多氯代苯胺亲电取代定位效应的密度泛函理论计算

采用密度泛函理论(DFT)的B3LYP方法,在6-311G**基组水平上计算多氯代苯胺脱氯加氢反应中苯环上各C原子电荷分布和亲电取代σ配位化合物能量的变化,研究多氯代苯胺脱氯加氢合成3,5-二氯苯胺反应的定位效应.结果表明:多氯代苯胺邻、对位C原子电荷数和σ配位化合物能量明显小于间位C原子的电荷数和σ配位化合物能量,邻、对位会优先发生脱氯取代反应,即理论上朝着生成3,5-二氯苯胺的方向进行.     

Nucleophilic substitution on PVC effect of the tacticity

Summary The substitution reaction with NaSC₆H₅ is studied for two samples of PVC having very different tacticities. From both the kinetic results and the thermal degradation of the modified polymers it is concluded that the observed very fast initial period of the reaction involves selectively a small fraction of isotactic and heterotactic triads, and that such triads are likely to be considered as labile structures in PVC. The results are therefore a contribution to the understanding of the thermal instability of PVC.     

Cocoa-butter long spacings and the memory effect

Depending on the maximum temperature before cooling, cocoa butter recrystallizes in the β(V) and/or β(VI) phase. In order to obtain structural information on the seed material initiating this recrystallization process, experiments with cocoa butter were performed at a small-angle X-ray scattering station. Crystallization of cocoa butter at various crystallization temperatures provided long d-spacing values (d>45 Å) that can be attributed to the polymorphic phases. From the recrystallization experiments it is concluded that the seeds initiating rapidstarting recrystallization have an 1,3-distearoyl-2-oleoylglycerol-dominated triple chain-length packing. Furthermore, the β(VI) phase of cocoa butter seems to adopt a similar packing. The seed crystals that initiate the slow-starting recrystallization and result in the β(V) phase are likely to be different from those giving the β(VI) phase.     

The effect of substitution of a thioether donor by a phosphine donor in thiacrown complexes of iron

Electrospray mass spectrometry and thermogravimetric analysis reveals that bis(1-phenyl-1-phospha-4,7-dithiacyclononane)iron(II) is more susceptible to ethene loss than bis(1,4,7-trithiacyclononane)iron(II). This is in accord with X-ray crystallographic studies, which show that the C–S bonds are longer in the former complex suggesting an increased population of the C–S σ^* π-acceptor orbitals.     

Density functional theory investigation of site predilection of Fe substitution in barium titanate

The site predilection and effects of the impurity in the form of Fe (II) and Fe (III) in the tetragonal barium titanate lattice were systematically determined by density function theory calculations. All electron calculation was carried out in the local density approximation (LDA). It is shown that both Fe (II) and Fe (III) prefer to substitute at the titanium site with oxygen vacancy compensation as a result of lowest substitution energy. Effect of Fe dopant both in form of Fe (II) and Fe (III) at titanium site with oxygen vacancy compensation lead to change in electrical property in terms of energy band structure of the tetragonal barium titanate lattice.     

Analysis of RNA phage fr coat protein assembly by insertion, deletion and substitution mutagenesis

A structure-function analysis of the icosahedral RNA bacteriophagefr coat protein (CP) assembly was undertaken using linker-insertion,deletion and substitution mutagenesis. Mutations were specificallyintroduced into either pre-existing or artificially createdrestriction enzyme sites within fr CP gene expressed in Escherichiacoli from a recombinant plasmid. This directs synthesis of wildtype protein that undergoes self-assembly and forms capsid-likeparticles indistinguishable morphologically and immunologicallyfrom native phage particles. A series of fr CP variants containingsequence alterations in the regions which are (i) exposed onthe external surface of capsid or (ii) located on the contactingareas between CP subunits were obtained and their assembly propertiesinvestigated. The majority of mutants demonstrated reductionof assembly ability and formed either CP dimers (mutations atresidues 2, 10, 63 or 129) or both dimer and capsid structures(residue 2 or 69). The exceptions were variants demonstratingnormal assembly and containing insertions at residues 2, 50or 129 of thefr CP. A third type of assembled structure wasformed by a variant with a single amino acid substitution I104T.The aA-helix region (residues 97-111) is particularly sensitiveto mutation and any alteration in this region decreases accumulationof mutant protein in E.coli. The relative contributions of particularfr CP domains in maintenance of capsid structural integrityas well as the possible capsid assembly mechanism are discussed.     

