2007 Vol. 20, No. 2

Article
Direct dynamics within the framework of DFT was used to study the long-time puzzling mechanism of the reaction between F2 and ethylene. Three types of reactions are widely accepted : F atom elimination reaction, HF elimination reaction and the addition reaction. Several reaction mechanisms have been proposed, but only the radical mechanism can reasonably explain the initial reaction at low temperature. In this article, our calculations support the radical mechanism and the reaction mechanisms of the three reactions, and they are described in detail by trajectory simulation. The reactions in a cryogenic matrix with the reaction mechanism were also discussed.
Rg·NO (Rg=He, Ne, Ar and Kr) complexes were studied using ab initio calculations. The neutral Rg·NO complex geometry and vibrational frequencies were calculated with the cc-pVDZ basis set at the CCSD(T) level of theory. The calculations show that the geometry of the Rg·NO complexes is a skewed T-shape with the Rg atom on the oxygen side of the NO molecule, and that the Rg{N{O bond angle increases with mass. The dissociation energies (DE) and ionization energies (IE) of the neutral Rg·NO complexes, and the dissociation energies of Rg·NO+ ionic complexes were calculated using Gaussian-2 (G2) methods and a high accuracy energy model. The ionization energies of the neutral Rg·NO complexes range from 9.265 eV for He·NO to 9.132 eV for Kr·NO and the dissociation energies of Rg·NO+ range from 0.017 eV for He·NO+ to 0.156 eV for Kr·NO+, in line with the expectation based on the increasing polarizability of the Rg atom.
Based on a deterministic mammalian circadian oscillator proposed recently, we have constructed the corresponding mesoscopic stochastic model, and studied the effect of internal noise on the genetic oscillations of such a system. It is found that the stochastic genetic oscillations can show best performance at an optimal internal noise level via a mechanism of internal noise stochastic resonance. Furthermore, it is found that there exists a moderate system size that makes the stochastic model show effective oscillation at more extended region than the deterministic description, which indicates enhanced robustness as the result of internal noise. The potential biological application of such an effect is also discussed.
The complex potential energy surface and reaction mechanisms for the unimolecular isomerization and decomposition of methyl-nitramine (CH3NHNO2) were theoretically probed at the QCISD(T)/6-311+G*//B3LYP/6-311+G* level of theory. The results demonstrated that there are four low-lying energy channels: (i) the N{N bond fission pathway; (ii) a sequence of isomerization reactions via CH3NN(OH)O; (IS2a); (iii) the HONO elimination pathway; (iv) the isomerization and the dissociation reactions via CH3NHONO (IS3). The rate constants of each initial step (rate-determining step) for these channels were calculated using the canonical transition state theory. The Arrhenius expressions of the channels over the temperature range 298-2000 K are k6(T)=1014:8e-46:0=RT , k7(T)=1013:7e-42:1=RT , k8(T)=1013:6e-51:8=RT and k9(T)=1015:6e-54:3=RT s-1, respectively. The calculated overall rate constants is 6.9£10-4 at 543 K, which is in good agreement with the experimental data. Based on the analysis of the rate constants, the dominant pathway is the isomerization reaction to form CH3NN(OH)O at low temperatures, while the N{N bond fission and the isomerization reaction to produce CH3NHONO are expected to be competitive with the isomerization reaction to form CH3NN(OH)O at high temperatures.
Comparative molecular field analysis (CoMFA) method was applied to study three-dimensional quantitative structure activity relationship (3D-QSAR) of a series of benzothiazole derivatives as potent anticancer agents. The CoMFA model of cross-validation and the partial-least-square (PLS) model of non cross-validation have been well established. The best CoMFA model gives a good cross-validation coe±cient of 0.642 and a conventional correlation coe±cient of 0.976. Moreover, the estimated standard error is 0.161 and the statistical square deviation ratio F(3;20) is 111.4. The statistical parameters of the best CoMFA model show this model is reasonable and has predictive ability. The CoMFA results suggest that an electron-withdrawing group or atom (e.g. F atom) linking to the first atom (C19) of substituent R can increase the positive charges of C19 and its fi-site atoms, which lie in the blue-colored regions in the electrostatic field contour map of CoMFA, and thus can improve the activity of the compound. Meanwhile, selecting an R with an appropriatevolume is also advantageous for improving the activity.
