{document}(0, 0) band of AgO has been reinvestigated by laser induced fluorescence spectroscopy with a spectral resolution of \begin{document}$ \sim $\end{document}0.02 cm\begin{document}$ ^{-1} $\end{document}. The AgO molecules are produced by discharging a gas mixture of O\begin{document}$ _2 $\end{document}/Ar with silver needle electrodes in a supersonic jet expansion. By employing a home-made narrowband single longitude mode optical parametric oscillator (SLM-OPO) as the laser source, high-resolution spectra of the \begin{document}$ C^2\Pi $\end{document}-\begin{document}$ X^2\Pi $\end{document}(0, 0) band have been recorded for both \begin{document}$ ^{107} $\end{document}Ag\begin{document}$ ^{16} $\end{document}O and \begin{document}$ ^{109} $\end{document}Ag\begin{document}$ ^{16} $\end{document}O isotopologues. The spectroscopic constants of the \begin{document}$ C^2\Pi $\end{document} state are consequently determined, with the \begin{document}$ ^{109} $\end{document}Ag\begin{document}$ ^{16} $\end{document}O one being reported for the first time. The nature of the spin-orbit coupling effect in the \begin{document}$ C^2\Pi $\end{document} state is proposed to be due to state mixing with the nearby repulsive \begin{document}$ ^{4}\Sigma^{-} $\end{document} and \begin{document}$ ^{4}\Pi $\end{document} states.     

Synthesis of the C29–C37 segment of spongistatin 1

Starting from racemic 2α-methyl-8-oxabicyclo[3.2.1]oct-6-en-3-one the spongistatin E segment has been prepared in nine steps (2.3 steps per stereogenic center) with umpolung of anomeric reactivity at C37. This 3,5-syn-diol sequence completes our methodology to all stereoisomers of 3,5,7-trihydroxy heptanoic ester building blocks functionalized for α-oxyanion chemistry.     

Synthesis of the C1–C13 fragment of eribulin mesylate

Synthesis of the C1–C13 fragment of eribulin mesylate has been accomplished. It features a highly stereoselective construction of a trans-dihydropyran framework using three key reactions: (1) Sharpless epoxidation, (2) regioselective ring opening, and (3) olefin metathesis.     

Kinetics of decomposition of the liquid phase of the ternary system LiOH-H2O2-H2O in the presence of the solid phase of Li2O2 · H2O

The kinetics of decomposition of hydrogen peroxide in the liquid phase of the ternary system LiOH-H₂O₂-H₂O was studied in the presence the solid phase of Li₂O₂·H₂O and without it. The main kinetic parameters of the processes studied were determined.     

Synthesis of the C8–C16 fragment of amphidinolide R

An asymmetric synthesis of the C8–C16 fragment with several stereogenic centers of amphidinolide R, is described. The key reaction, Sharpless asymmetric epoxidation has provided the basis for generation of the required stereocenters. The target fragment was accomplished in a convergent manner in nine steps (longest linear synthesis of the sequence) and 32% overall yield was obtained starting from 2-butene-1,4-diol.     

An improved synthesis of the C42–C52 segment of ciguatoxin 3C

In our previously reported method for the construction of the IJKLM-ring of ciguatoxin 3C (CTX3C), the lengthy synthetic process for the intermediate C42–C52 (L-ring) segment was problematic. Therefore, a new and improved procedure for the C42–C52 segment, having modified protecting groups, was developed. The new route includes a chirality transferring Ireland-Claisen rearrangement for the construction of the vicinal dimethyl branching at C47–48, a one-pot cyclization process for the establishment of the stereocenters at C45 and C46 as well as the γ-hydroxy δ-lactone framework corresponding to the L-ring, and Brown’s asymmetric crotylboration for the installation of the stereocenters at C43 and C44. The new C42–C52 segment was successfully coupled with the previously reported C32–C41 (I-ring) segment to produce the IJKLM-ring.     

Molecular weight distribution of A_2-B_2 type condensation polymers in the presence of capping monomer C

The molecular weight distribution of A2-B2 type condensation polymers in the presence of capping monomer C has been derived with statistical calculation and Monte Carlo simulation methods. The Monte Carlo simulation result agrees with that of statistical calculation. The number distribution function and weight distribution function of seven types of molecules existing in A2-B2-C system have been obtained. The effect of reactivity of capping monomer C on these distributions are discussed.     

Identification of the C2∏-X2Σ+ band system of AlO in the ultraviolet galvanoluminescence obtained during aluminum anodization

The first galvanoluminescence spectrum in the ultraviolet region obtained during anodization of high purity aluminum samples annealed at temperature above 525°C is presented. An intense broad peak with the maximum at about 31,900 cm(-1) is assigned to the transitions (some of them heretofore unobserved) between vibrational levels of the C(2)∏→X(2)Σ(+) spectral system of AlO, partly overlapped with the A(2)Σ(+)→X(2)∏ system of OH.     

Recent strategic advances for the activation of benzylic C–H bonds for the formation of C–C bonds

Alkylarenes, obtained from abundant hydrocarbon feedstock sources, are an attractive starting material for the formation of complex molecular architectures. Conventional activation strategies of the relatively inert sp³-hybridized benzylic C–H bonds usually require relatively harsh conditions and are difficult to apply to the synthesis of fine chemicals. The present review describes recent strategic advances for the activation of benzylic C–H bonds for the catalytic formation of C–C bonds. In particular, two activation methods, i.e., strategies that generate benzylic radicals or benzyl anions, are discussed.     

Kinetics of the Micellar Effects on the Alkaline Hydrolysis of 2-Chloro-quinoxaline in Acetonitrile-Water Mixtures at 35°C

Pseudo-first-order reaction rate for alkaline hydrolysis of 2-chloroquinoxaline (2-CQX) is carried out in acetonitrile (AN)-water (H₂O) mixtures at 35°C. Cationic surfactants as dodecyltrimethylammonium bromide (DOTAB) and an anionic surfactant as sodium dodecylsulphate (SDS) are used above their critical micelle concentration (cmc) to study the effect of micelles on reaction rate. When increasing the percentage of volume of AN, the rate profiles with DOTAB are shown to slightly increase with increasing surfactant concentration, while that with SDS are found to smoothly decrease. The micellar effect is explained in terms of a modified pseudo-phase ion exchange model. The binding constant (K_S) between 2-CQX and DOTAB as micelle showed a decrease by increasing percentage of volume of AN, while that with SDS increased. The counterion micellar coverage degrees (β) are found to be 0.55 and 0.85 with DOTAB and SDS systems, respectively, at all range of volume percentage of AN. Finally, the calculated ratio between rate constants in water to that in the micelle region k_w/k_M at different volume percentage of AN indicated that DOTAB enhances the reaction rate while SDS inhibits it.