2007 Vol. 20, No. 5

Article
A pair of peptide groups in space, as modeled by formamide dimer, was used to evaluate vibrational coupling between the amide-I modes and the spatial behavior of the coupling using ab initio quantum chemical calculations. It was found that the coupling between two C=O groups, which is electrostatic in nature, is still quite signiˉcant as the intermolecular distance reaches 10 oA. One- and two-dimensional infrared spectra of the dimer at several conˉgurations were calculated using a vibrational exciton model that utilizes the abinitio computation-obtained parameters. The distance dependence of the coupling is dramatically shown in both the 1D and 2D infrared spectral features. The results suggest that the C=O stretching modes in polypeptide can be coupled and their states can be delocalized over quite a long distance in space.
Determinations of the lowest energy structures and electronic properties of MgBen (n=2-12) clusters werecarried out by using density-functional theory. It was found that MgBe3 and MgBe9 clusters with higherbinding energy and larger HOMO-LUMO gap are more stable than the neighboring clusters. The electronicproperties from van der Waals to covalent and bulk metallic behavior in MgBen (n=2-12) clusters arediscussed with the evolution of the size, and the data indicates Magnesium-doped Beryllium clusters alreadyearly appear some metallic-like features than host Ben clusters. By analyzing electronic properties of MgBen(n=2-12) clusters, it can be concluded that Mg-doped reduces the stabilities of Be clusters.
The modied molecular structural mechanics method (MMSMM) is extended to analyze the dynamic characteristics of single-walled carbon nanotubes (SWCNTs). In MMSMM, the deformation potential of SWCNT is decomposed and it can be easily expressed as the function of the positions of carbon atoms in molecular mechanics, and so the sti?ness matrix of SWCNT can be obtained. The elemental mass matrix is a diagonal one, so the natural frequency and vibration mode of SWCNT can be calculated e?ectively. In this work, the form of cantilevered nanotubes is analyzed. The natural frequencies of SWCNT computed by this algorithm are discussed. The frequency dependence on the tube diameter and length of SWCNT is conˉrmed and, it is shown that when the diameter of tube is small the frequency can reach the the GHz level. The frequency results and the vibration modes are comparable with those of other researchers. Furthermore, a new FEM continuum-model is proposed to analyze the dynamic character of SWCNT to compare with these results by MMSMM.
Ab initio and density functional calculations were employed to investigate the bonding patterns in theadenine-5-bromouracil (AT+) complexes. It is shown that the Br atom in 5-bromouracil (T+) is involved in bonding both with the hydrogen atom of the amino group of adenine (A) and with N7(A) (or N1(A)). With this motif, the Br atom interacts with a nucleophile (H) in a "head-on" fashion and an electrophile(N) in a "side-on" fashion, forming both hydrogen and halogen bonds. Electrostatic attraction between the Br atom in T+ and N7 (or N1) of adenine was found via the electrostatic potential analysis. The existence of the Br···N interactions in the pairs was further conˉrmed by means of Bader's atoms in molecules theory. A bond critical point is identiˉed for the halogen bonds and the topological parameters at the bond critical point indicate the typical closed-shell interactions in the pairs. Natural bond orbital analysis suggests that the charge transfer from the lone pair of the nitrogen atom of adenine is mainly directed to the C-Br antibonding orbital. Finally, halogen bonds in the T+AT+A tetrads were also explored.
The photodetachment of H? near an elastic wall was investigated and an analytical formula of the crosssection was derived. It was found that the cross section is a superposition of a smooth background and sinusoidal oscillation, which depends on the distance between the ion and the elastic wall. The classical dynamics of the photo-detached electron and its quantum correspondence was analyzed. In this model, the semiclassical photodetachment cross section is exactly the same as the quantum result.
The relaxation of O-H bending of water molecule H2O in the liquid phase was studied with the molecular dynamics simulation approach. Both rigid and fexible solvents were used to identify the di?erent channels for the vibrational energy relaxation. It was observed that the relaxation time for the O-H bend overtone is 174 fs in the rigid solvent while it is 115 fs in the fexible solvent. The main pathway of the O-H bend overtone is transition to the bend fundamental. The relaxation time of the O-H bend fundamental was calculated as 204 fs which is comparable to the experimental value 170 fs.