A functional antibody mutant with an insertion in the framework region 3 loop of the VH domain: implications for antibody engineering

We have studied the effects of a four residue insertion intothe FR3 loop of the heavy chain variable region from the anti-NPantibody Bl-8. The insertion mutant is obtained as secretedantibody without major defects in biosynthesis, indicating thatantibody variable domains can accommodate length variation notonly in complementarity determining regions (CDRs), but alsoin framework region (FR) loops. The Bl-8 antigen binding siteis not affected by the change in a neighbouring loop. FR3 insertionsrepresent a new method of antibody engineering with a potentialto obtain strong antigen binding by designing additional antigencontacting residues.     

Temperature effect on the nitrogen insertion in carbon nitride films deposited by ECR

The impact of the temperature on the local structure of carbon nitride coating a-C_1-x N_x:H was investigated by spectroscopic analysis. A set of carbon nitride films were deposited at several substrate temperatures (77 K, 300 K, 673 K and 900 K) by electron cyclotron resonance (ECR) ion gun technique fed of CH₄/N₂ plasma.The films were in situ characterized by X-ray photoelectron spectroscopy (XPS). A drastic decrease of the nitrogen content was observed when increasing the deposition temperature from 77 K to 900 K. Qualitative structural and electronic changes were followed after air exposure by infrared (FTIR), near-edge X-ray absorption fine structure (NEXAFS) and Ultraviolet photoelectron (UPS) spectroscopy. Below 300 K, the films are hydrogenated with aliphatic structure and nitrogen is bonded to carbon in many kind of configuration. Between 300 K and 600 K, the nitrogen amount is reduced while both the aromatic and the aliphatic carbons increase. The local structure of the films radically changes at 900 K, whereas the nitrogen surrounding is the same at 673 K. In that case the hydrogen fraction into the films is reduced to zero. The increase of the sp³ carbon as well as the ratio π?/σ? on the nitrogen K edge can be observed. This behaviour may be explain by nitrogen substituted to sp² carbon which induces local changes in the distribution of the π? states.     

The effect of zirconia substitution on the high-temperature transformation of the monoclinic-prime phase in yttrium tantalate

Amorphous yttrium tantalate, as well as solid solutions containing zirconia, transform on heating to a monoclinic-prime phase and then, with further heating, to a crystalline tetragonal (T) solid solution phase at ～1450?°C. On subsequent cooling the tetragonal phase converts by a second-order displacive transformation to a different monoclinic phase not to the monoclinic-prime phase. On subsequent reheating and cooling, the phase transformation occurs between the monoclinic (M) and tetragonal phases, and the monoclinic-prime phase cannot be recovered. The limit of zirconia solubility in both the monoclinic-prime and monoclinic phases lies between 25 and 28?m/o ZrO₂, consistent with previous first-principles calculations. The monoclinic-prime phase is stable up to at least 1400?°C for 100?h for zirconia concentrations from 0 to ～60?m/o ZrO₂. This temperature exceeds the temperature of the equilibrium M-T phase transformation suggesting that the monoclinic-prime phase transforms directly to the tetragonal phase by a reconstructive transformation and is unaffected by the zirconia in solid solution.     

Charge transport in fibre-based perylene-diimide transistors: effect of the alkyl substitution and processing technique

We report a comparative study on the self-assembly from solution and electrical characterization of n-type semiconducting fibres obtained from five different perylenebis(dicarboximide) (PDI) derivatives. In particular we investigated the role of the nature of the alkyl chain covalently linked to the N,N' sites of the PDI in modulating the molecular solubility and aggregation capacity. We explored the morphologies of the self-assembled architectures physisorbed on dielectric surfaces and in particular how they can be modified by tuning the deposition and post-deposition procedures, i.e. by modulating the kinetics of the self-assembly process. To this end, alongside the conventional spin-coating, solvent vapour annealing (SVA) and solvent induced precipitation (SIP) have been employed. Both approaches led to fibres having widths of several hundred nanometres and lengths up to tens of micrometres. SVA formed isolated fibres which were tens of nanometres high, flat, and tapered at the ends. Conversely, SIP fibres exhibited nearly matching heights and widths, but organized into bundles. Despite these morphological differences, the same intermolecular packing is found by XRD in each type of structure, albeit with differing degrees of long-range order. The study of the electrical characteristics of the obtained low dimensional nano-assemblies has been accomplished by fabricating and characterizing organic field-effect transistors.     

