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141.
T. Schlathölter M.W. Newman T.R. Niedermayr G.A. Machicoane J.W. McDonald T. Schenkel R. Hoekstra A.V. Hamza 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2000,12(2):323-327
The emission of small (hydrogenated) carbon cluster ions CnHm
+
(n
=2-22) upon highly charged Xeq+ (q
=20-44) impact on C84 surfaces is studied by means of time-of-flight secondary ion mass spectrometry. The respective stage of hydrogenation/protonation
of a certain carbon cluster ion Cn
+
is a strong indication for its geometrical structure. From the cluster ion yield as a function of cluster size it can be
concluded, that the hydrogenation takes place after the initial carbon cluster formation. The carbon clusters seem to be emitted
as an entity in agreement with “equilibrium” and “shock wave” models.
Received 4 February 2000 相似文献
142.
143.
M. Rami Reddy Mark D. Erion Atul Agarwal Vellarkad N. Viswanadhan D. Quentin McDonald W. Clark Still 《Journal of computational chemistry》1998,19(7):769-780
The sensitivity of aqueous solvation free energies (SFEs), estimated using the GB/SA continuum solvent model, on charge sets, protocols, and force fields, was studied. Simple energy calculations using the GB/SA solvent model were performed on 11 monofunctional organic compounds. Results indicate that calculated SFEs are strongly dependent on the charge sets. Charges derived from electrostatic potential fitting to high level ab initio wave functions using the CHELPG procedure and “class IV” charges from AM1/CM1a or PM3/CM1p calculations yielded better results than the corresponding Mulliken charges. Calculated SFEs were similar to MC/FEP energies obtained in the presence of explicit TIP4P water. Further improvements were obtained by using GVB/6-31G** and MP2/6-31+G** (CHELPG) charge sets that included correlation effects. SFEs calculated using charge sets assigned by the OPLSA* force field gave the best results of all standard force fields (MM2*, MM3*, MMFF, AMBER*, and OPLSA*) implemented in MacroModel. Comparison of relative and absolute SFEs computed using either the GB/SA continuum model or MC/FEP calculations in the presence of explicit TIP4P water showed that, in general, relative SFEs can be estimated with greater accuracy. A second set of 20 mono- and difunctional molecules was also studied and relative SFEs estimated using energy minimization and thermodynamic cycle perturbation (TCP) protocols. SFEs calculated from TCP calculations using the GB/SA model were sensitive to bond lengths of dummy bonds (i.e., bonds involving dummy atoms). In such cases, keeping the bond lengths of dummy bonds close to the corresponding bond lengths of the starting structures improved the agreement of TCP-calculated SFEs with energy minimization results. Overall, these results indicate that GB/SA solvation free energy estimates from simple energy minimization calculations are of similar accuracy and value to those obtained using more elaborate TCP protocols. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 769–780, 1998 相似文献
144.
145.
146.
P. Ekanayake P. J. McDonald J. L. Keddie 《The European physical journal. Special topics》2009,166(1):21-27
A Peclet number, Pe, for the drying of colloidal films can be used as a predictor of the uniformity of water concentration in the direction normal
to the film. Uniform drying is predicted to occur when Pe < 1, whereas with , a layer of packed particles is expected to develop above a more dilute layer. Routh and Zimmerman have more recently proposed
that the particle concentration gradient between the packed and dilute layers, , will scale as . Here, this scaling relation is tested experimentally with magnetic resonance profiling data obtained from waterborne colloidal
films dried under conditions to yield a range of Pe. It is found that increases with Pe but scales as . This disagreement with the prediction can be attributed to an underestimate of Pe when there are greater non-uniformities of drying, because of an unquantified slowing down of the evaporation rate. 相似文献
147.
Xing L Kurumbail RG Frazier RB Davies MS Fujiwara H Weinberg RA Gierse JK Caspers N Carter JS McDonald JJ Moore WM Vazquez ML 《Journal of computer-aided molecular design》2009,23(1):13-24
Inducible, microsomal prostaglandin E synthase 1 (mPGES-1), the terminal enzyme in the prostaglandin (PG) biosynthetic pathway,
constitutes a promising therapeutic target for the development of new anti-inflammatory drugs. To elucidate structure–function
relationships and to enable structure-based design, an mPGES-1 homology model was developed using the three-dimensional structure
of the closest homologue of the MAPEG family (Membrane Associated Proteins in Eicosanoid and Glutathione metabolism), mGST-1.
