2008 Vol. 21, No. 4

Article
157 nm photodissociation of jet-cooled CH3OH and C2H5OH was studied using the high-n Rydberg atom time-of-flight (TOF) technique. TOF spectra of nascent H atom products were measured. Simulation of these spectra reveals three different atomic H loss processes: one from hydroxyl H elimination, one from methyl (ethyl) H elimination, and one from secondary dissociation of the methoxy (ethoxy) radical. The relative branching ratio indicates secondary dissociation of ethoxy is less important than that of methoxy. The average angular anisotropy parameter of methanol is negative (withβ≈-0.3), indicating the transition dipole moment is perpendicular to the C-O-H plane. The slightly more negative β value of ethanol (with β≈-0.4) implies that ethanol has a longer rotational period. These experimental results indicate that both systems undergo fast internal conversion to the 3s surface after it is excited to the 3px surface, and then dissociate on the 3s surface. The translational energy distribution of the CH3O+H products reveals extensive CH3 rocking or CH3 umbrella excitation in the CH3O radical. However the vibrational structures are not resolved in the C2H5O radical
The laser-induced fluorescence (LIF) excitation spectrum of NiH was recorded in the spectral region from 15000 cm-1 to 21400 cm-1, with the NiH molecules produced by the reaction of sputtered nickel atoms with methanol under supersonic jet conditions. The 19000-21400 cm-1 portion of the spectrum of NiH is reported for the first time. Twenty-four bands were observed and classified into seven electronic transitions. Every band was rotationally analyzed. Higher vibrational levels of many excited states, A, B, D, E, F, and G,were observed and the complete set of spectroscopic parameters, vibrational frequency, unharmonic constant, rotational constant, and equilibrium length of these states were obtained. Some bands were reassigned
The C-H stretch vibrational spectra of the trisiloxane superspreading surfactant Silwet L-77((CH3)3Si-O-Si(CH3)(C3H6)(OCH2CH2)7-8OCH3)-O-Si(CH3)3) at the air/water interface are measured with the surface Sum Frequency Generation Vibrational Spectroscopy (SFG-VS). The spectra are dominated with the features from the –Si-CH3 groups around 2905 cm-1 (symmetric stretch or SS mode) and 2957 cm-1 (mostly the asymmetric stretch or AS mode), and with the weak but apparent contribution from the -O-CH2- groups around 2880 cm-1 (symmetric stretch or SS mode). Comparison of the polarization dependent SFG spectra below and above the critical aggregate or micelle concentration (CAC) indicates that the molecular orientation of the C¡H related molecular groups remained unchanged at different surface densities of the Silwet L-77 surfactant. The SFG-VS adsorption isotherm suggested that there was no sign of Silwet L-77 bilayer structure formation at the air/water interface. The Gibbs adsorption free energy of the Silwet surfactant to the air/water interface is -42.2±0.8kJ/mol, indicating the unusually strong adsorption ability of the Silwet L-77 superspreading surfactant
The interconversion between the two distinct isomers of methyl vinyl ether (MVE), the formation of the primary ozonides from O3-initated reactions of MVE, the transformation between the primary ozonides, and the subsequent fragmentation were studied using quantum chemical methods at the BHandHLYP/6-311++G(d,p) level of theory for optimized geometries and frequency calculations and at the QCISD/6-31G(d,p) level for the single point energy calculations. The rate coe±cients were calculated for the temper-ature range 280-440 K by using the canonical transition state theory (TST). For ozone addition to MVE, there are two different possibilities discussed on the basis of two different possible orientations for ozone attack. The results of the theoretical study indicate that although the synperiplanar-MVE is 7.11 kJ/mol more stable than the antiperiplanar-MVE, the antiperiplanar-MVE plays a more important role in formation of the primary ozonides because the primary ozonides formed from the ozone addition antiperiplanar-MVE are more stable and the energy barriers corresponding to transition states are lower. The interconversion between the primary ozonides formed from the ozone addition to antiperiplanar-MVE is the most accessible compared with the transformations between other primary ozonides. The cleavage of the primary ozonides mainly leads to the formation of the CH2OO, which is in agreement with the experimental estimates. The calculated overall rate constant for the ozone-initiated reactions is 4.8×10-17cm3/(moleculec∙s) at 298.15 K, which agrees with the experimental value for ethyl vinyl ether
The defect formation and annealing behavior in as-grown and electron-irradiated 6H-SiC wafers were investi-gated by variable-energy slow positron beam. For the n-type as-grown samples, it was found that annealing decreased the defect concentration due to recombination with interstitial, and when it was annealed at 1400 ±C for 30 min in vacuum, a 20 nm thick Si layer was found on the top of SiC substrate, which is a direct proof of the Si atom diffusing to the surface when annealed at the high temperature stages. During the high temperature annealing stage, we found an obvious surface effect occurred that induced the higher S parameter close to the surface. This may be caused by the diffusion of the Si atoms to the surface during annealing. After 10 MeV electron irradiation of the n-type 6H-SiC, the positron effective diffusion length decreased from 86.2 nm to 39.1 nm. This shows that there are some defects created in n-type 6H-SiC. But in the p-type 6H-SiC irradiated by 10 MeV electrons, the change is very small. This may be because of the opposite charge of the vacancy defects. The same annealing behavior as that of as-grown 6H-SiC samples was also observed for the 1.8 MeV electron-irradiated 6H-SiC samples except that after being annealed at 300 ±C, its defect concentration increased. This may be explained as the generation of carbon vacancies, due to either the recombination between divacancies and silicon interstitial, or the charge of the charge states.