The effect of long chain branching on the processability of polypropylene in thermoforming

Linear polypropylene was modified by reaction with peroxydicarbonates in a twin screw extruder to obtain varied degrees of long chain branching. The melt strength and the elasticity of the modified polymers were found to increase with the modification. The processability in foaming and thermoforming processes improved with branching and showed an optimum, beyond which higher degrees of long chain branching appeared not to help any further. The branched PP samples showed distinct strain hardening in the elongational viscosity, which was absent from the original linear melts. Melt strength, elasticity and strain hardening increased with the increase of the number of long chain branches on the main chain. The effect of molecular weight and molecular weight distribution of the precursor on the improvement of the processability of the polymer was examined. Polym. Eng. Sci. 44:973–982, 2004. © 2004 Society of Plastics Engineers.     

Studies on the effect of substitution degree on the liquid crystalline behavior of cyanoethyl chitosan

An alkali–chitosan method was employed to prepare cyanoethyl chitosan (CNCS) with different degrees of substitution (DS) from chitosan by controlling the reaction time. The effect of the DS (from 0.36 to 1.21) on the liquid crystalline behaviors of CNCS was investigated. The critical concentration and texture of CNCS liquid crystalline in dichloroacetic acid and formic acid showed no obvious dependence on the DS. However, increase of the DS could enhance the birefringence of liquid crystalline solutions under a polarized microscope, which implied improved liquid crystallinity. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 2057–2061, 2000     

The effect of aluminum substitution on the mechanically induced self-sustaining reaction of molybdenum-silicon powders

The effect of Al concentration on the ignition of mechanically-induced self-sustaining reaction (MSR) in Mo + (2 ? x)Si + 2xAl powder mixtures has been investigated. When MoSi₂ is prepared by SHS method, Al addition results in the formation of Si-Al eutectic and consequently lowers the ignition temperature. In the case of MSR, replacing brittle Si with ductile Al also affects the activation process. The ignition of MSR was investigated as a function of the Al content in a SPEX 8000 Mixer Mill. The ignition time decreased by 30% between x = 0 and 0.08. At the same time, the grain size increased by about 50% according to XRD linewidth measurements. Comparison of SEM images suggests that Al substitution promotes the formation of agglomerates and lamellar structures, although the differences between the binary Mo-Si and Alcontaining samples are very small. Al substitution encourages the formation of hexagonal Mo(SiAl)₂ over the tetragonal form of MoSi₂. It also promotes more complete conversion during MSR.     

Estimating a change point in the long memory parameter

We propose an estimator of change point in the long memory parameter d of an ARFIMA(p, d, q) process using the sup Wald test. We derive the consistency and the rate of convergence of the estimator for the time of change. The convergence rate of our change point estimator depends on the magnitude of a shift. Furthermore, we obtain the limiting distribution of our change point estimator without depending on the distribution of the process. Therefore, we can construct confidence intervals for the change point. Simulations show the validity of the asymptotic theory of our estimator if the sample size is large enough. We apply our change point estimator to the yearly Nile river minimum water level.     

Density functional theory for the study of the multimode Jahn-Teller effect

The Jahn-Teller (JT) theorem states that in a molecule with a degenerate electronic state, a structural distortion must occur that lowers the symmetry, removes the degeneracy and lowers the energy. The multideterminental-DFT method performed to calculate the JT parameters for JT active molecules is described. Within the harmonic approximation the JT distortion can be analyzed as a linear combination of all totally symmetric normal modes in any of the low symmetry minimum energy conformation, which allows the intrinsic distortion path (IDP) to be calculated, exactly from the high symmetry point to the low symmetry configuration. Results obtained by the approach described here give direct insight into the coupling of electronic structure and nuclear movements.     

Electrochemical insertion of alkaline ions into polyparaphenylene: effect of the crystalline structure of the host material

The intercalation of lithium and sodium ions into different materials derived from polyparaphenylene (PPP) (as synthesized PPP without a thermal treatment, PPP annealed for 36 h at 400 °C and PPP pyrolyzed at 700 °C for half an hour in argon atmosphere) was studied using the electrolyte composed of ethylene carbonate (EC) and propylene carbonate (PC) and MClO₄ as the alkaline salt (M=Li or Na). These materials exhibit various degrees of crystallinity: PPP is a semi-crystalline polymer with about 30% of cristallinity, whereas pyrolyzed PPP exhibits a totally disorganized structure. Spectroscopic characterization indicates that the configurations of the polymer chains are similar in these two materials. Then, we present in this work a comparative study of the intercalation of alkaline ions into these materials in order to specify the effect of the crystalline structure on the intercalation processes. The electrochemical capacities are close whatever the degree of crystallinity contrary to the potentials profiles of the galvanostatic curves. That indicates different intercalation processes into these materials. A two-step mechanism of the intercalation into PPP is proposed. First, the intercalation of the alkaline ions into the crystalline parts of the polymer occurs and secondly the insertion of Li⁺ and Na⁺ into the amorphous regions takes place at very low potentials.