The ensuing model of mPGES-1 is a homo-trimer, with each monomer consisting of four membrane-spanning segments. Extensive
structure refinement revealed an inter-monomer salt bridge (K26-E77) as well as inter-helical interactions within each monomer,
including polar hydrogen bonds (e.g. T78-R110-T129) and hydrophobic π-stacking (F82-F103-F106), all contributing to the overall
stability of the homo-trimer of mPGES-1. Catalytic co-factor glutathione (GSH) was docked into the mPGES-1 model by flexible
optimization of both the ligand and the protein conformations, starting from the initial location ascertained from the mGST-1
structure. Possible binding site for the substrate, prostaglandin H2 (PGH2), was identified by systematically probing the refined molecular structure of mPGES-1. A binding model was generated by induced
fit docking of PGH2 in the presence of GSH. The homology model prescribes three potential inhibitor binding sites per mPGES-1 trimer. This was
further confirmed experimentally by equilibrium dialysis study which generated a binding stoichiometric ratio of approximately
three inhibitor molecules to three mPGES-1 monomers. The structural model that we have derived could serve as a useful tool
for structure-guided design of inhibitors for this emergently important therapeutic target. 相似文献
148.
Evert J. Elzinga Yuanzhi Tang Jason McDonald Stephanie DeSisto Richard J. Reeder 《Journal of colloid and interface science》2009,340(2):153-159
The interaction of selenate, selenite, and chromate with the hydrated surface of γ-Al2O3 was studied using a combination of macroscopic pH edge data, electrophoretic mobility measurements, and X-ray absorption spectroscopic analyses. The pH edge data show generally increased oxyanion adsorption with decreasing pH, and indicate ionic strength-(in)dependent adsorption of chromate and selenate across the pH range 4–9, and ionic strength-(in)dependent adsorption of selenite in this pH range. The adsorption of chromate peaks at pH 5.0, whereas for selenate and selenite no pH adsorption maxima are observed. Electrophoretic mobility measurements show that all three oxyanions decrease the zeta potential of γ-Al2O3 upon adsorption; however, only selenite decreased the pHPZC of the γ-Al2O3 sorbent. EXAFS data indicate that selenite ions are coordinated in a bridging bidentate fashion to surface AlO6 octahedra, whereas no second-neighbor Al scattering was observed for adsorbed selenate ions. Combined, the results presented here show that pH is a major factor in determining the extent of adsorption of selenate, selenite, and chromate on hydrated γ-Al2O3. The results point to substantial differences between these anions as to the mode of adsorption at the hydrated γ-Al2O3 surface, with selenate adsorbing as nonprotonated outer-sphere complexes, chromate forming a mixture of monoprotonated and nonprotonated outer-sphere adsorption complexes, and selenite coordinating as inner-sphere surface complexes in bridging configuration. 相似文献
149.
Aidan R. McDonald 《Journal of organometallic chemistry》2009,694(14):2153-2759
We present the synthesis, via Adler condensation reactions, of mono- and tetrakis-4-(ethynyl-phenyl)porphyrin ligands and the zinc and manganese complexes thereof. The formed complexes were immobilised on silica by reacting the ethynyl groups with azide-functionalised silica in a copper(I) catalysed Huisgens 1,3-dipolar cycloaddition reaction. The synthesised metallo-porphyrin containing materials were thoroughly characterised using various solid-state techniques (NMR, IR, UV-Vis, elemental content analysis). The manganese containing materials were applied as catalysts in the epoxidation of various alkenes (cyclooctene, cyclohexene, styrene) with various oxidants (iodosylbenzene, tert-butylperoxide). The heterogenised homogeneous catalysts show diminished activity and yields compared to the analogous homogeneous catalysts (71% yield cf. 92% for cyclooctene epoxidation, TOF 82 h−1 cf. 230 h−1). Upon recycling, the heterogenised catalysts become gradually less active over five cycles until they are catalytically inactive. The deactivation process is discussed, with spectroscopy suggesting that the catalysts themselves are intact and thus stable to the reaction conditions and recycling, however there is likely some decomplexation, and also both chemical and mechanical decomposition of the silica support resulting in inaccessibility to the catalytic site. 相似文献
150.
J.M. McDonald 《Discrete Mathematics》2009,309(8):2077-2214
Let G be a multigraph with maximum degree Δ and maximum edge multiplicity μ. Vizing’s Theorem says that the chromatic index of G is at most Δ+μ. If G is bipartite its chromatic index is well known to be exactly Δ. Otherwise G contains an odd cycle and, by a theorem of Goldberg, its chromatic index is at most , where go denotes odd-girth. Here we prove that a connected G achieves Goldberg’s upper bound if and only if G=μCgo and (go−1)∣2(μ−1). The question of whether or not G achieves Vizing’s upper bound is NP-hard for μ=1, but for μ≥2 we have reason to believe that this may be answerable in polynomial time. We prove that, with the exception of μK3, every connected G with μ≥2 which achieves Vizing’s upper bound must contain a specific dense subgraph on five vertices. Additionally, if Δ≤μ2, we prove that G must contain K5, so G must be nonplanar. These results regarding Vizing’s upper bound extend work by Kierstead, whose proof technique influences us greatly here. 相似文献