Effects of noise on rate oscillations during CO oxidation on Pt(110) surface were investigated, both theo-retically and numerically, by focusing on the interplay of internal noise (IN) due to stochasticity in reaction events, and external noise (EN) resulting from parameter perturbation. The surface is divided into cells of variable size which are assumed to be well mixed, and we consider the behavior inside a single cell. At-tention is paid to parameter regions subthreshold of the deterministic Hopf bifurcation, where noise can induce stochastic oscillations, the signal-to-noise ratio (SNR) of which shows a maximum with the variation of noise intensity, known as coherent resonance (CR). By stochastic normal theory, we show that IN and EN contribute in a weighted additive way to an effective noise that lead to CR, such that SNR shows a ridge shape in the D-1/N plane, where D and 1/N measures the strength of EN and IN, respectively. It is shown that for too large IN (EN), CR behavior with EN (IN) no longer exists. Numerical simulations show good agreements with the theoretical results
Herein we give a theoretical study of the hydrolysis processes of a novel anticancer drug trans-[PtCl2(3-pico)(ipa)] (3-pico=3-methylpyridine, ipa=isopropylamine). Two different models, model 1 relative to isolated reactant/product (R/P, wherein R=platinum complex+H2O, P=platinum complex+Cl-) and model 2 relative to reactant complex/product complex (RC/PC, wherein RC=(platinum complex)(H2O),PC=(platinum complex)(Cl-) are employed and the geometric structures are optimized at the B3LYP level of DFT method. It is found that the processes of the reactions follow the established theory for ligand substitution in square planar complexes; the geometries of the transition states (TS) agree with the previous related work and all of the reactions are endothermic. The effects originating from the inclusion of the attacking water/released chloride into the second coordination shell of platinum in RC/PC play an important role in the thermodynamic and kinetic profiles of the reactions, that is, the barrier heights of the reactions of model 2 are increased by ~6.3 and ~23.8 kJ/mol for step1 and step2 respectively, and the endother-micity is considerably decreased by ~420.5 and ~771.2 kJ/mol compared to model 1 in the gas phase. Theconsideration of the bulk solvation effects increase the barrier heights for both steps of model 1 by ~27.6 and ~6.7 kJ/mol respectively, whereas it reduces the barrier heights by ~7.9 and ~29.3 kJ/mol for model 2.The reaction energies are all decreased, especially for model 1, indicating more stable complexes solvated in the bulk aqueous solution than in the gas phase. Additionally, to get an accurate energy picture of the title complex, the relative free energies derived from the DFT-SCRF (density functional theory self-consistent field) calculations are compared with the relative total energies. The results are that activation energies rise for the first hydrolysis and fall for the second hydrolysis for all the systems, and for all the systems,the barrier height of the second hydrolysis is always higher than that of the first step. The rate constants indicate that transplatin analogue is kinetically comparable to cisplatin and its analogue in the hydrolysis process
A three dimensional nano-scale finite element model (FEM), called the chemical bond element model, is proposed for the simulation of mechanical properties of single-walled carbon nanotubes (SWCNTs) based upon molecular mechanics method. Chemical bonds between carbon atoms are modeled by chemical bond elements. The constants of a sub-stiffness matrix are determined by using a linkage between molecular mechanics and continuum mechanics. In order to evaluate the correctness and performance of the proposed model, simulation was done to determine the influence of nanotube wall thickness, radius and length on the elastic modulus (Young's modulus and shear modulus) of SWCNTs. The simulation results show that the choice of wall thickness significantly affects the Young's modulus and shear modulus. The force field constants is also very important, because the elastic modulus is sensitive to force field constants and the elastic properties of SWCNT are related to the radii of the tubes. The contribution of length to elastic modulus is insignificant and can be ignored. In comparison with the Young's modulus and shear modulus reported in the literature, the presented results agree very well with the corresponding theoretical results and many experimental measurements. Furthermore, if the force constants are properly chosen, the present method could be conveniently used to predict the mechanical behavior of other single-walled nanotubes such as boron nitride nanotubes. The results demonstrate the value of the proposed model as a valuable tool in the study of mechanical behaviors of carbon nanotubes and in the analysis of nanotube-based equipments.
