2011 Vol. 24, No. 6

Special Issue
2011, 24(6): 0-0. doi: 10.1088/1674-0068/24/6/0-0
Benzene dimer (bz2) is the simplest prototype of the π-π interactions. Such interactions are ubiquitous in diverse areas of science and molecular engineering. In the present work, we have made assessment on some modern density functional methods including B97-D, BLYP-D3, M06-2X, XYG3, and force field models including CHARMM, AMBER, MM3, AMOEBA on six important interaction modes of bz2. Our results not only highlight the usefulness of these cost-effective methods, which can be used as economic substitutes of the expensive CCSD(T) for complex real-world systems, but also indicate their weakness in the description of the π-π interactions, which points to the future direction for further improvements.
The intra- and inter-band relaxation dynamics of CdSe/CdS/ZnS core/shell/shell quantum dots are investigated with the aid of time-resolved nonlinear transmission spectra which are obtained using femtosecond pump-probe technique. By selectively exciting the core and shell carrier, the dynamics are studied in detail. Carrier relaxation is found faster in the conduction band of the CdS shell (about 130 fs) than that in the conduction band of the CdSe core (about 400 fs). From the experiments it is distinctly demonstrated the existence of the defect states in the interface between the CdSe core and the CdS shell, indicating thatultrafast spectroscopy might be a suitable tool in studying interface and surface morphology properties in nanosystems.
The photodissociation dynamics of 2-bromobutane has been investigated at 233.62 and 233.95 nm by ion-velocity map imaging technique coupled with resonance-enhanced mul-tiphoton ionization. The speed and angular distribution of Br and Br* fragments were determined from the map images. The two Gaussian components, shown in the speed dis-tributions of Br and Br* atoms, are suggested to attribute to the two independent reaction paths of photodissociation for 2-bromobutane at 233.62 and 233.95 nm. The high-energy component is related to the prompt dissociation along the C-Br stretching mode, and the low-energy component to the dissociation from the repulsive mode with bending and C-Br stretching combination. The contributions of the excited 3Q0, 3Q1, and 1Q1 states to the products (Br and Br*) were discussed. Relative quantum yield of 0.924 for Br(2P3/2) at about 234 nm in the photodissociation of 2-bromobutane is derived.
The electrochemical and the mass transport behavior of ABTS2-/ABTS·- (2,20-azinobis(3-ethylbenzothiazoline-6-sulfonate)) redox couple at glassy carbon electrode (GCE) in phos-phate buffer solution (PBS, pH=4.4) is studied in detail by cyclic voltammetry combined with rotating disk electrode system. From the i-E curves recorded at different electrode ro-tating rate, rate constant, and transfer coefficient for ABTS2-?ABTS·-+e reaction at GCE electrode and the diffusion coefficient of ABTS2- in PBS are estimated to be 4.6×10-3 cm/s, 0.28, and 4.4×10-6 cm2/s, respectively. The transfer coefficient with a value of ca. 0.28 dif-fers largely from the value of 0.5 that is always assumed in the literature. The origins for the difference of the rate constant determined and the challenges for estimating the stan-dard rate constant are discussed. The performance for such ABTS2- mediated bio-cathode toward oxygen reduction reaction is discussed according to the over-potential drop as well as current output limit associated with the charge transfer kinetics of ABTS2-?ABTS·-+eredox reaction and/or the mass transport effect.
We have investigated the self-assembly and light emission properties of organic α-sexithiophene (α-6T) molecules on Ag(100) under different coverage by scanning tunneling microscopy (STM). At very low coverage, the α-6T molecules form a unique enantiomer by grouping four molecules into a windmill supermolecular structure. As the coverage is increased, α-6T molecules tend to pack side by side into a denser stripe structure. Fur-ther increase of the coverage will lead to the layer-by-layer growth of molecules on Ag(100) with the lower-layer stripe pattern serving as a template. Molecular fluorescence for α-6T molecules on Ag(100) at a coverage of five monolayers has been detected by light excitations,which indicates a well decoupled electronic states for the top-layer α-6T molecules. How-ever, the STM induced luminescent spectra for the same sample reveal only plasmonic-like emission. The absence of intramolecular fluorescence in this case suggests that the elec-tronic decoupling is not a sufficient condition for generating photon emission from molecules. For intramolecular fluorescence to occur, the orientation of the dynamic dipole moment of molecules and the energy-level alignment at the molecule-metal interface are also important so that molecules can be effectively excited through efficient dipolar coupling with local plasmons and by injecting holes into the molecules.