The interaction potential plays an important role in molecular dynamics (MD) simulations. Pair potentialhas been used to simulate the melting temperature of MgSiO3 perovskite in previous studies, but considerablediscrepancy of melting temperature exists between these simulations. Comparisons of potential energy curvesare performed to explain the discrepancy. To further investigate the infuence of the interaction potentialparameters on the MD simulation result, a new set of potential parameters is developed based on two fitting potential parameters of previous studies, and is applied in the present study. The simulated molar volume MgSiO3 perovskite agrees well with the study by Belonoshko and Dubrovinsky at ambient condition. The equations of state, constant-pressure heating capacity and the constant-pressure thermal expansivity of MgSiO3 perovskite are close to the experimental data. Calculated melting temperatures are also comparable with those derived from previous studies.
The structural and binding properties of diatomic molecules CO, NO and O2 to P450 heme were investigatedin two different models (labeled as M1 and M2) using density functional method at the B3LYP/6-31G(d)level. The e?ects of the serine residue near diatomic molecules XO were considered in the model M2. Theresults show that the serine residue near the heme enforced the binding of XO to heme. Frequency analysisindicates that the stretching vibrational frequency decreased as CO, NO, and O2 complex with heme.
The catalytic properties of CuCr2O4 with the cubic normal spinel-type structure were discussed by means of studying CO adsorption on the CuCr2O4 (100) surface in the framework of density functional theory. The results of geometry optimization show that CO prefers to adsorb at a Cu site with the adsorption energy of 133.2 kJ/mol. The adsorptions at all sites lead to a decrease in C-O stretching frequency, an increase in C-O bond length and a net positive Mulliken charge for the CO molecule. Population analysis indicates that the charges transfer from the CO molecule to substrate. The density of states for CO molecule before and after adsorption are also computed to discuss the bonding mechanism of CO.
A new dynamic theory of die swell for entangled polymeric liquids in a steady simple shear °ow is proposed which can be used to predict the correlation of the time-dependent and time-independent extrudate swelling behaviors to the molecular parameters of polymers and the operational variables. The theory is based on the O-W-F constitutive equation and the free recovery from Poioeuille flow with different ratios. Experimentsshow that the magnitudes of the simple shear in the steady simple shear flow may be resolved into the free recoil resulting from the recoverable elastic strains and the viscous heating resulting from the unrecoverable viscoelastic strains. For distinguishing the recoil from the viscous heating, a partition function and twoexponential fractions of conformation for the recoil and the viscous heating were defined. Thus the instantaneous, delayed and ultimate recoverable strain, and recoil in the free recovery were correlated to the partition function, the fraction of recoverable conformation, the molecular parameters, and the operational variables. Also the dynamics of the growth equation on the delayed viscoelastic strain and the delayed recoil in freestate were deduced. After introducing the condition of uniform two dimensional extensions, the definition ofswell ratio and the operational variables into the above correlation expressions and growth equations, then the correlations of the delayed extrudate swelling effect and the ultimate extrudate swelling effects to the molecular parameters and the operational variables were derived. Finally, two new sets of equations on the growth variables and ultimate extrudate swelling ratios under the dynamic and equilibrium states were also deduced from this dynamic theory of die swell. The first set of equations on the ultimate extrudate swelling ratio under the free and equilibrium states was verified by HDPE experimental data at two temperatures and different operational variables. The second set of equations on the growth extrudate swelling ratios under free and dynamic states was verified by PBD experimental data with different molecular weights and different operational variables. An excellent agreement is obtained, which shows that the two sets of equations for the growth and ultimate extrudate swelling ratios can be used directly to predict the correlation of extrudate swelling ratios to the molecular parameters and the operational variables.
