2003 Vol. 16, No. 2

Letter
Article
NCO radicals were produced by laser photolysis of CHBr3at 266 nm followed by the reaction of CH with N2O. The electronic ground state NCO radicals were electronically excited toA2Σ+(0000) state by laser irradiation at 438.6 nm. The quenching rate constants of NCO (A2Σ+) by CH4,n-C5H12,c-C6H12,n-C6H14, n-C7H16,n-C8H18were measured by investigating the time-resolved fluorescence from the excited NCO in room temperature (298 K). Removal was found to be very efficient where the rate constants with the above gases were measured as 2.35±0.14, 4.74±0.11, 4.65±0.12, 4.84±0.21, 5.15±0.19, 5.51±0.26, respectively, in units of 10~10 cm3/molec s. It is shown that the quenching rate constants and cross sections of NCO (A2Σ+) increase almost linearly with increasing the number of C-H bonds contained in the alkane molecules. By analysis, it is thought that chemical reactions play an important role in the quenching of the electronically excited NCO radicals and in addition to chemical reaction, collisional removal of NCO (A2Σ+) can take place via an E-V energy transfer.
The photodissociation process of SO2+ molecular ions has been investigated by measuring the photofragment SO+excitation (PHOFEX) spectrum in visible (563~660 nm) wavelength range under the condition of supersonic molecular beam. The SO2+molecular ion were prepared purely by [3+1] multiphoton ionization of the neutral SO2 molecules at 380.85 nm. The SO+ PHOFEX spectrum in 563~660 nm range is continuum without any clear structure and the dissociation efficiency producing SO+ decreases with the laser wavelengths toward to red direction. These facts means that there should be a repulsive state ofα2A2around SO2+ (E, D , C). The [1+1] photodissociation mechanism of producing SO+ is suggested. Firstly, SO2+(~X2A1) ions is excited into the energy range with high dense levels of the middle state~B2B2, then SO2+(~B2B2) ions is excited into theα2A2repulsivestate by absorbing another photon, and finally dissociates to SO+(X2Π) +O(3Pg) directly .
Using the MP2 method at the 6-311G(d,p) level, the geometries of the reactants, transition state and products of the hydrogen abstraction reaction of isocyanic acid (HNCO) with formyl radical (HCO) have been optimized. Changes of bond lengths of interacting molecules and the stationary point structures were discussed along the minimum energy paths (MEP). It shows that the process of the N-H bond breaking and the C-H bond forming is a concerted reaction. The stationary point energies are corrected by QCISD(T,full) and the potential barrier calculated 91.47 kJ/mol is close to the experimental barrier 108.9 kJ/mol. The rate constants calculated by the CVT method have been carefully compared with the experimental data over the range temperature 500~2500 K.
The potential energy function, force constant and spectroscopic data for the ground state X6Σ+ of PuN have been derived by the Gaussian 98 program with the B3LYP method, based on the approximation of relativistic effective core potential for Pu atom and all-electron 6-311G* basis set for N atom. The separated atomic group method is used to derive the possible electronic states for PuN. The ground states for Pu and N are4Su, respectively, which are the components of irreducible representation of the SU(n) group. PuN belongs to the C∞v group. The irreducible representations of the SU(n) group can be resolved into those of the C∞v group. That is, the possible electronic states of PuN are given. Pu(7Fg) and N(4Su) are resolved into the direct sum of C∞v. Their direct product and its reduction are the possible electronic states of PuN. Dissociation limit is derived by atomic and molecular reaction statics. The potential energy function of the ground state for PuN molecule is derived byab initiowith the B3LYP method, based on the approximation of relativistic effective core potential for Pu atom and all-electron 6-311G* basis set for N atom and fitting the Murrell-Sorbie (M-S ) function. In addition, the thermodynamic functionΔfH0、ΔS0andΔfG0of PuN(g) in the standard state are also calculated, which are -487.239 kJ/mol, 95.345 J/mol K and -515.6661 kJ/mol, respectively.
