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51.
Luo MF Chiang CI Shiu HW Sartale SD Wang TY Chen PL Kuo CC 《The Journal of chemical physics》2006,124(16):164709
We present a scanning tunnel microscopy study of Co clusters grown through vapor deposition on Al(2)O(3) thin films over NiAl(100) at different coverages and temperatures. Formation of Co clusters was observed at 90, 300, 450, and 570 K. At the three lower temperatures, we find narrow cluster size distributions and the mean sizes (with a diameter of 2.6 nm and a height of 0.7 nm) do not change significantly with the coverage and temperature, until the clusters start to coalesce. Even on 3-4-nm-wide crystalline Al(2)O(3) strips where the deposited Co atoms are confined, the same features sustain. Only at 570 K the normal growth mode where the cluster size increases with the deposition coverage is observed, although the data are less conclusive. A simple modeling of kinetic surface processes on a strip confirms the normal growth mode, but fails to show a favored size unless additional energetic constraints are applied on the cluster sizes. Increasing Co coverages to cluster coalescence, a larger preferable size (mean diameter of 3.5 nm and height of 1.4 nm) appears for growth at 450 K. These two sizes are corroborated by morphology evolution of high Co coverages deposited at 300 K and annealed to 750 K, in which the coalescence is eliminated and the two preferable geometries appear and coexist. 相似文献
52.
Let k be a global function field over a finite field and let A be the ring of the elements in k regular outside a fixed place ∞. Let K be a global A-field of finite A-characteristic and let ? be a rank one Drinfeld A-module over K. Given any α∈K, we show that the set of places P of K for which α is a primitive root modulo P under the action of ? possesses a Dirichlet density. We also give conditions for this density to be positive. 相似文献
53.
The forcing number or the degree of freedom of a perfect matching M of a graph G is the cardinality of the smallest subset of M that is contained in no other perfect matchings of G. In this paper we show that the forcing numbers of perfect matchings in a fullerene graph are not less than 3 by applying the 2-extendability and cyclic edge-connectivity 5 of fullerene graphs obtained recently, and Kotzig’s classical result about unique perfect matching as well. This lower bound can be achieved by infinitely many fullerene graphs. 相似文献
54.
This paper presents a distance invariant manifold that preserves the neighborhood relations among data patterns. All patterns have their corresponding cells in the manifold space. The constellation of neighborhood cells closely resembles that of patterns. The manifold is invariant under the translation, rotation and scale of the pattern coordinates. The neighborhood relations among cells are adjusted and improved in each iteration according to the reduction of the distance preservation energy. 相似文献
55.
The title compound crystallizes in space group P 21/c of the monoclinic system in a cell of dimensions: a = 9.231(2), b = 15.931(2), c = 12.420(2) Å, β = 108.37(1)°. The observed density is 1.60 g/cm3 and the density calculated for four molecules in the cell is 1.58 g/cm3. The refinement converged with R = 0.029 and Rw = 0.028 based on 2569 reflections with I > 2.5 σ. The molecule has approximate Cs symmetry with a mirror plane roughly containing the central Mo atom and two cis-carbonyls, C(1)O(1) and C(4)O(4), while bisecting the bidentate, H2CPz2, and two trans-carbonyls. The four carbonyls and tine bidentate adopt a quasi-octahedral arrangement around the Mo atom. The C(NN)2Mo linkage of the (H2CPz′2)Mo fragment is observed to be in the boat form. The two cis-carbonyls are bent away by the bulky H2CPz′2 bidentate giving ∠C(1)-Mo-C(4) ? 167.3(1)°. The distortion explains the facile allyl bromination and decarbonylation observed for the title compound. 相似文献
56.
Three new η2-acyl complexes, TMo(CO)2(η2-COR) (T = Tp', Tp'; R= Me, Bun; Tp' = hydridotris(3,5-dimethyl-pyrazol-1-yl)borate; Tp' = hydridotris(3,4,5-trimethylpyrazol-l-yl)borate) prepared from the corresponding alkyl iodides, RI, and the anions, TMo(CO)?3. The preparation of Et4N+Tp Mo(CO)3? is also described. Using solution IR spectra to monitor the reactions between RI and TMo(CO)3?, it was found that Tp'Mo(CO)2(η2-COMe) was formed more readily than Tp'Mo(CO)2(η2-COMe) which was obtained more easily than Tp'Mo(CO)2(η2-COBun). This finding suggests that the mechanism is probably an ionic substitution rather than a radical mechanism. The different times required for the complete conversion from the anions to TMo(CO)2(η2-COR) is rationalized in terms of the electronic and steric influences of T and RI. 相似文献
57.
