D+HCN反应的SVRT含时波包理论研究

基于Horst的势能面,用SVRT(SemirigidVibratingRotorTarget)方法对D+HCN反应进行了含时波包动力学研究,计算得到了不同初始振转态的总反应几率和积分反应截面,采用UniformJ-shifting方法得到该反应的热速率常数.计算结果与H+HCN反应进行了比和讨论.     

H+HF(v,j)→H+HF(v′,j′)的传能动力学理论研究

本文运用非含时量子动力学方法研究了H+HF(v=1,j)→H+HF(v'=0,j')传能过程在295~500 K的振动弛豫速率常数.在此温度范围内,所有转动分辨的振动弛豫速率常数随着温度升高而单调递增,速率常数最大的末态转动量子数随着初态转动量子数的增加而增加.在室温下,振动态分辨的振动弛豫速率常数与实验值符合较好.同时,我们也计算了H+HF(v=1,j)→H+HF(v'=1,j')纯转动传能过程在500 K的速率常数,发现它们整体上比振动弛豫速率常数大了几个数量级,并且△j=-1的速率常数一般大于△j=-1的速率常数.     

二级反应速率常数与活化能计算问题的讨论

针对物理化学参考书中个别有关化学反应速率常数的计算问题进行讨论。计算二级反应的速率常数k时,套用半衰期公式要注意与反应计量方程式的表达形式相对应;对于理想气体反应使用Arrhenius公式计算活化能时,要注意反应速率常数k_c和k_p以及相应活化能E_ac和E_ap的区别。以帮助学生在计算过程中有正确的理解和认识。     

O(3P)+HBr(DBr)反应的含时量子散射计算

基于LEPS势能面, 用三维含时量子波包法对O(³P)+HBr(DBr)反应进行了准确的动力学计算. 计算的结果表明, 振动激发对这个反应是有效的, 而转动激发在某一能量范围内具有方位效应. 计算得到了该反应的速率常数和反应截面, 速率常数kO+HBr的计算值同实验值符合得很好. 通过对相应结果的对比, 可以发现这个反应具有比较明显的同位素效应.     

聚酯型Gd(Ⅲ)配合物的合成与表征

将二乙三胺五乙酸双酐与五种分子量不同的聚乙二醇共缩聚，制得聚酯型大分子配体，配体与ＧｄＣｌ３·６Ｈ２Ｏ反应，生成相应的配合物，并测试了配合物的弛豫速率．结果表明这些配合物比现用于临床的磁共振成像造影剂Ｇｄ（ＤＴＰＡ）具有更高的弛豫速率．     

较快一级化学反应的热动力学研究法: 热弛豫法

分析了非稳态传热情况下热导式热量计的动态性能,通过对输出信号的剖析,建立了热导式热量计冷却常数的经验动态模型。通过对热弛豫现象的分析,建立了可用于研究快速化学反应的热动力学研究法-热弛豫法,应用此种量热研究法可用于研究速率常数小于5s^-^1的一级化学反应。研究了25℃下乙酸甲酯、甲酸乙酯和甲酸甲酯在水溶液中的皂化反应,其计算结果与文献值相吻合,从而验证了热弛豫法的正确性。     

反应N+NH→N2+H的态-态量子动力学研究

采用在MRCI/aug-cc-pVQZ水平上构建的N₂H基态势能面, 并运用Chebyshev实波包法研究了N + NH→N₂ + H反应的量子动力学, 如反应几率、 积分截面以及产物振转态分布等. 在50~500 K温度范围内, 该反应的速率常数随着温度升高而递增, 与基于其它势能面的理论结果吻合. 然而, 在室温条件下, 所有理论计算的速率常数均显著大于实验值.     

土卫六大气中H2和N+反应的动力学研究

依据Titan大气的压强和温度条件计算了N++H2→NH++H反应的热化学函数. 结果发现, 该反应是一个吸热反应, 在Titan的低温环境中不具有反应自发性. 运用量子化学理论计算研究了反应的动力学性质, 发现该反应在300 K温度下的反应速率k=4.16×10−10 cm3•mol−1•s−1, 在实验室温度下(298.15 K)的反应势垒是109.847 kJ•mol−1. 依据Titan电离层气压温度条件计算了90 Pa压强条件下1 K到5000 K温度范围内(极低温度和极高温度)的 反应活化能和反应速率, 研究发现低温下该反应的反应速率非常低, 而且, 随着温度的降低, 反应速率急剧降低. 理论计算值和文献中的实验值也符合得较好, 理论计算数据可以为星际分子的模拟实验提供一定的参考.     

