2013 Vol. 26, No. 5

Content
Content
2013, 26(5): 0-0. doi: 10.1088/1674-0068/26/5/0-0
Article
The photodissociation dynamics of 2-bromopentane at ~234 nm has been investigated by utilizing ion-velocity map imaging technique. The mapped images of Br(2P3/2) (denoted as Br) and Br(2P1/2) (denoted as Br*) fragments were analyzed by means of the speed and angular distributions, respectively. The speed distributions can be fitted with two Gaussian components which are correlated to the two independent reaction paths on the excited po-tential energy surfaces (PES). The high-energy component is from the prompt dissociation along the C-Br stretching mode, while the low-energy one is related to the dissociation from the coupling of the C-Br stretching and bending modes. Relative quantum yield is measured to be 0.892 for Br in the photodissociation of 2-bromopentane at 234 nm. Com-bining the anisotropy parameter with the relative quantum yield of Br and Br* fragments, the contributions of the excited 3Q03Q1, and 1Q1 states to the products Br and Br* were derived. The effect of alkyl branching on the mechanism of photodissociation was discussed by comparing the photodissociation processes of four isomers of bromopentane.
Electron transfer (ET) reactions between 1,8-dihydroxyanthraquinone (DHAQ) and two DNA bases, adenine (A) and cytosine (C), have been investigated in CH3CN/H2O solution with nanosecond time-resolved laser flash photolysis. After irradiation at 355 nm, the triplet DHAQ is produced via intersystem crossing and reacts with two nucleobases. ET processes for both reactions have been definitely identified, in which two bases play a significant role of electron donor. Based on the measured decay dynamics of various intermediates and the corresponding quenching rates, an initial ET process followed by a secondary proton-transfer reaction is suggested for both the overall reactions. By plotting the observed quenching rate against the concentration of two DNA bases, the bimolecular quenching rate constants are determined as 9.0×108 L/(mol·s) for the 3DHAQ*+C reaction and 3.3×108 L/(mol·s) for the 3DHAQ*+A reaction, respectively.
The geometries of one-electron reduced/oxidized species ([TOP]-/[VOP]+) of vanadyl por-phyrin (VOP) have been calculated with PBE1PBE method. The results show that for both [VOP]- and [VOP]+ the ground states are triplet, in which one of the two unpaired electron occupies the dxy orbital of the V atom while the other occupies the π-orbital of porphyrin ring. Thus both [VOP]- and [VOP]+ can be considered as π-radicals. The ground state of neutral VOP molecule is doublet with the unpaired electron occupying dxy orbital of V atom. In contract to the C4v symmetry of neutral VOP molecule, [VOP]- anion has a "rectangular" distorted C2v structure due to Jahn-Teller effect. The linear vibronic coupling constants for the Jahn-Teller active modes of [TOP]- were evaluated and the node patterns of frontier KS orbitals are used to explain the reason why the distortion occurs along specific modes. The ground state [VOP]+ has a porphyrin ring with pronounced bond length alternation due to pseudo-Jahn-Teller effect, causing its symmetry declined from C4v to C4. The bond length alternation is well explained with the node patterns of re-constructed frontier KS orbitals.
Laser-induced fluorescence excitation spectra of NiO have been recorded in the wavelength region of 510-650 nm under supersonic molecular beam conditions. More than fifty bands have been observed and rotationally analyzed to determine the molecular constants. The excited states exhibit highly irregular variations in terms of isotopic shifts, vibrational inter-vals, and rotational constants. Twenty-six bands attributed to [Ω=0, 1]-X3-0 transitions have been tentatively grouped into five vibrational progressions. Furthermore, dispersed fluorescence and lifetimes of the strong bands have also been measured.
The potential energy surfaces for butanone isomerization have been investigated by density function theory calculation. Six main reaction pathways are confirmed using the intrinsic reaction coordinate method, and the corresponding isomerization products are 1-buten-2-ol, 2-buten-2-ol, butanal or 1-buten-1-ol, methyl 1-propenyl ether, methyl allyl ether, and ethyl vinyl ether, respectively. Among them, there are three pathways through butylene oxide, indicating butylene oxide is an important intermediate product during butanone isomer-ization. The calculated vertical ionization energies of the reactant and its products are in a good agreement with the experimental values available. From the consideration for the relative energies of transition states and the number of high-energy barriers we infer that the reaction pathway butanone→1-buten-2-ol→2-buten-2-ol is the most competitive. The obtained results are informative for future studies on isomerization of ketone molecules.
