2013 Vol. 26, No. 1

Content
2013, 26(1): 0-0. doi: 10.1088/1674-0068/26/1/0-0
Article
The kinetics and dynamics of photocatalyzed dissociation of ethanol on TiO2(110) sur-face have been studied using the time-dependent and time-resolved femtosecond two-photon photoemission spectroscopy respectively, in order to unravel the photochemical properties of ethanol on this prototypical metal oxide surface. By monitoring the time evolution of the photoinduced excited state which is associated with the photocatalyzed dissociation of ethanol on Ti5c sites of TiO2(110), the fractal-like kinetics of this surface photocatalytic reaction has been obtained. The measured photocatalytic dissociation rate on reduced TiO2(110) is faster than that on the oxidized surface. This is attributed to the larger defect density on the reduced surface which lowers the reaction barrier of the photocatalytic reaction at least methodologically. Possible reasons associated with the defect electrons for the acceleration have been discussed. By performing the interferometric two-pulse corre-lation on ethanol/TiO2(110) interface, the ultrafast electron dynamics of the excited state has been measured. The analyzed lifetime (24 fs) of the excited state is similar to that on methanol/TiO2(110). The appearance of the excited state provides a channel to mediate the electron transfer between the TiO2 substrate and its environment. Therefore studying its ultrafast electron dynamics may lead to the understanding of the microscopic mechanism of photocatalysis and photoelectrochemical energy conversion on TiO2.
Preparation of a high flux of hydrogen molecules in a specific vibrationally excited state is the major prerequisite and challenge in scattering experiments that use vibrationally excited hydrogen molecules as the target. The widely used scheme of stimulated Raman pumping suffers from coherent population return which severely limits the excitation efficiency. Re-cently we successfully transferred D2 molecules in the molecular beam from (v=0, J=0) to (v=1, J=0) level, with the scheme of Stark-induced adiabatic Raman passage. As high as 75% of the excitation efficiency was achieved. This excitation technique promise to be a unique tool for crossed beam and beam-surface scattering experiments which aim to reveal the role of vibrational excitation of hydrogen molecules in the chemical reaction.
Accurately known energy level structure of the Cs2 A1+u -b3Πu complex of states from a recent global de-perturbation of these states has enabled additional assignments of 140 per-turbation facilitated infrared-infrared double resonance (PFIIDR) transitions to the 231g state from collisionally populated intermediate A1+u levels. Together with the 221 previ-ously observed 231g←A1+u← X1+g double resonance lines [J. Chem. Phys. 128, 204313 (2008)], molecular constants and the Rydberg-Klein-Rees potential energy curve of the 231g state have been recalculated (excluding 54 perturbed levels). The centrifugal distortion con-stant has been determined and agrees well with the value calculated based on standard empirical formulas. The hyperfine structure of the 231g state, which has not resolved in our sub-Doppler excitation spectra of the 231g state, has been interpreted with a preliminary simulation.
Vacuum ultraviolet (VUV) photoionization and dissociative photoionization of capecitabine and its metabolites, 5'-deoxy-5-fluorocytidine (5'-DFCR) and 50-deoxy-5- fluorouridine (5'-DFUR), were investigated with infrared laser desorption/tunable synchrotron VUV pho-toionization mass spectrometry. Molecular ions (M+) with small amounts of fragments can be found for these compounds at relatively low photon energies, while more fragment ions would be produced by increasing the photon energies. (M-H2O)+, (base+H)+, (base+2H)+,(base+30)+, (base+60)+, and sugar moiety were proposed for these nucleoside drugs with similar backbones. Decomposition channels for the major fragments were discussed in de-tail. Moreover, ab initio calculations were introduced to study the dehydration pathways of three fluoro-nucleosides. Corresponding appearance energies for the (M-H2O)+ ions were computed.
Cell membranes play a crucial role in many biological functions of cells. A small change in the composition of cell membranes can strongly influence the functions of membrane-associated proteins, such as ion and water channels, and thus mediate the chemical andphysical balance in cells. Such composition change could originate from the introduction of short-chain alcohols, or other anesthetics into membranes. In this work, we have applied sum frequency generation vibrational spectroscopy (SFG-VS), supplemented by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), to investigate interaction between methanol and 1,2-dimyristoyl-d54-sn-glycero-3-phosphocholine (d54-DMPC) lipid bilayers. Lipid's hydrocarbon interior is deuterated while its head group is hydrogenated. At the same time, CH3 symmetric stretch from methanol and lipid head amine group has different frequency, thus we can distinguish the behaviors of methanol, lipid head amine group, and lipid hydrocarbon interior. Based on the spectral feature of the bending mode of the water molecules replaced by methanol, we determined that the methanol molecules are intercalated into the region between amine and phosphate groups at the lipid hydrophilic head. The dipole of CH3 groups of methanol and lipid head, and the water O-H all adopt the same orientation directions. The introduction of methanol into the lipid hydrophilic head group can strongly perturb the entire length of the alkyl chains, resulting that the signals of CD3 and CD3 groups from both leaflets can not cancel each other.
