共线散射体系A+BC的平-振能量传递──无限维李代数处理方法

利用无限维李代数方法处理了在BC分子能谱中含有二级与三级非简谐项的散射体系A＋BC的平-振能量传递问题，获得了散射过程的含有主要动力学参量的跃迁矩阵元和跃迁几率的解析表达式     

手性氮磷配体与钌的配位过程的研究—原位变温 31P核磁及分子模拟计算

用原位变温 ³¹P NMR和分子模拟研究了手性氮磷配体与金属钌的配位过程。首先确定了配体及其与金属的配合物的化学位移信号分别在δ=-13.0ppm和δ=48.2ppm。配位过程在293～343K温度范围内,相继生成四配位和五配位中间体。四配位体在 ³¹P NMR上对应于30.8ppm和-15.0ppm的两个信号;五配位中间体对应于35.1ppm和-16.5ppm的两个信号;最后在343K,产物中只有六配位的配合物存在,其信号出现在48.2ppm。为了得到各种中间体的结构,用分子模拟方法进行了结构优化计算,并比较了它们的构象能。计算结果表明,四配位中间体有两种构象,能量相差9kcal·mol^-1,它们可能同时存在于平衡状态。而五配位中间体只有一种绝对优势构象,尚有一个磷原子未参与配位。六配位的钌金属配合物的结构优化结果显示,两个氯原子分别位于PNNP原子所构成平面的两侧。其构象能为162.0kcal·mol^-1,其中键角能的贡献是112.5kcal·mol^-1,而非键静电作用是-41.4kcal·mol^-1,这表明分子内的静电吸引力对于形成完全配位产物是十分有利的,但是收敛的配体分子却承受了较大的键角张力。     

Effect of enzyme concentration on the dynamic behavior of a membrane-bound enzyme system

The dynamic behavior of the reaction-diffusion system, composed of glucose oxidase (EC 1.1.3.4) immobilized at a uniform concentration in a membrane, used as a glucose electrode is represented by a diffusion equation with a nonlinear reaction-term in one-dimensional space. The mathematical model is analyzed by computer simulation, that is, numerical integration of the equation under various initial and boundary conditions, to examine the effect of enzyme concentration on the response characteristics (responsiveness and linearity in response) of the electrode. The analysis of the responses of the system to stepwise changes in the boundary value (glucose concentration in simple solution) infers that the enzyme concentration governs the patterns of the spatial distributions of the substrates (glucose and dissolved oxygen) in steady states and transient responses. It is also revealed that the response characteristics of the electrode are optimized with concentration of immobilized enzyme and that the system establishes the steady states at the same spatial distributions of the substrates, regardless of the boundary value. The diffusion of the substrates and the oxygen concentration also have significant effects on the response characteristics of the electrode.     

Computational studies of carbon nanotube-hydrocarbon bond strengths at nanotube ends: effect of link heteroatom and hydrocarbon structure

Semiempirical and density functional electronic structure theory methods were used to study SWNT-X--R bond strengths, where the single-walled carbon nanotube (SWNT) had an armchair or zigzag structure, the link heteroatom X was O, N(H), or S and the hydrocarbon chain R was CH(2)CH(3), CH(OH)CH(3), CHCH(2), or CH(CF(3))CH(3). In all systems the hydrocarbon was bonded to the end of the nanotube. The SWNT-X--R bond (that is, the bond joining the link atom to the hydrocarbon) is more than 0.4 eV stronger for armchair than for zigzag nanotubes with the same diameters, irrespective of whether O, N, or S are used as link atoms or whether OH, C==C, or CF(3) groups are present in the hydrocarbon chain. This raises the possibility for selective manipulation of armchair/zigzag nanotubes using a variety of link atoms and hydrocarbon structures. The SWNT-O--CH(CF(3))CH(3) bond is weaker than the SWNT-O--CH(2)CH(3) bond (for both armchair and zigzag nanotubes), while inclusion of a double bond in the ethyl chain increases the bond strengths. Also, SWNT-S--CH(2)CH(3) and SWNT-N(H)--CH(2)CH(3) bonds are stronger than SWNT-O--CH(2)CH(3) bonds.     