The lattice parameters, elastic constants, cohesive energy, structural energy differences, as well as the properties of point defects and planar defects of hexagonal close-packed yttrium (hcp-Y) have been studied with ab initio density functional theory for constructing an ex-tensive database. Based on an analytical bond-order potential scheme, empirical many-body interatomic potential for hcp-Y has been developed. The model is fitted to some properties of Y, e.g., the lattice parameters, elastic constants, bulk modulus, cohesive energy, vacancy formation energy, and the structural energy differences. The present potential has ability toreproduce defect properties including the self-interstitial atoms formation energies, vacancy formation energy, divacancy binding energy, as well as the bulk properties and the thermal dynamic properties.
The interacting patterns of the luteolin and guanine have been investigated by using the density functional theory B3LYP method with 6-31+G* basis set. Eighteen stable structures for the luteolin-guanine complexes have been found respectively. The results indicate that the complexes are mainly stabilized by the hydrogen bonding interactions. Meanwhile, both the number and strength of hydrogen bond play important roles in determining the stability of the complexes which can form two or more hydrogen bonds. Theories of atoms in molecules and natural bond orbital have also been utilized to investigate the hydrogen bonds involved in all the systems. The interaction energies of all the complexes which were corrected by basis set superposition error are 6.04-56.94 kJ/mol. The calculation results indicate that there are strong hydrogen bonding interactions in the luteolin-guanine complexes. We comparedthe interaction between luteolin and four bases of DNA, and found luteolin-thymine was the strongest and luteolin-adenine was the weakest. The interaction between luteolin and DNA bases are all stronger than luteolin-water.
The C-I bond dissociation enthalpies (BDE) of various organic iodides were calculated using high-level theoretical methods including MP2 and CCSD(T) with extrapolated basis set as well as a number of density functional theory methods. After systematic evaluation of the theoretical results against available experimental C-I BDEs, it was found that the MPW-LYP1M method gave the lowest root mean square error. We, therefore, used this method to examine the substituent effects on different categories of C(sp3)-I and C(sp2)-I bonds. Fur-thermore, the remote substituent effects on the C-I BDEs of substituted iodobenzenes and substituted (iodomethyl)benzenes were also investigated at the same level. The C-I BDEs of typical heteroaromatic iodides including five-membered and six-membered heterocyclic iodides were also examined.
We consider a bistable mesoscopic chemical reaction system and calculate entropy produc-tion along the dominant pathway during nonequilibrium phase transition. Using probability generating function method and eikonal approximation, we first convert the chemical master equation into the classical Hamilton-Jacobi equation, and then find the dominant pathways between two steady states in the phase space by calculating zero-energy trajectories. We find that entropy productions are related to the actions of the forward and backward dominant pathways. At the coexistence point where the stabilities of the two steady states are equiv-alent, both the system entropy change and the medium entropy change are zero; whereas at non-coexistence point both of them are nonzero.
The adsorption sites and diffusion mechanism of CO2 molecules in the flexible Zn(MeIM)2 (MeIM=2-methylimidazole) (ZIF-8) have been investigated by grand canonical Monte Carlo and molecular dynamics simulations. A reasonable time correlation function is for the first time constructed to explore the mean residence time of CO2 molecules in the ZIF-8 cages, suggesting that CO2 molecules can remain in the same cage for up to several tens of picosec-onds. Furthermore, we find that the mean residence time almost linearly increases with the increasing pressure (or loading) at 273 and 298 K.
Proteins adsorption at solid surfaces are of paramount important for many natural processes. However, the role of specific water in influencing the adsorption process has not been well understood. We used molecular dynamics simulation to study the adsorption of BPTI on Au surface in three water environments (dielectric constant model, partial and full solvation models). The result shows that a fast and strong adsorption can occur in the dielectric environment, which leads to significant structure changes, as confirmed by great deviation from the crystal structure, largely spreading along the Au surface, rapid lose in all secondary structures and the great number of atoms in contact with the surface. Compared to the dielectric model, slower adsorption and fewer changes in the calculated properties above are observed in the partial solvation system since the specific water layer weakens the adsorptioneffects. However, in the partial solvation system, the adsorption of polar Au surface causes a significant decrease in the specific hydration around the protein, which still results in large structure changes similar to the dielectric system, but with much less adsorption extent. Enough water molecules in the full solvation system could allow the protein to rotate, and to large extent preserve the protein native structure, thus leading to the slowest and weakest adsorption. On the whole, the effects of non-specific and specific solvation on the protein structure and adsorption dynamics are significantly different, highlighting the importance ofthe specific water molecule in the protein adsorption.