The simplest intermolecular potential for C60 is Lennard-Jones 6-12 type potential, but the L-J potential couldn’t consider the volume of C60 molecules, so the potential is not correct in most cases. Kihara type interaction potential between two C60 molecules has been proposed recently. This intermolecular potential for C60 includes the volume of C60 molecules, so it is better than L-J potential. According to the Kihara type interaction potential for C60 molecules, lattice vibrations of fcc C60 crystal have been studied. Distribution of phonon dispersion frequencies of fcc C60 crystal along [111], [110] and [100] directions has been obtained. Distribution of state-density of fcc C60 crystal has been gained too. The Debye temperature ΘD of fcc C60crystal has been calculated, because the fcc C60 crystal is molecular crystal and the ΘD is 75.5 K lower than that of common metal crystals. According to the distribution of state-density of fcc C60 crystal, heat capacity at 298 K and 101 kPa has been obtained here, its value is 526.23 J/Kmol and the result accords with the experimental value.
A molecule of straight-chain alkanes was regarded as a multi-dimension system in mechanic vibration. A atom, or atomic group was regarded as the dispersion mass-point in it. The chemical bond in molecules was like the spring combining the dispersion mass-point. The intrinsic vibrational frequency was calculated by transmit matrix method. The change relation of the intrinsic vibration with the structure unit was discussed. It is found that there are good relationships between the fundamental frequencies and the structural properties of straight-chain alkanes, and between the sum-frequencies and the additivity properties of straight-chain alkanes as well. Combining the fundamental frequency and the sum-frequency, the relation model of the condensation properties with the fundamental frequency and sum-frequency can reflects the change law between the properties and intrinsic frequencies awfully nice. The quantitative relation model was established between the fundamental or sum frequency and properties of straight-chain alkanes. Some properties of straight-chain alkanes were predicted by the model. The results showed that there was very close relation between the frequency and the properties, and the coefficient r>0.999.
Based on the dipolar approximation, the change of volume and shape of particles suspended in electrorheological fluids(ER) are investigated by using the electric polarization method. The relative variational rate is calculated for the volume and shape of particles. The results of computation show that the relative variationa rate of the volume and shape of particles is in direct proportion to the square of electric field intensity, and relates to the character of ER. The relative variational rate of the volume and shape of particles is 8.4% and 12.5%, and the contribution to integral volume is 1.68% under the general condition. The contribution is devised by the change for electrodilatancy and electric heating of ER. The free energy is in direct proportion to the square of electric field intensity when particles are imagined as rigid balls.
A new effective and fast minimization approach is proposed for the prediction of protein folding, in which the“relative entropy”is used as minimization function. Unlike the energy minimization method, the essence of this approach is to search the conformation with high occupation probability, which corresponds to the state with low free energy instead of low energy. The off-lattice model is used, and the prediction just focuses on the frame of the main chain of protein. In this approach, only the distances between the consecutive Cα atoms along the peptide chain and a generalized form of the contact potential for 20 types of amino acids are used. Tests of the prediction algorithm are performed on real proteins with the initial structure fully denatured. The root mean square deviations (RMSD) of the structures of four folded target proteins versus the native structures are from 5 to 7. The advantage of this approach is its simple potential function and fast performance. Moreover, it can be considered as an improvement on the energy minimization method in principle.
The phase-separation kinetics of liquid-crystalline polymer/flexible polymer (LCP/FP) blend wa studied with the coupled time-dependent Ginzberg-Landau equations for compositional order parameterand orientational order parameter Sij, which were numerically solved by means of the cell dynamical system in two dimensions. The influence of coupling between compositional and orientational ordering processes on the phase separating morphology was investigated. It was found that the orientational ordering promoted the compositiona ordering, i.e., the increasing of the magnitude of orientational order equaled the decreasing of the compatibility between two phases. Moreover, the spatial distribution of phase-separated structures is determined by both the thermodynamic factor, which means the nematic direction of LCP domains parallel to the interface is preferable, and kinetic factors, which imply LCP has a tendency to diffuse parallel to the nematic direction. The small-angle light scattering patterns are numerically reproduced by means of the optical Fourier transformation of spatial variation of the polarizability tensoraij, and the azimuthal dependence of the scattering intensity distribution i interpreted.