Surface-enhanced Raman scattering (SERS) and surface-enhanced resonance Raman scat-tering (SERRS) spectra of the 1,4-benzenedithiol molecule in the junction of two Au3 clus-ters have been calculated using density functional theory (DFT) and time-dependent DFT method. In order to investigate the contribution of charge transfer (CT) enhancement, the wavelengths of incident light are chosen to be at resonance with four representative excited states, which correspond to CT in four different forms. Compared with SERS spectrum, SERRS spectra are enhanced enormously with distinct enhancement factors, which can be attributed to CT resonance in different forms.
The composition of products formed from photooxidation of the aromatic hydrocarbon toluene was investigated. The OH-initiated photooxidation experiments were conducted by irradiating toluene/CH3ONO/NO/air mixtures in a smog chamber, the gaseous products were detected under the supersonic beam conditions by utilizing vacuum ultraviolet pho-toionization mass spectrometer using synchrotron radiation in real-time. And an aerosol time-of-flight mass spectrometer was used to provide on-line measurements of the individ-ual secondary organic aerosol particle resulting from irradiating toluene. The experimen-tal results demonstrated that there were some differences between the gaseous products and that of particle-phase, the products of glyoxal, 2-hydroxyl-3-oxo-butanal, nitrotoluene, and methyl-nitrophenol only existed in the particle-phase. However, furane, methylglyoxal, 2-methylfurane, benzaldehyde, cresol, and benzoic acid were the predominant photooxidation products in both the gas phase and particle phase.
The time-dependent quantum wave packet method is used to study the dynamics of the pho-todissociation processes for the isotopomers 14N14N16O, 14N15N16O, 15N15N16O, 15N15N16O, 14N14N17O, and 14N14N18O. In general, the computed isotopic fractionation factors derived from the absorption cross sections of five heavy isotopomers are in good agreement with the experimental results. Relative to the 14N14N16O isotopomer, the N2 rotational state distributions for the isotopically nitrogen substituted N2O are found to be entirely shifted to higher rotational states. Similar to its isotopic fractionation factors, the N2 rotational state distributions for the asymmetric isotopomers 14N15N16O and 15N14N16O are found to be observably different.
The vector correlations in Ca+HCl, Ca+DCl, and Ca+TCl reactions have been investigated by means of the quasi-classical trajectory calculations on PES constructed by means of multireference configuration interaction. The distributions of P(θr), P(Φr) and the PDDCSs of (2π/σ)(dσ00/dωt), (2π/σ)(dσ20/dωt), (2π/σ)(dσ22+/dωt), (2π/σ)(dσ21-/dωt) have been calculated based on the surface. The remarkable isotopic effects in the reactions are observed, and the mechanism which may be ascribed to different mass factors is discussed.
All-atom molecular dynamics (MD) simulation and the NMR spectra are used to investi-gate the interactions in N-glycylglycine aqueous solution. Different types of atoms exhibit different capability in forming hydrogen bonds by the radial distribution function analysis. Some typical dominant aggregates are found in different types of hydrogen bonds by the statistical hydrogen-bonding network. Moreover, temperature-dependent NMR are used to compare with the results of the MD simulations. The chemical shifts of the three hydrogen atoms all decrease with the temperature increasing which reveals that the hydrogen bonds are dominant in the glycylglycine aqueous solution. And the NMR results show agreement with the MD simulations. All-atom MD simulations and NMR spectra are successful in revealing the structures and interactions in the N-glycylglycine-water mixtures.