The water-miscible room temperature ionic liquid 1-butyl-3-methylimidazolium tetrafluorob-orate ([bmim][BF4]) is a model system for studying the interactions between ionic liquid and water molecules. In this work the orientational structure of the low concentrated aqueous solution of [bmim][BF4] at the air/liquid interface was investigated by sum frequency gener-ation vibrational spectroscopy. It has been found that at very low concentrations, the butyl chain exhibited a significant gauche defect, indicating a disordered conformation; and the cation ring oriented with a fairly small tilting angle at the surface. When the concentration increased, the cation ring tended to lie flat at the surface, and the gauche defects of the butyl chain decreased due to the intermolecular chain-chain interactions and the consequent more ordered interfacial molecular arrangement. Additionally, the anti-symmetric stretching mode in the PPP and SPS spectra exhibited a peak shift, showing that there exists more than one kind of orientation or chemical environment for the butyl CH3 group. These results may shed new light on understanding the surface behavior of water-miscible ionic liquids as well as the imidazolium based surfactants.
Three-Coulomb-wave method is employed to treat the process of (e, 2e) simultaneous ion-ization and excitation to the n=2 state of helium, with radial and angular correlated wave-function of He target. The triple differential cross sections are calculated and analyzed in very asymmetric coplanar geometry at incident energies of 5.50, 1.50 and 0.57 keV. Results are compared with the absolute measurements and the theoretical first and second Born approximation. The present triply differential cross section (TDCS) is found to be in good agreement with experimental data qualitatively. The distinguishing feature noted in TDCS structure is the presence of intense recoil peak that for certain parameters is even larger than the binary peak, an unusual feature for the single-ionization process at high and intermediate energies.
The hydrogen adsorption (storage) studies upon Ni/Al2O3 nano-composite prepared by metal organic chemical vapor deposition technique (MOCVD) exploiting single source molec-ular precursor (SSP) approach were carried out. The Ni/Al2O3 nano-composite is prepared in cold walled MOCVD reactor by the decomposition of SSP, [H2Al(OtBu)]2, on a substrate holding Ni(acac)2 powder. The SSP is a reducing agent which reduces Ni+2 to Ni0 and works as source for Al2O3 matrix in which the Ni0 is dispersed. The resulting Ni/Al2O3 nano-composite is characterized by XRD, SEM, TEM, and EDX. The hydrogen adsorption (storage) studies are performed using home-made Sievert's type apparatus. The hydrogen storage studies reveal that approximately 2.9% (mass ratio) hydrogen can be stored in the Ni/Al2O3 nano-composite. The results show that Ni/Al2O3 nano-composite can be a po-tential candidate for hydrogen storage which can be used for onboard fuel purposes.
We report a systematic study on wrinkling and CuO nanowires (NWs) growth in the thermal oxidation of copper foil. Copper foils with thickness of 0.5 mm were thermally oxidized in air at 500 oC for 0.5-10 h. It is found that all the samples have wrinkles and the size of the wrinkles increases with the oxidation time increasing. CuO NWs can grow on both the sidehill and hilltop of wrinkle. The CuO NWs on sidehill are longer and denser than those on hilltop. The growth direction of the CuO NWs on sidehill is not vertical to the substrate but vertical to their growth surfaces. The process of wrinkling and CuO NWs growth can be divided into three stages: undulating, voiding, and cracking. The CuO NWs on both sidehill and hilltop grow at the undulating stage. However, only the CuO NWs on sidehill grow and those on hilltop stop growing at the voiding and cracking stages because of the void in hilltop. The local electric field in a wrinkle at undulating stage was calculated, and it is found that the difference of local electric field strengths between hilltop and sidehill is small, which indicates that the predominant driving force for the diffusion of Cu ion during CuO NWs growth is internal stress.