Carbon nanotubes and metal nanowires were formed in situ during the electrolysis of graphite cathode using LiCl, LiOHand LiCl/SnCl2 melts as electrolyte, respectively. The structure and morphology were characterized by TEM, XRD and EDS analyses. TEM images showed that the prepared materials varied with the nature and composition of the melts. Carbon nanotubes were formed electrolytically in the LiCl melt with the diameters in the range of 75~100 nm, but β-Sn-filled carbon nanotubes were generated in molten LiCl+1.0%SnCl2 mixtures. Followed by subsequent oxidation in the air, β-Sn nanowires were converted into SnO2 in carbon nanotubes, which was identified by XRD analysis. The obtained SnO2 nanowires were of tetragonal structure ranging from 20~50 nm in diameter. It was implied that the occurrence of LiC6produced from cathodic reactions had significant influence on the formation of carbon nanostructure during electrolysis.
The AgCl/PMMA nano-composite was prepared by polymerization of MMA in site through reverse microemulsion as template, in which the methyl methacrylate(MMA) was designated as oily phase instead of non-reactive functional group solvents. The advantage of this method is that nanocomposite are prepared without the process of modification of nano-particles with organic substances and dispersing them in the matrix. TEM shows that the particle size of nano-AgCl is 20~80 nm, and the scanning electron microscope (SEM) shows that nano-AgCl is dispersed in the PMMA. FTIR spectrum reveals that the micro-environment of H2O and carboxyl group has changed after the polymerization of MMA, which indicates that the interaction of nano-particle with polymer exists. DMTA spectrum results show that there is strong interaction of nano-particles with polymer chains, which restricts the mobility of polymer chains and forms an interphase layer. Ripening process affects the interaction of polymer with nano-particles. At the higher temperature ripening process, the interphase layer is overlapped and forms a continuous phase. DMTA reveals that the aqueous phase layer around the nanoparticles does not apparently affect the interaction of polymer with the nano-particles and the nano-particles acts as a cross-linker in the nanocomposite.
Two new Schiff base complexes, ZnL(ClO4)·4H2O (A) and CdL(ClO4)·3H2O (B), where L=2-{[2-(Aminomethyl-amino)-ethylimino]-methyl}-phenol, have been synthesized and characterized by elemental analysis and infrared spectra. Further, two new bimetallic coordination polymers, {[ML][FeIIFeIII(ox)3·H2O]}∞ were synthesized and characterized, in which ox2-=oxalate, M=Zn2+(C) or Cd2+(D). The bonds of νas(C=O), δ(C=O) were at 1635, 818 cm-1 for complex C and 1646, 820 cm-1for complex D, respectively. The results of Mussbauer spectra of C and D revealed that there exist two kinds of valence ions. The two bimetallic coordination polymers feature the 2-D layer structure in the solid state, and their anions layer was formed by the [FeIIFeIII(ox)3]-unit.