On the basis of a general model of fuel cells, the entropy production rates of a fuel cell system under different conditions are derived by using theories of electrochemistry and thermodynamics. In order to analyze the influence of the irreversible losses existing in an actual fuel cell, the equivalent circuit of the fuel cell is introduced, so that the irreversible factor of the fuel cell may be determined directly as a function of the internal, leak and load resistances. Moreover, the maximum power output and efficiency of the fuel cell are calculated, the optimal operation of the fuel cell is discussed, and the matching condition of the load resistance is determined.
Density functional theory (DFT) was applied to study the ground state geometries and isomerization processes of 1,1'-binaphthalene-8,8'-diol. Three isomers, denoted as ISO1, ISO2, and ISO3, were found, distinguished by different orientations of the OH groups, and each OH-orientational isomer has R- and S- enantiomer. The conformational stabilities of these isomers were investigated by tracking the energy change with respect to the ring-to-ring torsion. The inter-conversions between the three OH-orientational S-isomers were found to have quite low barriers owing to the nearly free rotation of OH groups around the O-C single bonds. The S-R enantiomerization of ISO1 and ISO2 can take place through the ring-ring torsion around the C1-C10 single bond, either in the anti-rotation manner or in the syn-rotation manner. The barriers of the anti routes are lower than those of the corresponding syn routes by 87.95 and 75.04 kJ/mol. For the S-R enantiomerization of ISO3, only the anti route was found. The barriers for the anti route enantiomerizations of ISO1, ISO2, and ISO3 are 119.61, 120.43, and 121.59 kJ/mol, respectively. A parallel reaction mechanism via three anti enantiomerization routes was proposed for the racemization of 1,1'-binaphthalene-8,8'-diol.
By using metal nitrates as starting materials and citric acid as a complexing agent, Y2Si2O7:Re3+ (Re=Eu, Tb) phosphors were prepared by a sol-gel method. X-ray diffraction was employed to characterize the resulting samples. The results of XRD indicate that the ff-Y2Si2O7 nanocrystal with size of 27 nm is obtained at 1000 oC and the doping ion content does not influence the structure. The excitation spectra in the UV and VUV ranges and the emission spectra of Re3+ doped samples were measured. The excitation spectra in the VUV range is due to absorption of host, that in the UV range is ascribed to absorption transitions from 4f to 5d state of the Tb3+ and the charge transfer in the Eu3+-O2- bond. The spectral energy distribution of the Tb3+ emission depends strongly on the Tb3+ concentration. The dependence of photoluminescence intensity on Re3+ concentration is also discussed in detail. The fluorescent decay curves at room temperature were measured and analyzed.
Ferrites having general formula Ni1-xZnxFe2O4 with x=0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7 were prepared by wet chemical co-precipitation method. The structural and magnetic properties were studied by means of X-ray diffraction, magnetization, and AC susceptibility measurements. The X-ray analysis confirmed the single-phase formation of the samples. The lattice parameter obtained from XRD data was found to increase with Zn content x. The cation distribution was studied by X-ray intensity ratio calculations. Magnetization results exhibit collinear ferrimagnetic structure for x<0.4, and which changes to non-collinear for x>0.4. Curie temperature TC obtained from AC susceptibility data decreases with increasing x.
Inclusion complexes between β-cyclodextrin (β-CD) and a series of dicarboxylic acids (DAn, n=11-15) were prepared by co-grinding and co-precipitation methods and the [3]pseudorotaxane structure of them was eluci-dated by FTIR, DTA and XRD characterizations. Inclusion complexes of β-CD and α,w-alkanedicarboxylate anions (DAn2-) were acquired by neutralizing β-CD/DAn different inclusion complexes with sodium hydrox-ide and the structure was also proved to be a pseudorotaxane structure by 1H-NMR spectra and NOESY spectrum. Both the inclusion complexes of β-CD/DAn and β-CD/DAn2- adopt the [3]pseudorotaxane structure with β-CD arranged in dimers threaded onto one aliphatic chain and the binding mode of 1:1 inclusion complex was excluded based on the consideration of chain conformations
Copper iron composite oxides (CuO/Fe2O3) and copper cobalt composite oxides (CuO/Co3O4) for the catalytic reduction of NO with CO at low temperature were prepared by co-precipitation. The catalytic activity and thermal stability of the catalysts were evaluated by a microreactor-GC system. The 100% conversion temperatures of NO are 80 oC for CuO/Fe2O3 and 90 oC for CuO/Co3O4. The catalysts possess high catalytic activity and favorable thermal stability for NO reduction with CO in a wide temperature range and long time range. A systematic study of the molar ratios of the reactants, the volume of NaOH, aging time, and calcination temperature/time was carried out to investigate the influence preparation conditions on the catalytic activity of the catalysts.
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