Table of contents

The PDF contains Chinese-language abstracts for the articles in the June 2007 issue.

An imaginary-time split operator approach is proposed to study electron transfer (ET) dynamics using Sumi-Marcus theory. The approach is applied to evaluate the intermolecular ET rate between oxazine 1 and N,N-dimethlaniline. By measuring the two average survival times of the donor state probability and the rate constant in long time limit, the full kinetics of the ET is revealed with a variety of sink functions. Several new properties for the ET have been found in numerical simulations.

The decomposition mechanisms of azoisobutyronitrile were systematically investigated. Density function theory B3LYP/6-311G^*, B3LYP/6-311+G^* and BHandH/6-31+G^** methods were employed to optimize the geometry parameters of the reactants, transition states, possible intermediates and products based on the detailed potential energy surfaces scanned with AM1. The reaction mechanisms were also analyzed theoretically. The results indicate that the decomposition of azoisobutyronitrile is a two-bond (three body) synchronous cleavage process in the ground state (CH₃)₂CNC-N=N-CCN(CH₃)₂ →2(CH₃)₂CNC·+N₂; and is a two-bond asynchronous cleavage process while in the triple state (CH₃)₂CNCN=N-CCN(CH₃)₂ →(CH₃)₂CN–CN=N · · · CCN(CH₃)₂ →2(CH₃)₂CNC·+N₂. The calculations rationalize and verify all experimental facts.

A shell model molecular dynamics method is used to investigate the behavior of the pressure-volume relationship, heat capacities at constant pressure and constant volume, Grüneisen parameter for GaN with zinc-blende cubic structure at high pressures and high temperatures. The interactions between Ga-Ga, Ga-N, and N-N are described with polarizable potential models which have assigned two different partially ionic charges to Ga and N by taking into account of the ionic character of GaN. It is shown that the calculated thermodynamic parameters at ambient condition are in good agreement with the available theoretical results. Compared with the results from first-principles calculations, the discrepancy of constant-volume heat capacity at lower temperature may be explained well with different approximation mechanisms. The properties of GaN with zinc-blende structure are summarized in the temperature range of 300-2000 K and pressure up to 40 GPa.

Molecular dynamics (MD) simulations with a quantum correction were used to study the 0.5 mol% V⁵⁺/TiO₂ rutile under conditions of 300 K and 101 kPa. The interatomic potential function in MD simulations is composed of Coulomb, short-range repulsion, van der Waals, and Morse interactions. The topology of rutile was found to undergo a large deformation when Ti⁴⁺ is replaced by V⁵⁺, which can be related to the difference of valence and ionic radius between V⁵⁺ and Ti⁴⁺. The graphed distributions of Ti-O and O-O bond lengths, and O-Ti-O angles are all broaden. The simulations showed that when Ti⁴⁺ is replaced by V⁵⁺, the V⁵⁺ moves out of the O₆ polyhedron and toward the interstice between TiO₆ octahedra, which results in serious distortion of the octahedra near the interstice, but the phase with 0.5 mol% V⁵⁺ doping still remains in rutile. The structural features, such as the bond lengths, migration of the dopant, and crystal phase in the MD simulation are consistent with the experimental observations by FTIR, Raman, XRD and ESR.

The reactions of CH₃CF₂O₂ with HOO are important chemical cyclic processes of photochemical contamination. In this paper, the reaction pathways and reaction mechanism of CH₃CF₂O₂+HOO are investigated extensively with the Gaussian 98 package at the B3LYP/6-311++G^** basis sets. The use of vibrational mode analysis and electron population analysis to reveal the reaction mechanism is firstly reported. The study shows that CH₃CF₂CO₂+HOO → IM1 → TS1 → CH₃CF₂O₂H+O₂ channel is the energetically most favorable, CH₃CF₂CO₂H and O₂ are the principal products, and the formation of CH₃OH and CF₂O is also possible.

The potential energy surface (PES) for the reaction of E,E-pentadienyl with molecular oxygen was theoretically studied at the G3B3//B3LYP/6-311G(d,p) level of theory. The first step of the reaction was found to be the direct adduction of molecular O₂ on either the C1 or the C3 atoms of E,E-pentadienyl, forming two C₅H₇O₂ · isomers. These two C₅H₇O₂ · isomers undergo a series of isomerization processes through either the hydrogen-transfer or cyclization pathway. In the final step, the hydrogen-transferred and cyclized isomers decompose into unsaturated aldehydes, unsaturated ketones, and hydroxyl radicals. Involves 20 stable species and 14 transition states, and the energies and structures of all reactants, products and transition states were calculated. Based on the calculated barriers and heats of formation, the authors suggest that the C₂H₃O·+C₃H₄O formation channel is the dominant channel for the C₅H₇ · +O₂ reaction. The possible existence of C₅H₇O₂ · radicals as long lifetime intermediates is also proposed, which is consistent with the recent photoionization mass spectrometric experiments by Zils etal.