We study the geometries, stabilities, electronic and magnetic properties of (MgO)n (n=2-10) clusters doped with a single Mn atom using the density functional theory with the gener-alized gradient approximation. The optimized geometries show that the impurity Mn atom prefers to replace the Mg atom which has low coordination number in all the lowest-energy MnMgn-1On (n=2-10) structures. The stability analysis clearly represents that the aver-age binding energies of the doped clusters are larger than those of the corresponding pure (MgO)n clusters. Maximum peaks of the second order energy differences are observed for MnMgn-1On clusters at n=6, 9, implying that these clusters exhibit higher stability than their neighboring clusters. In addition, all the Mn-doped Mg clusters exhibit high total magnetic moments with the exception of MnMgO2 which has 3.00 μB. Their magnetic be-havior is attributed to the impurity Mn atom, the charge transfer modes, and the size of MnMgn-1On clusters.
The H2Ge=Ge:, as well as and its derivatives (X2Ge=Ge:, X=H, Me, F, Cl, Br, Ph, Ar, : : :) is a kind of new species. Its cycloaddition reactions is a new area for the study of germy-lene chemistry. The mechanism of the cycloaddition reaction between singlet Me2Ge=Ge: and acetaldehyde was investigated with the B3LYP/6-31G* method in this work. From the potential energy profile, it could be predicted that the reaction has one dominant re-action pathway. The reaction rule is that the two reactants firstly form a four-membered Ge-heterocyclic ring germylene through the [2+2] cycloaddition reaction. Because of the 4p unoccupied orbital of Ge: atom in the four-membered Ge-heterocyclic ring germylene and the π orbital of acetaldehyde forming a π→p donor-acceptor bond, the four-membered Ge-heterocyclic ring germylene further combines with acetaldehyde to form an intermedi-ate. Because the Ge atom in intermediate happens sp3 hybridization after transition state, then, intermediate isomerizes to a spiro-Ge-heterocyclic ring compound via a transition state. The research result indicates the laws of cycloaddition reaction between Me2Ge=Ge: and ac-etaldehyde, and lays the theory foundation of the cycloaddition reaction between H2Ge=Ge: and its derivatives (X2Ge=Ge:, X=H, Me, F, Cl, Br, Ph, Ar, : : :) and asymmetric π-bonded compounds, which are significant for the synthesis of small-ring and spiro-Ge-heterocyclic ring compounds.
Potential energy scan for uranium oxide (UO) was performed by ab initio configuration inter-action (CI) method and density functional theory methods at the PBE1 and the B3LYP levels in combination with the (ECP80MWB_AVQZ+2f) basis set for uranium and 6-311+G* foroxygen. The dissociation energies of UO, after being corrected for the zero-point vibrational energy, are 2.38, 3.76, and 3.31 eV at the CI, PBE1, and B3LYP levels, respectively. The calculated energy was fitted to potential functions of Morse, Lennard-Jones, and Rydberg. Only the Morse function is eligible for the potential. The anharmonicity constant is 0.00425. The anharmonic frequency is 540.95 cm-1 deduced from the PBE1 results. Thermodynamic properties of entropy and heat capacity at 298.2-1500 K were calculated using DFT-UPBE1 results and Morse parameters. The relationship between entropy and temperature was es-tablished.
The enamines, iminium ions, and oxazolidinones are thought to be the key intermediates in the proline-catalyzed reactions of aldehydes or ketones, but there is an extensive contro-versy about their roles. Here, the corresponding transition states connecting any two of the three kinds of species are located at the ωB97XD/6-311++G** level of theory. The calcula-tions demonstrate that the oxazolidinones are the predominant species in both the gas phase and solvents; there exists tautomeric equilibrium among these species and the equilibriums are controlled by the employed solvents and temperature in the reaction. These results demonstrate that the concentration and role of the mentioned species are controlled by the employed solvent and temperature. A new reaction pathway is presented herein for the trans-formation between iminium ions and oxazolidinones through iminium ion-water complex and oxazolidinone-water complex. The calculations demonstrate that the rate-limiting step in proline-catalyzed Mannich reaction between acetaldehyde/keteones and N-Boc imines is the formation of the C-C bond rather than the intermediates tautomerization. These calcula-tions rationalize the available experimental observations and can be valuable in optimizing the experimental conditions of asymmetric organic-catalyzed reactions of aldehydes or ke-tones.