Evaporation of sessile water-droplets on superhydrophobic polymer surfaces has been simulated in recent research. Models based on the ellipsoidal cap geometry and spherical cap geometry, which were originally put forward to describe the profile of a droplet during its evaporation process on a solid surface with a contact angle <90±, are developed to reveal the issue with an initial contact angles larger than 150±. To verify the validity of the model, experiments on superhydrophobic polycarbonate, and °uorinated polyurethane and poly (methyl methacrylate) blend surfaces were carried out. It was observed that the change trends of contact angle and height of the droplet against evaporation time on the superhydrophobic surfaces experimentally are consistent with the simulated results by ellipsoidal and spherical cap models. The ellipsoidal cap model shows the better fits due to the shape distortions of droplets.
The optical transition probability of Tm3+ (7.0 mol%) doped PLZT was investigated because of its great interest in many fields such as optical communications in the mid-infrared waveband and medical instrumentation. The absolute intensities of its forced electric dipole transitions between 420 and 2000 nm were measured. According to Judd-Ofelt (J-O) theory, three phenomenological parameters, ?2=9.133£10-21 cm2, ?4=1.529£10-21 cm2 and ?6=1.712£10-21 cm2, were obtained. The J-O intensity parameters were used to calculate the radiative lifetime (7.493 ms) of the excited 3F4 level. The stimulated emission cross-section for the 3F4!3H6 transition was also evaluated. Analysis reveals that Tm3+-doped PLZT is promising for use as e±cient optical amplification devices or zero-loss electro-optical devices.
Based on the comprehension of the specific structural features affecting the refractive indices of the compounds, two descriptors PX1CC and PX1CH extracted from the bond orbital-connection matrix (BOCM) method were employed to develop a QSPR model for predicting the refractive indices of alkanes, chloroalkanes and bromoalkanes. The obtained results confirmed the usefulness of the BOCM method. PX1CC and PX1CH re°ect the ability of the electronic cloud of the alkanes to be polarized; such ability is correlated with the refractive indices of substances. Therefore, the physical meaning of the obtained model can be rationally interpreted from the physical point of view. The present descriptors obtained by the BOCM method have the merit of topological indices (i.e. facility and rapid calculation of the descriptors) and the advantage of quantum descriptors (i.e. explicitly physical meaning of the parameters), which lead to an expectation of wide use in QSAR studies.
The semi-empirical AM1 and INDO/CIS methods as well as density function theory were used to study equilibrium geometries and spectroscopic properties of the possible isomers of C78O5 based on C2v-C78. The most stable geometry of C78O5 is 28,29,30,31,52,53,70,71,73,78-C78O5(A) with one annulene-like structure and four epoxide structures. Compared with that of C2v-C78, the blue-shift in the electronic absorption spectra of C78O5 isomers is predicted. The reason for the blue-shift effect is discussed and the electronic transitions are assigned. The IR and NMR spectra of C78O5 are explored with the AM1 and B3LYP/6-31G methods based on the B3LYP/6-31G optimized geometries.
The equation of state of ZnO with rocksalt phase under high pressure and high temperature was calculated by using the molecular dynamics method with effective pair potentials which consist of the Coulomb, dispersion, and repulsion interaction. It was shown that molecular dynamics simulation is very successful in accurately reproducing the measured molar volumes of the rocksalt phase of ZnO over a wide range of temperatures and pressures. The simulated P-V -T data matched experimental results up to 10.4 GPa and 1273 K. In addition, the linear thermal expansion coe±cient, isothermal bulk modulus and its pressure derivative were also calculated and compared with available experimental data and the latest theoretical results at ambient condition. At extended temperature and pressure ranges, the P-V -T relationship, linear thermal expansion coe±cient, and isothermal bulk modulus were predicted up to 2273 K and 50 GPa. The detailed knowledge of thermodynamic behavior and equations of state at extreme conditions are of fundamental importance to the understanding of the physical properties of ZnO.