Cooling rate dependence of the cooperative relaxation in 1,2-propanediol was investigated in terms of the nonlinear Adam-Gibbs (AG) enthalpy relaxation theory using differential scanning calorimetry. The AG parameters were obtained using a curve-fitting method. The results indicated that the model parameters show strong dependence on the cooling rates. Those obtained at higher cooling rates are in good agreement with the published data. The fitting results were used to estimate the microscopic parameters of the cooperative rearranging region (CRR), in particular the size of the CRR (z*) and the configurational state available to it (W*). It was found that the W* recommended for polymers led to physically meaningless z* for 1,2-propanediol. Johari's method, which was based on the AG theory, yielded around 3 molecules in the CRR, but the W* was anomalistically higher than those of polymers. It is difficult to reconcile the Adam-Gibbs' z* with the results obtained using Donth's formula. An argument is presented that a new physical meaning should be given to conventional Adam-Gibbs' z* for molecular H-bond liquids.

Spin-allowed transition probabilities between energy levels above the 3s3p state of Si III are reported by employing a coupled equation within WBEPM theory. The results show a good agreement with critical values, with the derivations mostly less than 15%. The method can readily be applied to highly excited states without any extra effort.

The absorption spectral properties of para-aminobenzophenone (p-ABP) were investigated in gas phase and in solution by time-dependent density functional theory. Calculations suggest that the singlet states vary greatly with the solvent polarities. In various polar solvents, including acetonitrile, methanol, ethanol, dimethyl sulfoxide, and dimethyl formamide, the excited S₁ states with charge transfer character result from π → π* transitions. However, in nonpolar solvents, cyclohexane, and benzene, the S₁ states are the result of n → π* transitions related to local excitation in the carbonyl group. The excited T₁ states were calculated to have ππ* character in various solvents. From the variation of the calculated excited states, the band due to π → π* transition undergoes a redshift with an increase in solvent polarity, while the band due to n → π* transition undergoes a blueshift with an increase in solvent polarity. In addition, the triplet yields and the photoreactivities of p-ABP in various solvents are discussed.

A lattice gas model was proposed to explore the effect of inert surface impurities on the oscillation in the NO+CO reaction system on Pt(100). It was found that when the fraction of the impurities is small, the (1×1) phase resulting from the surface restructuring can form a connected phase and the system exhibits a global sustained oscillation. With the fraction of the impurities increasing, the (1×1) phase only can form many isolated patches and the spatial coherence between the local oscillators with a random phase relationship lost, and as a result, the sustained oscillation changes into a damped one. When the diffusion rate of adsorbed CO and NO increases, the synchronization between local oscillators is enhanced and the global sustained oscillation can appear again.

The (2, 0) band of the A²Π_u-X²Σ_g⁺ system of N₂⁺ was rotationally studied via optical heterodyne detected velocity modulation spectroscopy. Owing to the high sensitivity of the spectroscopy employed, the frequencies of 310 lines of this band were accurately determined. Moreover, those overlapped lines were also well determined via deconvolution method. A nonlinear least-squares fitting procedure using a standard Hamiltonian was applied to analyze this band. Therefore, the most accurate molecular constants were obtained.

The new half-metals Fe₂ScO₄ and FeSc₂O₄ were designed and their spinel structures were optimized based on the first-principle pseudo-potential method. Their electric and magnetic properties including molecular magnetic moments and electronic structures were calculated and analyzed, and then were compared with those of Fe₃O₄. The calculation showed that Fe₂ScO₄ and FeSc₂O₄ were both new ferromagnetic II B-type half-metals, but Fe₃O₄ was ferrimagnetic. The molecular magnetic moment of Fe₂ScO₄ is about 7.28 μ_B, which is much larger than the 4.0 μ_B of Fe₃O₄ and 3.96 μ_B of Fe₂ScO₄. The molecular magnetic moment of Fe₂ScO₄ mainly came from the spin-polarization of Fe3d electrons. Also, the conductance of Fe₂ScO₄ was a little larger than that of Fe₃O₄. For Fe₂ScO₄, the average electronic structure of Sc on A-sites was Sc⁺3s²3p⁴3d² and that of Fe on B-sites was Fe²⁺t_2g^3↑t_g^2↑t_2g^↓. It can be predicted that the new half-metal Fe₂ScO₄ has wider application ground in spin electronic instruments because of its larger magnetoresistance compared to Fe₃O₄ and FeSc₂O₄.

The benzene conversion and phenol selectivity from C₆H₆/O₂/H₂O over [Ca₂₄Al₂₈O₆₄]⁴⁺·4O⁻(C12A7-O⁻) catalyst were investigated using a flow reactor. The benzene conversion increases with the increase of temperature, and the phenol selectivity mainly depends on both reaction temperature and the composition of the mixtures. The changes of the catalyst structure before and after the reactions and the intermediates on the catalyst surface and in the bulk were investigated by XRD, EPR and FT-IR. The catalytic reactions do not cause any damage to the structure of the positively charged lattice framework C12A7-O⁻, but part of the O⁻ and O₂⁻ species in the bulk of C12A7-O⁻ translate to OH⁻ after the reactions. The neutral species and anion intermediate were investigated by Q-MS and TOF-MS respectively. It is suggested that the active O⁻ and OH⁻ species played a key role in the process of phenol formation.