The electronic and magnetic properties of Ce doped SrMnO3 have been investigated us-ing the pseudo-potential plane wave method within the generalized gradient approximation method by first principles. The different Mn-O bond lengths indicate that there is a strong Jahn-Teller distortion of the MnO6 octahedron, which associates with a structural phase transition from cubic symmetry (Pm3m) to tetragonal symmetry (I4/mcm), and the Jahn-Teller ordering stabilizes a chain like (C-type) antiferromagnetic ground state. The electronic structures indicate that SrMnO3 and Sr1-xCexMnO3 (x=0.125 and 0.25) are semiconductor and metallic, respectively. The doping of SrMnO3 with cerium induces simultaneously a decrease in the electrical resistivity, which can be attributed to the formation of Mn3+ as a result of charge compensation. The density of states and charge density map present that hybridization exists between some of O bands with those of Mn and Ce bands, the bonding between Sr and O is mainly ionic. Density of states and magnetic moment calculations show that the formal valence state of the Ce ion is trivalence.
The thermodynamic and elastic properties of magnesium silicate (MgSiO3) perovskite at high pressure are investigated with the quasi-harmonic Debye model and the first-principles method based on the density functional theory. The obtained equation of state is consis-tent with the available experimental data. The heat capacity and the thermal expansion coefficient agree with the observed values and other calculations at high pressures and tem-peratures. The elastic constants are calculated using the finite strain method. A complete elastic tensor of MgSiO3 perovskite is determined in the wide pressure range. The geo-logically important quantities: Young's modulus, Poisson's ratio, Debye temperature, and crystal anisotropy, are derived from the calculated data.
We examined the puzzling mechanism for Cu-catalyzed meta-C-H arylation reaction of anilides by diaryliodonium salts through systematic theoretical analysis. The previously proposed anti-oxy-cupration mechanism featuring anti-1,2- or anti-1,4-addition of cuprate and oxygen to the phenyl ring generating a meta-cuprated intermediate was excluded due to the large activation barriers. Alternatively, a new amide-directed carbocupration mechanism was proposed which involves a critical rate- and regio-determining step of amide-directed addition of the Cu(III)-aryl bond across the phenyl C2=C3 double bond to form an ortho-cuprated, meta-arylated intermediate. This mechanism is kinetically the most favored among several possible mechanisms such as ortho- or para-cupration/migration mechanism, direct meta C-H bond cleavage mediated by Cu(III) or Cu(I), and Cu(III)-catalyzed ortho-directed C-H bond activation mechanism. Furthermore, the predicted regioselectivity based on this mechanism has been shown to favor the meta-arylation that is consistent with the experimental observations.
Crystalline Co nanocomposites in the rutile TiO2 were synthesized by 180 keV Co ion beam implantation at temperature of 623 K with the fluence of 4×1016 cm-2. The structural and magnetic properties of samples with different thermal treatment were characterized by synchrotron radiation X-ray diffraction (SR-XRD), high resolution transmission electron mi-croscopy (HRTEM), Rutherford backscattering/channeling and the superconducting quan-tum interference device magnetometer. The SR-XRD results reveal the formation of hcp and fcc phases of Co clusters, and the SR-XRD and HRTEM show that Co nanocrystals (NCs) have been formed in TiO2 after ion implantation. With increasing of annealing tem-perature, the transition of hcp to fcc Co is observed, and the Co NCs sizes were increased with increasing post-annealing temperature. At annealing temperature 1073 K, the lattice damaged is significantly removed compared with the virgin sample. The Co NCs forming inside TiO2 are the major contribution of the measured ferromagnetism.
Mo(CO)6 adsorption on the clean, oxygen-precovered and deeply oxidized Si(111) surfaces was comparatively investigated by high-resolution electron energy loss spectroscopy. The downward vibrational frequency shift of the C-O stretching mode in adsorbed Mo(CO)6 illustrates that different interactions of adsorbed Mo(CO)6 occur on clean Si(111) and SiO2/Si(111) surfaces, weak on the former and strong on the latter. The strong interac-tion on SiO2/Si(111) might lead to the partial dissociation of Mo(CO)6, consequently the formation of molybdenum subcarbonyls. Therefore, employing Mo(CO)6 as the precursor, metallic molybdenum could be successfully deposited on the SiO2/Si(111) surface but not on the clean Si(111) surface. A portion of the deposited metallic molybdenum is transformed into the MoO3 on the SiO2/Si(111) surface upon heating, and the evolved MoO3 finally desorbs from the substrate upon annealing at elevated temperatures.