The structure-activity relationship of °uoroquinolones, which show anti-K. pneumoniae activity, was studied by using principal component analysis (PCA) and hierarchical cluster analysis (HCA). The PCA results showed that the lowest unoccupied molecular orbital energy, energy difference between the highest occupied and the lowest unoccupied molecular orbital, dipole moment, net atomic charge on atom I, molecular polarizability, partition coe±cient and molecular refractivity of these compounds are responsible for the separation between high-activity and low-activity groups. The HCA results were similar to those obtained with PCA. By using the chemometric results, four synthetic compounds were analyzed through PCA and HCA, and three of them are proposed as active molecules against K. pneumoniae which is consistent with the results of clinical experiments. The methodologies of PCA and HCA provide a reliable rule for classifying new °uoroquinolones with anti-K. pneumoniae activity.
In efforts to develop a new fabrication method for improvement of the MREs' performance, the bound-rubber phenomenon was observed in MREs. Further experiments indicate the existance of bound-rubber and it in°uences the MRE performance as well as the particle size. Both theoretical analysis and experimental results indicated that MRE performance can be improved by enhancing the ratio of particle radius to bound-rubber thickness.
Thermodynamic studies were carried out for the vapor complex of sodium chloride with hafnium tetrachloride at 712-778 K and 0.5-3.1 kPa by using high temperature phase equilibrium-quenching experiments, with closed Pyrex glass ampoules as the reaction containers. The results show that the sole predominant vapor complex is Na2HfCl6 for the HfCl4-NaCl system under the experimental conditions. The thermodynamic equilibrium constants and other thermodynamic functions of the reaction 2NaCl(s)+HfCl4(g)=Na2HfCl6(g) have been derived from the measurements. The results for the changes in enthalpy and entropy are -65.5§1.5 kJ/mol and -99.6§2.0 J/(mol K) in the temperature range.
Pure polystyrene (PS) and PS composites containing magnesium hydroxide (Mg(OH)2) were burned in alaboratory-scale combustion chamber. The analyses of combustion products were carried out by gas chromatography and gas chromatography/mass spectrometry. The results indicate that the effects of Mg(OH)2 on combustion products of PS are obvious. With the increase of Mg(OH)2, the concentration of CO2 gradually reduces, and the majority of the volatile and semivolatile organic products, especially styrene monomer and some polycyclic compounds, take on increasing tendencies. Furthermore, the content of coke in residue also obviously increases because of the enhancement of Mg(OH)2. All the changes seem to imply that the presence of Mg(OH)2 alters reaction pathways during the combustion of PS, promotes incomplete combustion and decreases combustibility of PS.
X-ray diffraction (XRD) was used to investigate the microstructure and phase separation of mixed multilayers of cadmium stearate (SA) and behenate (BA) deposited onto hydrophilic glass by the Langmuir-Blodgett technique. No unitary fatty acid diffraction peaks in the XRD spectra of the mixed LB films, which reveals that domains in these samples are small and uniform. The interplanar spacing of the mixed LB films changes with the ratio of BA to SA in a step-shaped curve, which suggests that with the changing of the ratio between BA and SA, only three kinds of ordered structure form in the mixed system, and each microstructure, in particular, the short chains against long chains meshed microstructure can be maintained in a certain ratio range . In addition, for the meshed microstructure, the alkyl chains of both SA and BA stand straight or may have exactly the same small tilted angle from the substrate normal, as is indicated by the symmetric and asymmetric stretching vibration of methylene (CH2) peaks which are at 2847.80 and 2914.37 cm-1respectively in the Fourier transform infrared (FT-IR) transmission spectra. The mixed system always goes through a longitudinal regularity decreasing process when BA/SA is out of the intermediate ratio range (BA/SA: 1/5-1/1), which is illustrated by the relative XRD intensity changing with the ratio of BA/ SA in a \W" shape. These results for the mixed LB films of BA/SA provide meaningful data for choosing the mixture ratio when fabricating composite films with special structure.