Cubic SiC (3C-SiC) films were deposited on on-axis 6H-SiC (0001) substrates by low-pressure chemical vapor deposition (LPCVD). The result of X-ray diffraction patterns shows that the 3C-SiC films were of good crystalline quality. The influence of the growth parameters (flow rates of the gas sources and growth temperature) on the growth rate of the SiC films is discussed. The results show that the transport of silane or its reaction products is the limiting factor for the growth. The surface morphology of the SiC films was observed by atomic force microscope imaging. From these results it can be concluded that the growth of the films is in agreement with a Stranski-Krastanov growth mode.

A large quantity of Zinc oxide (ZnO) comb-like structure and high-density well-aligned ZnO nanorod arrays were prepared on silicon substrate via thermal evaporation process without any catalyst. The morphology, growth mechanism, and optical properties of the both structures were investigated using XRD, SEM, TEM and PL. The resulting comb-teeth, with a diameter about 20 nm, growing along the [0001] direction have a well-defined epitaxial relationship with the comb ribbon. The ZnO nanorod arrays have a diameter about 200 nm and length up to several micrometers growing approximately vertical to the Si substrate. A ZnO film was obtained before the nanorods growth. A growth model is proposed for interpreting the growth mechanism of comb-like zigzag-notch nanostructure. Room temperature photoluminescence measurements under excitation wavelength of 325 nm showed that the ZnO comb-like nanostructure has a weak UV emission at around 384 nm and a strong green emission around 491 nm, which correspond to a near band-edge transition and the singly ionized oxygen vacancy, respectively. In contrast, a strong and sharp UV peak and a weak green peak was obtained from the ZnO nanorod arrays.

A series of Co₃O₄-loaded SnO₂ nanocomposite thick films were prepared by grinding, screen-printing and sintering at 700 °C for 3 h. XRD data showed the nanocomposite thick films were rutile structure of SnO₂ and cubic Co₃O₄. The composite films were found to exhibit good response to alcohol and acetone at 300 °C. The film went through a sharp sensitivity maximum at 5 mol%CoO_4/3 with a change in Co₃O₄ content. At 300 °C, the maximum sensor response to alcohol and acetone, each 1000 ppm in air, was 301 and 235, respectively, which was about 7 and 5 times as large as that of the pure SnO₂ respectively. The selectivity to alcohol and acetone over H₂ and CO also was promoted by the addition of Co₃O₄ to SnO₂. The mechanism of such strong promotion of sensor response (electronic sensitization) is discussed.

Core-shell structural Li and Ti co-doped NiO/polystyrene samples (LTNO/PS) were synthesized by a sol-gel method and studied by TEM, X-ray diffraction, IR spectroscopy and LCR meter. It was found that the core-shell LTNO/PS particles are nearly smooth spheres with an average size of around 4.0 μm, while the LTNO particles alone have a size of around 3.5 μm. The electrorheological activity of LTNO particles with PS coating is larger than that of the bare ones with the same components. This research reveals that the high dielectric constant and corresponding dielectric loss of the LTNO is related to the Li and Ti contents. The increase of electrorheological activity of LTNO particles with PS coating is caused by the increase of dielectric permittivity, the surface structural change, and the reduction of leakage current of PS-coated samples due to the high resistivity and soft contacting of PS shell. By the preparation of core/shell structural materials and taking the advantage of the shell to reduce the leakage current between the particles, the electrorheological effect can be effectively increased.

The effects of neutron irradiation with a fluence of 10¹⁵ n/cm² on the superconducting properties of GdBa₂Cu₃O_7−δ single domain sample were studied. The point and cascade defects produced by the neutron irradiation were observed by high-resolution transmission electron microscopy. The cascade defects were found to have the sizes of about 4-7 nm which is comparable to the coherence lengths of high temperature superconductors. The point defects disappear while the cascades still exist through thermal annealing. The temperature dependence of magnetization for the magnetic field parallel to the crystallographic c axis shows that the neutron irradiation leads to a dramatic degradation of superconductivity for the as-irradiated sample, a decrease of critical current density (J_c), and the weakening of the fishtail effect in the J_c versus magnetic induction B curve. However, for the as-irradiated sample annealed in the flowing oxygen atmosphere, it shows that J_c under high magnetic fields is greatly enhanced, the fishtail shifts towards higher magnetic fields, and its superconductivity is partially recovered as well due to the remaining effective pinning centers of the cascades. These results suggest a prospective application for such a treated GdBa₂Cu₃O_7−δ superconductor.