The photo-physical properties of oligo(fluorene-vinylene) functionalized anthracene linear oligomers (An-OFVn (n=1-4)) have been systemically investigated through experimental and theoretical methods. The steady-state spectral measurement shows that the increas-ing of fluorene-vinylene (FV) group could lead to the red shift of absorption spectra and restrain the excimer formation between oligomers. Quantum chemical calculations exhibit that the energy levels of HOMO, LUMO, and the band gap gradually converge to a con-stant in accompany with the increasing of FV unit. Meanwhile, the electronic cloud which distributes on the branch arms, also gradually enhances and makes the absorption spectral shape of oligomers become similar to that of branch arms step by step. The time-resolved fluorescence tests exhibits that the lifetime of excimer emission would be almost invariable after the number of FV group in oligomer is ≥2. In nonlinear optical test, the two-photon photoluminescence efficiency and two-photon absorption cross-section will both gradually enhance and be close to an extremum after the number of FV unit is equal to 4. These results will provide a guideline for the design of novel photo-electronic materials.
All-atom molecular simulations and two-dimensional nuclear overhauser effect spectrum have been used to study the conformations of carnosine in aqueous solution. Intramolecular distances, root-mean-square deviation, radius of gyration, and solvent-accessible surface are used to characterize the properties of the carnosine. Carnosine can shift between extended and folded states, but exists mostly in extended state in water. Its preference for extension in pure water has been proven by the 2D nuclear magnetic resonance (NMR) experiment. The NMR experimental results are consistent with the molecular dynamics simulations.
Quantum dynamics for the D+OD+ reaction at the collision energy range of 0.0-1.0 eV was studied on an accurate ab initio potential energy surface. Both of the endothermic abstrac-tion (D+OD+→O++D2) and thermoneutral exchange (D+OD+→D+OD+) channels were investigated from the same set of time-dependent quantum wave packets method under cen-trifugal sudden approximation. The reaction probability dependence with collision energy, the integral cross sections, and the thermal rate constant of the both channels are calculated. It is found that there is a convex structure in the reaction path of the exchange reaction. The calculated time evolution of the wave packet distribution at J=0 clearly indicates that the convex structure significantly influences the dynamics of the exchange and abstraction channels of title reaction.
Dimethyl ether carbonylation to methyl acetate was comparatively investigated over mor-denite supported copper (Cu/HMOR) catalysts prepared by different methods including evaporation, urea hydrolysis, incipient wetness impregnation and ion-exchange. The resultsshowed that Cu/HMOR prepared via iron-exchange method exhibited the highest catalytic activity due to the synergistic effect of active-site metal and acidic molecular sieve support. Conversion of 95.3% and methyl acetate selectivity of 94.9% were achieved under conditions of 210 oC, 1.5 MPa, and GSHV of 4883 h-1. The catalysts were characterized by nitrogen absorption, X-ray diffraction, NH3 temperature program desorption, and CO temperature program desorption techniques. It was found that Cu/HMOR prepared by ion-exchange method possessed high surface area, moderate strong acid centers, and CO adsorption cen-ters, which improved catalytic performance for the reaction of CO insertion to dimethyl ether.
The Eu3+-doped La2Zr2O7 phosphor with rod-like morphology was successfully synthesized by conventional solid state reaction and hydrothermal method. X-ray diffraction patterns, transmission electron microscopy, and photoluminescence spectra were employed to charac-terize its structure and morphology as well as luminescent properties. The results indicated that the red-emitting phosphor La2Zr2O7:Eu3+ had well crystallized and belonged to the cubic structure with space group of Fd3m. The as-obtained product mainly appeared as straight nanorods with an average diameter of 47 nm and length of 50-700 nm. The pos-sible growth mechanism was also discussed. It was found that under blue excitation with a wavelength of 466 nm, the La2Zr2O7:Eu3+ phosphor exhibited a characteristic red emission at 616 nm that was attributed to the hypersensitive 5DO7F2 electric dipole transition of Eu3+ ions. Meanwhile, it was more interesting to note that the emission of 5D17FJ (J=0, 1, 2) transitions and the splitting patterns of 5D07FJ (J=1, 2, 4) transitions of Eu3+ ions can be observed in the luminescent spectra of La2Zr2O7:Eu3+. It was demonstrated that Eu3+ preferred to occupy a low symmetry site.