Electrochemical reduction of silver ions at the interface of monolayer prepared from stearic acid resulted in the two-dimensional formation of silver film. The morphology of the deposits and their growth rate were studied at the surface of silver nitrate solution as a function of several physical and chemical parameters, namely, the pH of subphase, the surface pressure of the monolayer and the electric voltage were studied. The working electrode (0.33 mm diameter silver wire) just touched the solution surface, coating the surfactant monolayer and producing a small concave depression. The anode (0.33 mm diameter silver wire) was placed at the circumference of the Langmuir trough. A standard calomel electrode served as the reference electrode, via a luggin capillary whose tip was set near the cathode. The experiment was carried out under potentiostatic condition controlled by the TD 3690 potentiostat and the electrochemical system software. The silver film formed at the monolayer/solution interphase under different conditions was transferred to solid substrates by horizontal lifting in order to be investigated by SEM or TEM images. High concentration of silver ions localized at the monolayer interface, as opposed to those present in relatively low concentrations in the subphase, assured preferentially two-dimensional growth. In neutral or alkazid solutions, the silver film could be obtained under the surfactant monolayers. In contrast, the silver film could not be observed at the surface of the acid solution. The importance of the existence and the state of the negatively charged monolayer on the formation and structure of silver deposits are presented. Furthermore, the growth rate rises along with the increase of the electric voltage, making for the change in the shape of silver films from roundness to irregularity. It was found that silver ions, which nucleated heterogeneously firstly induced by monolayer at the surface of subphase, develop star-shaped particles from octahedral framework and become the pine-structured silver films gradually.
The morphological structures shown in scanning electron microscope (SEM) images were investigated for polytetrafluoro ethylene (PTFE) porous membranes prepared from PTFE fine powder by mechanical processes, including extrusion, rolling and stretching. Fiber net-like structures with fiber-connected nodes could be observed in membranes. The morphology of membranes was significantly changed by stretching with unixial and biaxial and also by the fiber shrinkage if the stretched membrane was not treated under high temperatures. It is proposed that nodes are formed by PTFE particles in extrusion and rolling and then bunches of fibers were pulled out of particles in stretching to make porous formed. Since the SEM image shown for membrane surface is just 1 nm in depth, the porous distribution in 10μm-thick membranes should be very complicated.
The supported dinuclear nickel methoxide complex Ni2(OCH3)2/SiO2was prepared by surface reaction modification and ion-exchange method. Its chemical composition and surface structure were characterized by element analysis and infrared spectroscopy (IR) techniques. The results showed that, in Ni2(OCH3)2/SiO2, Ni2+ bonds with surface O2-of SiO2 supported in bidentate fashion andbis-μ-OCH3bridged structure Ni2 (OCH3)2 was formed between the two metal ions. The chemisorption properties of CO2 and CH3OH on the catalyst were studied by chemisorption infrared spectroscopy and chemisorption temperature programmed desorption (TPD) techniques. The experimental results showed that CO2chemisorbs on the catalyst as two states: bridged absorption state and methyl carbonate species. At room temperature, the bridged absorption state is formed between Ni2+ and OCH3 ligand , in which metal ions adsorb with the O atom and OCH3 ligand adsorbs with the carbon atom in CO2. At higher temperature, it transforms to the methyl carbonate species, the amount of which can be increased by formation of the bridged OCH3. Whilst CH3OH only forms molecular adsorption states with O atom in CH3OH adsorbing on Ni2+ and desorbs molecularly during 110~140℃, the synergic relationship between Ni2+ and OCH3ligand determines the activation process and the adsorption state of CO2 and CH3OH. The catalytic reactivity for Ni2(OCH3)2/SiO2 was characterized by temperature programmed surface reaction mass spectroscopy(TPSR-MS) and micro-reactor evaluation techniques. The results revealed that CO2and CH3OH could react on Ni2(OCH3)2/SiO2 with good reactivity and high DMC selectivity. At 100~200℃, reaction products were mainly DMC and H2O and the reactivity was determined by surface methoxyl carbonate species and CH3OH in molecular adsorption states. Based on the results above, the activation processes of CO2 and CH3OH and the mechanism for DMC synthesis were analyzed theoretically. The formation and amount of methyl carbonate species of CO2 on Ni2(OCH3)2/SiO2 control the procedure of the reaction. The methoxylation of intermediate surface hydroxide is necessary to the catalytic circle.