Clusters,metastability, and nucleation: Kinetics of first-order phase transitions

We describe and interpret computer simulations of the time evolution of a binary alloy on a cubic lattice, with nearest neighbor interactions favoring like pairs of atoms. Initially the atoms are arranged at random; the time evolution proceeds by random interchanges of nearest neighbor pairs, using probabilities compatible with the equilibrium Gibbs distribution at temperatureT. For temperatures 0.59T_c, 0.81 T_c, and 0.89T _c, with density of A atoms equal to that in the B-rich phase at coexistence, the density C₁ of clusters ofl A atoms approximately satisfies the following empirical formulas: C₁ w(1 –)³ andC ₁, (1 –)⁴Q₁w¹ (2 l 10). Herew is a parameter and we defineQ _l = _K e ^–E(K) , where the sum goes over all translationally nonequivalentl-particle clusters andE(K) is the energy of formation of the clusterK. Forl > 10,Q ₁ is not known exactly; so we use an extrapolation formulaQ _l Aw _s ^–l l ^– exp(–bl ), wherew _s is the value ofw at coexistence. The same formula (withw > w _s) also fits the observed values of C, (for small values ofl) at densities greater than the coexistence density (forT=0.59T_c): When the supersaturation is small, the simulations show apparently metastable states, a theoretical estimate of whose lifetime is compatible with the observations. For higher supersaturation the system is observed to undergo a slow process of segregation into two coexisting phases (andw therefore changes slowly with time). These results may be interpreted as a more quantitative formulation (and confirmation) of ideas used in standard nucleation theory. No evidence for a spinodal transition is found.Supported by AFOSR Grant No. 73-2430D and by ERDA Contract No. EY-76-C-02-3077*000.     

Computer Simulation of the Structure of Porous Electrodes with an Immobilized Enzyme and Nanosized Support Particles

The efficiency of the operation of a porous electrode with an immobilized enzyme is defined, in particular, by a lucky structure of its active layer, which can contain nanosized particles of the support. The composites of such a kind are prepared with the aid of methods of colloidal chemistry. The aim of this particular investigation is to perform a computer simulation of processes of coagulation of particles of the support and their possible heterocoagulation with molecules of the enzyme. Algorithms of the formation of nanocomposite structures in solution are suggested. Calculations show that the concentration of the enzyme molecules in the nanocomposite structures cannot exceed a certain critical value. On the other hand, at a fixed value of the concentration of the enzyme molecules, the concentration of the support particles must not fall below a certain threshold quantity, which provides for the passing of current through the active layer. In order for all the enzyme molecules, rather than for a fraction of these, in the composite to take part in the process of bioelectrocatalysis, the concentration of support particles must be increased even higher, to an optimum value.__________Translated from Elektrokhimiya, Vol. 41, No. 6, 2005, pp. 738–747.Original Russian Text Copyright © 2005 by Chirkov, Rostokin.     

Methodology for the calculation of the potential of mean force for a cation-pi complex in water.

We report potential of mean force (PMF) calculations on the interaction between the p-sulfonatocalix[4]arene and a monovalent cation (Cs(+)). It has been recently shown from microcalorimetry and (133)Cs NMR experiments that the association with Cs(+) is governed by favourable cation-pi interactions and is characterized by the insertion of the cation into the cavity of the macrocycle. We show that the PMF calculation based upon a classical model is not able to reproduce both the thermodynamic properties of association and the insertion of the cation. In order to take into account the different contributions of the cation-pi interactions, we develop a new methodology consisting of changing the standard PMF by an additional contribution resulting from quantum calculations. The calculated thermodynamic properties of association are thus in line with the microcalorimetry and (133)Cs NMR experiments and the structure of the complex at the Gibbs free-energy minimum shows the insertion of the cation into the cavity of the calixarene.     

Computing the Kekulé structure count for alternant hydrocarbons

A fast computer algorithm brings computation of the permanents of sparse matrices, specifically, molecular adjacency matrices. Examples and results are presented, along with a discussion of the relationship of the permanent to the Kekulé structure count. A simple method is presented for determining the Kekulé structure count of alternant hydrocarbons. For these hydrocarbons, the square of the Kekulé structure count is equal to the permanent of the adjacency matrix. In addition, for alternant structures the adjacency matrix for N atoms can be written in such a way that only an N/2 × N/2 matrix need be evaluated. The Kekulé structure count correlates with topological indices. The inclusion of the number of cycles improves the fit. When comparing with previous results, the variance decreases 74%. The calculated standard heat of formation correlates with the logarithm of the Kekulé structure count. This heat increments 349 kJ/mol each time the Kekulé structure count increases by one order of magnitude. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002