Myers-Saito cyclizations of a series of enyne-allenes and enyne-butatrienes have been studied by density functional methods. The pure DFT method, BPW91, in conjunction with the 6-311 basis set is demonstrated to be suitable to study these systems. Geometry optimizations and harmonic frequency calculations were applied for every reactant, transition structure, as well as product. It has been shown that the cyclic structure of reactant lowers significantly the critical distance and reaction barrier. For the Myers-Saito product of (5Z)-1,2,3,5-cyclononatetraen-7-yne (10R), the confinement of ring leads to an essential change of the biradical character from sigma-pi type to sigma-sigma type. The through-bond coupling is therefore involved in this product as in the Bergman products. With the enlargement of the ring, the geometrical distortion weakens the through-bond coupling and raises the stability of the products. As a consequence, 1,5-didehydroindene (10P) presents a particularly long critical distance and lower thermodynamic stability. Detailed comparisons of the reactivities of 10R, (Z)-1-cyclononene-3,8-diyne (13R), and (Z)-1-cyclodecene-3,9-diyne (14R) that represent the core structure of a category of natural antitumor drugs have also been made. It reveals that the reactivity of these three systems is quite similar, despite the fact that the thermochemical properties of the prototypical Myers-Saito and Bergman cyclizations are significantly different from each other. 相似文献
58.
用聚合反应动力学模型, 详细分析了ABg型缩聚体系中各种结构单元的演变, 对超支化聚合物支化度的各种定义作了比较, 并对ABg型反应体系的支化度提出了一个改进的定义, 使得支化度随g的增加而增加, 当g从2增加到无穷大, 最大支化度相应地从0.5增加到1-e-1. 对均方回转半径的数值计算表明, 在确定的A基团转化率(x), z-均均方回转半径几乎与g无关, 当x=0.999时, 随着g从2增加到无穷大, z-均回转半径只增加0.1%. 但是, 同样的条件下, z-均聚合度增加到2倍. 这一现象表明在任何指定的A转化率, ABg型缩聚中形成的超支化聚合物的分子轮廓与g无关, 随着g 增大而在超支化聚合物中增加的结构单元(或聚合度)必定分布在分子内部. 相似文献
59.
Kom-Bei Shiu Li-Yuan Yeh
Shie-Ming Peng
Ming-Chu Cheng 《Journal of organometallic chemistry》1993,460(2):203-211The neutral nitrogen-bidentate ligand, diphenylbis(3,5-dimethylpyrazol-1-yl)methane, Ph2CPz′2, can readily be obtained by the reaction of Ph2CCl2 with excess HPz′ in a mixed-solvent system of toluene and triethylamine. It reacts with [Mo(CO)6] in 1,2-dimethoxyethane to give the η2-arene complex, [Mo(Ph2CPz′2)(CO)3] (1). This η2-ligation appears to stabilize the coordination of Ph2CPz′ 2 in forming [Mo(Ph2CPz′2)(CO)2(N2C6H4NO2-p)][BPh4] (2) and [Mo(Ph2CPz′2)(CO)2(N2Ph)] [BF4] (3) from the reaction of 1 with the appropriate diazonium salt but the stabilization seems not strong enough when [Mo{P(OMe)3} 3(CO)3] is formed from the reaction of 1 with P(OMe)3. The solid-state structures of 1 and 3 have been determined by X-ray crystallography: 1-CH2Cl2, monoclinic, P21/n, a = 11.814(3), b = 11.7929(12), c = 19.46 0(6) Å, β = 95.605(24)°, V = 2698.2(11) Å3, Z = 4, Dcalc = 1.530 g/cm3 , R = 0.044, Rw = 0.036 based on 3218 reflections with I > 2σ(I); 2 (3)-1/2 hexane-1/2 CH3OH-1/2 H2O-1 CH2Cl2, monoclinic, C2/c, a = 41.766(10), b = 20.518(4), c = 16.784(3) Å, β = 101.871(18)°, V = 14076(5) Å3, Z = 8, Dcalc = 1.457 g/cm3, R = 0.064, Rw = 0.059 based on 5865 reflections with I > 2σ(I). Two independent cations were found in the asymmetric unit of the crystals of 3. The average distance between the Mo and the two η2-ligated carbon atoms is 2.574 Å in 1 and 2.581 and 2.608 Å in 3. The unfavourable disposition of the η2-phenyl group with respect to the metal centre in 3 and the rigidity of the η2-arene ligation excludes the possibility of any appreciable agostic C---H → Mo interaction. 相似文献
60.
The effect of pressure, temperature, HD isotopes, and C isotopes on the kinetics of the OH+CO reaction are investigated using Rice-Ramsperger-Kassel-Marcus theory. Pressure effects are treated with a step-ladder plus steady-state model and tunneling effects are included. New features include a treatment of the C isotope effect and a proposed nonstatistical effect in the reaction. The latter was prompted by existing kinetic results and molecular-beam data of Simons and co-workers on incomplete intramolecular energy transfer to the highest vibrational frequency mode in HOCO(*). In treating the many kinetic properties two small customary vertical adjustments of the barriers of the two transition states were made. The resulting calculations show reasonable agreement with the experimental data on (1) the pressure and temperature dependence of the HD effect, (2) the pressure-dependent (12)C(13)C isotope effect, (3) the strong non-Arrhenius behavior observed at low temperatures, (4) the high-temperature data, and (5) the pressure dependence of rate constants in various bath gases. The kinetic carbon isotopic effect is usually less than 10 per mil. A striking consequence of the nonstatistical assumption is the removal of a major discrepancy in a plot of the k(OH+CO)k(OD+CO) ratio versus pressure. A prediction is made for the temperature dependence of the OD+CO reaction in the low-pressure limit at low temperatures. 相似文献