The graphene-gold nanoparticles composite film modified glassy carbon electrode (EG-AuNPs/GCE) was prepared by one-step coelectrodeposition and employed for determination of trace mercury in environmental water with differential pulse stripping voltammetry. Such a nanostructured composite film combined with the advantages of gold nanoparticles and graphene, can greatly promote the electron-transfer process and increase accumulation abil-ity for Hg(II), leading to a remarkably improved sensitivity. The linear calibration curve ranged from 0.2 μg/L to 30 μg/L for Hg(II) and the detection limit (S/N=3) was found to be 0.03 μg/L at a deposition time of 300 s. Moreover, the stablity of the as-prepared electrode and interferences from other substances were evaluated. The modified electrode was successfully applied to the direct detection of Hg(II) in real water samples.
The rod-like and bundle-like γ-LiV2O5 were synthesized via a simple solvothermal process-ing. The rod-like γ-LiV2O5 with diameter of 500-800 nm and the bundle-like architectures are composed of several of order-attached rods with diameter of 100-600 nm. γ-LiV2O5 were synthesized using LiOH·H2O, NH4VO3, HNO3, C2H5OH without and with PVP as raw materials. At the same time, the actual formation mechanism of γ-LiV2O5 was also investigated. As the cathode materials for lithium ion batteries, the bundle-like γ-LiV2O5 prepared with PVP delivers a better electrochemical performance, which has an initial dis-charge capacity of 269.3 mAh/g at a current density of 30 mA/g and is still able to achieve 228 mAh/g after the 20th cycle. The good electrochemical properties of the as-synthesized γ-LiV2O5 coupled with the simple, relatively low temperature, and low cost of the prepara-tion method may make this material a promising candidate as a cathode material for lithium ion batteries.
Monodisperse nanoparticle assembly with tunable structure, composition and properties can be taken as a superstructured building block for the construction of hierarchical nanostruc-tures from the bottom up, which also represents a great challenge in nanotechnology. Here we report on a facile and controllable method that enables a high yield fabrication of uniform gold nanoparticle (AuNP) core-satellites with definable number (in average) of the satellite particles and tunable core-to-satellite distance. The formation of the core-satellite nanostruc-tures is driven by programmable DNA-basepairing, with the resulting nanocomplexes beingisolatable via gel electrophoresis. By rationally controlling the DNA coverages on the core and shell particles, high production yields are achieved for the assembly/isolation process. As well, benefiting from a minimum DNA coverage on the satellite AuNPs, a strong affinity is observed for the as-prepared core-satellites to get adsorbed on protein-coated graphene ox-ide, which allows for a two-dimensional hierarchical assembly of the core-satellite structures. The resulting hierarchical nanoassemblies are expected to find applications in various areas, including plasmonics, biosensing, and nanocatalysis. The method should be generalizable to make even more complicated and higher-order structures by making use of the structural programmability of DNA molecules.
We have developed a simple, productive, and effective poly(dimethysiloxane) (PDMS) trans-fer method to fabricate highly conductive Pd nanowires following DNA scaffolds on various substrates, based on ethanol-reduction at low temperature. Pd nanoparticles were selectively deposited and confined onto the DNA templates on a PDMS sheet to form Pd nanowires and then the nanowires were transferred to other various substrates with a low occurrence of par-asitic nanoparticles. The structure, morphology and the conductance of Pd nanowires were characterized with transmission electron microscopy, field emission scanning electron mi-croscopy, and electrical transport measurement, respectively. Moreover, the growth process of the Pd nanowires was investigated by varying the incubation time and reaction temper-ature. The present strategy provides a new method to fabricate extremely dense, highly conductive, and well aligned Pd nanowires on various substrates, which make it promising for building nanosensors and nanoelectronic devices.
By means of density functional calculations, the structural and electronic properties of chem-ical modification of pristine and Ca-doped BeO nanotubes were investigated with NH3 and H2O molecules. It was found that the NH3 and H2O molecules can be adsorbed on the Be atom of the tube sidewall with the adsorption energies of about 36.1 and 39.0 kcal/mol, respectively. Density of states analysis shows that the electronic properties of the BeONT are slightly changed after the adsorption processes. Substitution of a Be atom in the tube surface with a Ca atom increases the adsorption energies by about 7.4 and 14.7 kcal/mol for NH3 and H2O, respectively. Unlike the pristine tube, the electronic properties of Ca-doped BeONT are sensitive to NH3 and H2O molecules. Also, the Ca-doped tube is much more sensitive to H2O molecule than NH3 one.
Chinese abstract