NO adsorption on Ag/Pt(110)-(1×2) bimetallic surfaces at room temperature was inves-tigated by means of Auger electron spectroscopy, X-ray photoelectron spectroscopy and thermal desorption spectroscopy. An unexpected formation of nitrite/nitrate surface species on Ag/Pt(110)-(1×2) bimetallic surfaces is observed, then decompose at elevated tempera-tures to form N2. However, such nitrite/nitrate surface species do not form on clean Pt(110) and Ag-Pt alloy surfaces upon NO exposure at room temperature. The formation of ni-trite/nitrate surface species on Ag/Pt(110)-(1×2) bimetallic surfaces is attributed to highreactivity of highly coordination-unsaturated Ag clusters and the synergetic effect between Ag clusters and Pt substrate.
Functionalizing and patterning of the silicon surface can be realized simultaneously by the chemomechanical method. The oxide-coated crystalline silicon (100) surface is scratched with a diamond tool in the presence of aryldiazonium salt (C6H5N2BF4). Scratching activates the silicon surface by removing the passivation oxide layer to expose fresh Si atoms. The sur-face morphologies before and after chemomechanical reaction are characterized with atomic force microscopy. Time-of-flight secondary ion mass spectroscopy confirms the presence of C6H5 and provides evidence for the formation of self-assembled monolayer (SAM) on silicon surface via Si-C covalent bonds by scratching the silicon in the presence of C6H5N2BF4. C6H5 groups further bond with surface Si atoms via Si-C covalent bonds as confirmed from infrared spectroscopy results. We propose that chemomechanical reaction, which occurred during scratching the silicon surface, produce C6H5 groups from aryldiazonium salt. The rel-evant adhesion of SAM is measured. It is found that SAM can reduce the adhesion of silicon. The monolayer can be used as anti-adhesion monolayer for micro/nanoelectromechanical sys-tems components under different environments and operating conditions.
Highly efficient synthesis of clean biofuels using the bio-syngas obtained from biomass gasi-fication was performed over Fe1.5Cu1Zn1Al1K0.117 catalyst. The maximum biofuel yield from the bio-syngas reaches about 1.59 kg biofuels/(kgcatal·h) with a contribution of 0.57 kg alcohols/(kgcatal·h) and 1.02 kg liquid hydrocarbons/(kgcatal·h). The alcohol products in the resulting biofuels were dominated by the C2+ alcohols (mainly C2-C6 alcohols) with a content of 73.55%-89.98%. The selectivity of the liquid hydrocarbons (C5+) in the hy-drocarbon products ranges from 60.37% to 70.94%. The synthesis biofuels also possess a higher heat value of 40.53-41.49 MJ/kg. The effects of the synthesis conditions, including temperature, pressure, and gas hourly space velocity, on the biofuel synthesis were investi-gated in detail. The catalyst features were characterized by inductively coupled plasma and atomic emission spectroscopy, X-ray diffraction, temperature programmed reduction, and the N2 adsorption-desorption isotherms measurements. The present biofuel synthesis with a higher biofuel yield and a higher selectivity of liquid hydrocarbons and C2+ alcohols may be a potentially useful route to produce clean biofuels and chemicals from biomass.
We provide a new way to prepare ZnO nanorods pattern from the solution composed of hexamethylenetetramine (HMT) and Zn(NO3)2. The substrate is ITO substrate covered by well ordered Au islands. Since Au and the underneath ITO substrate have two different nucleation rates in the initial stage of heterogeneous nucleation process, the subsequent ZnO growth on the quick nucleating area takes place under diffusion control and is able to confine the synthesis of ZnO nanorods to specific locations. The concentrations of zinc nitrate and HMT are well adjusted to show the possibility of the new route for the patterning of the ZnO nanorods. Furthermore, the nanorods pattern was characterized by X-ray diffraction and photoluminescence and the performance of field emission property from ZnO nanorod patterns was investigated. The ZnO nanorods pattern with a good alignment also shows a good field enhancement behavior with a high value of the field enhancement factor.