A series of polyimide/SnO2 hybrid membranes supported on TiO2/kieselguhr-mullite were prepared from polyimide with a large amount of carboxyl and SnO2 sol via a sol-gel process. The SnO2 phase chemically linked with the polyimide through the pendant carboxyl along the polyimide. The hybrid membranes were highly homogeneous, and when the SnO2 contents reached 15wt% the SnO2 phase was observed as particles with a diameter of 5 nm dispersed in the hybrid membranes . The cross-linking between the SnO2 phase and polyimide effectively enhanced the glass temperature of the hybrid films. With the increasing of the SnO2 contents, the pore sizes of the membranes decreased, and their pore sizes were mainly focused on 3.8, 3.1, 2.8 and 2.4 nm. The hybrid membranes showed higher permeability for H2, CO2, CO and H2O when compared to the pure polyimide. The separation factors of the polyimide/SnO2 hybrid membranes with 15wt% SnO2 content for H2/N2, CO2/N2, CO/N2 and H2O/N2 were 54.1, 30.2, 35.9 and 40.1, respectively.
A convenient approach is reported for the synthesis of spherical maghemite (°-Fe2O3) nanoparticles. The process was realized by the controlled oxidation of Fe3O4 precursor, which originated from a facile partialreduction co-precipitation process. The starting material of hydrosulfurous sodium (Na2S2O4), which can allow reaction to proceed without any deoxygenated protection, was proven to be important in the formation of the precursor. A series of techniques, including X-ray diffraction, transmission electron microscopy and a vibrating sample magnetometer were used to characterize the product. The resultant °-Fe2O3 nanoparticles exhibited ferromagnetism at 300 K and the values of saturation magnetization and coercivity were 70 emu/g and 164 Oe, respectively. The electrochemical properties of lithium ions intercalation into °-Fe2O3 nanoparticles were tested in Te°on cells. A specific capacity of 933 mAh/g was delivered at a current density of 0.2 mA/cm2 (voltage range 3.0-0.3 V vs. Li), corresponding to the reaction of 5.7 Li+ per Fe2O3. A possible mechanism of the reaction of lithium with maghemite spinel was discussed.
Vertically aligned carbon nanotubes (CNTs) were synthesized on Fe-deposited silicon substrates using chemical vapor deposition. Scanning electron microscope investigations reveal that the morphology of the CNTs depends on several growth parameters including the reaction temperature, the size of catalyst nanoparticles, and the partial pressure of the reaction gas. When the reaction temperature rises or the concentration of carbon source gas increases, the diameter of CNTs gets larger, but the length becomes shorter. With decreasing thickness of the catalyst film, the diameter of the CNTs shrinks monotonically, but the length of the CNTs increases first, reaches a maximum and then decreases afterwards. These results indicate that the diameter and the length of the vertically aligned CNTs can be manipulated by selecting appropriate growth parameters.
A high-density well-aligned Zinc Oxide nanorod array was synthesized on Si (100) substrate by a simplevapor deposition under normal pressure using neither a catalyst and nor pre-deposition of ZnO film. Various different morphologies were obtained in different deposition regions. Si substrate put over the Zn source was the key factor in getting a well-aligned sample. Field emission scanning electron microscope (FESEM) observations and X-ray diffraction were carried out to characterize the surface morphology and crystalline quality of the samples. The growth mechanism is discussed. The photoluminescence properties of the ZnO samples were also investigated. It is suggested that the green band is related to oxygen vacancies and thekinetic process involving transition from shallow donor to deep acceptor level.