CuO/Al2O3, CuO/CeO2-Al2O3, and CuO/La2O3-Al2O3 (denoted as Cu/Al, Cu/CeAl, and Cu/LaAl) catalysts were prepared by an impregnation method. CuO species and CuO/Al2O3 thermal solid-solid interaction were characterized by in situ XRD, Raman spectroscopy and H2-TPR techniques. For the Cu/Al catalyst, a CuAl2O4 phase exists between the CuO and Al2O3 layer and the CuO phase exists on the surface in both highly dispersed and bulk forms. For the Cu/CeAl catalyst, there is highly dispersed and bulk CuO on the surface, but most of the CuO has transferred into the internal layer of CeO2 as bulk CuO and CuAl2O4. For the Cu/LaAl catalyst, only bulk CuO is present on the surface of the catalyst and no CuAl2O4 is formed. The catalytic activity order for CO oxidation is Cu/CeAl>Cu/Al>Cu/LaAl. The highly dispersed CuO on the catalyst surface may be the active phase for CO oxidation. The results show that the addition of CeO2 not only promotes both the transference of CuO and the formation of CuAl2O4 but also favors the CO oxidation due to the association of highly dispersed CuO with CeO2, while La2O3 hinders the transference of CuO and the formation of CuAl2O4.
An atomic group model of the disordered binary alloy NixCu1-x (x=0.4) was constructed to investigate surface segregation. According to the model, the electronic structure of the NixCu1-x alloy surface was calculated by the Recursion method when glycine atoms are adsorbed on the NixCu1-x (110) surface under the condition of 0.33 coverage. The calculation results indicate that Cu is segregated on the surface of the NixCu1-x alloy, and the chemisorption of glycine restrains the segregation. In addition, the chemical adsorption of glycine greatly changes the density of states of the alloy surface near the Fermi level, and there is electric charge transfer between the alloy surface and the glycine.
Poly(butylene terephthalate)/poly(butylene terephthalate-e-caprolactone) is a new A/AxB1-x binary crystalline blend with intra-molecular repulsion interaction. Using the mean-field binary interaction model, the value of interaction parameter between the butylene terephthalate and caprolactone structural unit was first reported to be 0.305. This blend exhibited different crystallization behavior from a typical homopolymer/copolymer blend, which was carefully investigated by di?erential scanning calorimetry. It was found that poly(butylene terephthalate-e-caprolactone) copolymers have a great effect on the pure poly(butylene terephthalate) chain mobility and poly(butylene terephthalate) crystalline lattice packing. In the meantime, the crystallization of butylene terephthalate segments in copolymers was restricted by the previously formed poly(butylene terephthalate) crystallites. The two constituents for blending can not form a co-crystal in the range of composition even if they have the same butylene terephthalate unit. It can be concluded that longersegments in a copolymer would be beneficial for the formation of a co-crystal in blends.
Co3O4 nanoflowers were prepared through a sequential process of a hydrothermal reaction and heat treatment. The as-synthesized products were characterized by powder X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, and infrared spectrum. These nanoflowers consist of numerous Co3O4 nanofibers, which have diameters of 20-40 nm, and lengths ranging from 100 nm to 500 nm. They have pore structures and Brunauer-Emmett-Teller surface area of ?34.61 m2/g. The temperature dependence curves of magnetization in zero-field-cooled conditions and field-cooled indicate mainly antiferromagnetism and weak ferromagnetism of Co3O4 nanoflowers at blocking temperature of ?34 K respectively.
Mn2+-doped ZnSe ethylenediamine-intercalated precursor nanoribbon bundles (MnxZn1-xSe·en3, en=ethylenediamine) were prepared through an improved solvothermal route, which is time-saving and more efficient compared with the traditional route. XPS, TGA, and FESEM were used to characterize morphology and composites of the precursor and it was proved that the doping process was successful. Electron paramagnetic resonance and photoluminescence (PL) spectra were used in a detailed experiment to characterize and study the Mn2+ doping state and Mn2+-Mn2+ interaction in MnxZn1-xSe·en3. Six well-resolved hyperfine lines, which are attributed to the Mn2+ allowed transition imply that the Mn ions were embedded inside MnxZn1-xSe·en3 lattices and partly replaced Zn. As shown in PL spectrum; the introduction of Mn2+ strongly increases the intensity of the PL peak. The internal Mn transition (4T1 →6A1), which strongly increases the peak intensity depends on the average number of Mn atoms in the nanoparticals. As the Mn-doped concentration increases, the PL peak intensity is enhanced. A series of samples with different Mn-doping concentrations were prepared and studied. Combining the EPR results with PL spectra of the samples prepared, it is believed that under conditions of heating 40 h at 180 ℃ and controlling Mn2+