The electrochemical synthesis silicon wires by electrorefining metallurgical grade silicon in thermally dried and pre-electrolyzed molten KF-NaF eutectic were studied at temperatures 800-900 oC using cyclic voltammetry and ac impedance. One oxidation peak at -0.14 V could be attributed to the reaction of Si to Si4+. A cathodic peak occurred at -0.56 V in the cyclic voltammogram and one response semicircle in the ac impedance spectrum was observed, supporting a one-step electrochemical reduction process of Si4+→Si. The electrochemical reaction of silicon was controlled by the diffusion process. The purity of electrorefined silicon wires was up to 99.999% by ICP-MS analysis.
A polymerized lyotropic liquid crystal monomer of sodium 3,4,5-tris(11-acryloxyundecyloxy)-benzoate was synthesized by a convenient route starting from 3,4,5-trihydroxybenzoic acid via esterification followed by etherification, acylation and finally neutralization. The chemi-cal structure was confirmed by Fourier transform infrared (FT-IR) and 1H nuclear magnetic resonance spectral analysis. The self-organization behavior of the monomer with deionized water in methanol at room temperature was also demonstrated. The assemblies were char-acterized by polarized optical microscope and X-ray diffraction. The results show that a solution containing 80:20 of the monomer to water was found to be able to self-organize into Lamellar (La) phase and 92:8 with inverted hexagonal (HII) phase, which was in ac-cordance with the theoretical calculation of critical packing parameter. It suggests that the concentration of the monomer was the key factor to influence assembly structure. Addi-tionally, the acrylate conversion with different photoinitiators and nanostructure retention after polymerization were investigated. The research shows that the acrylate conversion of the monomer with Darocur2959 could reach up to 78% when irradiated by 30 mW/cm2 UV light of 365 nm for 30 min characterized by Real-time FT-IR as well as the sol-gel method. Meanwhile, the La and HII phase nanostructures were both retained after polymerization.
Currently, a conventional two-step method has been used to generate black silicon (BS) surfaces on silicon substrates for solar cell manufacturing. However, the performances of the solar cell made with such surface generation method are poor, because of the high surface recombination caused by deep etching in the conventional surface generation method for BS. In this work, a modified wet chemical etching solution with additives was developed. A homogeneous BS layer with random porous structure was obtained from the modified solution in only one step at room temperature. The BS layer had low reflectivity and shallow etching depth. The additive in the etch solution performs the function of pH-modulation. After 16-min etching, the etching depth in the samples was approximately 200 nm, and the spectrum-weighted-reflectivity in the range from 300 nm to 1200 nm was below 5%. BS solar cells were fabricated in the production line. The decreased etching depth can improve the electrical performance of solar cells because of the decrease in surface recombination. An efficiency of 15.63% for the modified etching BS solar cells was achieved on a large area, p-type single crystalline silicon substrate with a 624.32-mV open circuit voltage and a 77.88% fill factor.
A new kind of multiple metal (Cu, Mg, Ce) doped Ni based mixed oxide catalyst, synthesized by the co-precipitation method, was used for efficient production of hydrogen from bio-oil reforming at 250-500 oC. Two reforming processes, the conventional steam reforming (CSR) and the electrochemical catalytic reforming (ECR), were performed for the bio-oil reforming. The catalyst with an atomic mole ratio of Ni:Cu:Mg:Ce:Al=5.6:1.1:1.9:1.0:9.9 exhibited very high reforming activity both in CSR and ECR processes, reaching 82.8% hydrogen yield at 500 oC in the CSR, yield of 91.1% at 400 oC and 3.1 A in the ECR, respectively. The influences of reforming temperature and the current through the catalyst in the ECR were investigated. It was observed that the reforming and decomposition of the bio-oil were significantly enhanced by the current. The promoting effects of current on the decomposition and reforming processes of bio-oil were further studied by using the model compounds of bio-oil (acetic acid and ethanol) under 101 kPa or low pressure (0.1 Pa) through the time of flight analysis. The catalyst also shows high water gas shift activity in the range of 300-600 oC. The catalyst features and alterations in the bio-oil reforming were characterized by the ICP, XRD, XPS and BET measurements. The mechanism of bio-oil reforming was discussed based on the study of the elemental reactions and catalyst characterizations. The research catalyst, potentially, may be a practical catalyst for high efficient production of hydrogen from reforming of bio-oil at mild-temperature.