氟代乙炔氢迁移反应微正则系综速率常数的计算

以氟代乙炔的氢迁移反应为例, 计算了隧道效应、曲率因子和转动选态对微正则系综单分子反应速率常数的影响。结果表明, 在低能区域, 隧道效应和曲率因子对反应速率有较大影响, 均使反应速率增加, 而在高能区域作用不明显。与此同时还研究了不同对称态下的反应速率常数, 结果表明, 本例中的两对称态(A'态和A"态)的反应速率常数在能量较低时差异较大, 随能量升高, 两者趋于相等。     

O(~3P)+HBr(DBr)反应的含时量子散射计算(英文)

基于LEPS势能面, 用三维含时量子波包法对O(3~P)+HBr(DBr)反应进行了准确的动力学计算. 计算的结果表明, 振动激发对这个反应是有效的, 而转动激发在某一能量范围内具有方位效应. 计算得到了该反应的速率常数和反应截面, 速率常数kO+HBr的计算值同实验值符合得很好. 通过对相应结果的对比, 可以发现这个反应具有比较明显的同位素效应.     

Pseudo-bimolecular [2+2] cycloaddition studied by time-resolved photoelectron spectroscopy

The first study of pseudo‐bimolecular cycloaddition reaction dynamics in the gas phase is presented. We used femtosecond time‐resolved photoelectron spectroscopy (TRPES) to study the [2+2] photocycloaddition in the model system pseudo‐gem‐divinyl[2.2]paracyclophane. From X‐ray crystal diffraction measurements we found that the ground‐state molecule can exist in two conformers; a reactive one in which the vinyl groups are immediately situated for [2+2] cycloaddition and a nonreactive conformer in which they point in opposite directions. From the measured S₁ lifetimes we assigned a clear relation between the conformation and the excited‐state reactivity; the reactive conformer has a lifetime of 13 ps, populating the ground state through a conical intersection leading to [2+2] cycloaddition, whereas the nonreactive conformer has a lifetime of 400 ps. Ab initio calculations were performed to locate the relevant conical intersection (CI) and calculate an excited‐state [2+2] cycloaddition reaction path. The interpretation of the results is supported by experimental results on the similar but nonreactive pseudo‐para‐divinyl[2.2]paracyclophane, which has a lifetime of more than 500 ps in the S₁ state.     

The dependence of the branching ratio in the F+HD reaction on the initial rotational state of HD

Both classical trajectory and quantal scattering calculations indicate that the branching ratio in the F+HD reaction varies considerably with the initial rotational state of HD. Information theory argues that this variation must be reflected in the distribution of the reaction products. Hence, given the (normalized) product distribution for each reaction path one should be able to predict the dependence of the branching ratio on the state of the reagents. The trajectory computations of Muckerman are used to illustrate the procedure. First the dynamic constraint is identified and then the reaction probability matrix is constructed. The determination (“synthesis”) of the matrix, in terms of the given constraint invokes information theory and, in particular, the procedure of maximising the entropy. The branching ratio is readily computed from the elements of the probability matrix. Very good agreement is obtained between the trajectory-computed and the synthetic branching ratio for all initial rotational states of HD.The F+HD reaction has three possible final arrangement channels (one nonreactive and two reactive ones) and is used to illustrate the structure of the reaction probability matrix and the associated entropy measures.     

Product branching between reactive and nonreactive pathways in the collisional quenching of OH A 2Sigma+ radicals by H2

The collisional quenching of OH radicals in their excited A 2Sigma+ electronic state by molecular hydrogen is examined to determine the partitioning between reactive and nonreactive pathways. This is achieved using a pump-probe laser technique to compare the population prepared in the excited OH A 2Sigma+ state with that produced in the OH X 2Pi ground state from nonreactive quenching. Only a small fraction of the products, less than 15%, arise from nonreactive quenching; reactive quenching is the dominant product channel. The branching between the product channels provides a new dynamical signature of the conical intersection region(s) that couple the excited state potential for OH A 2Sigma++H2 with OH X 2Pi+H2 and H2O+H products.     

Nonadiabatic time-dependent wave packet study of the D+ + H2 reaction system

A theoretical investigation on the nonadiabatic processes of the D(+) + H(2) reaction system has been carried out by means of exact three-dimensional nonadiabatic time-dependent wave packet calculations with an extended split operator scheme (XSOS). The diabatic potential energy surface newly constructed by Kamisaka et al. (J. Chem. Phys. 2002, 116, 654) was employed in the calculations. This study provided quantum cross sections for three competing channels of the reactive charge transfer, the nonreactive charge transfer, and the reactive noncharge transfer, which contrasted markedly to many previous quantum theoretical reports on the (DH(2))(+) system restricted to the total angular momentum J = 0. These quantum theoretical cross sections derived from the ground rovibrational state of H(2) show wiggling structures and an increasing trend for both the reactive charge transfer and the nonreactive charge transfer but a decreasing trend for the reactive noncharge transfer throughout the investigated collision energy range 1.7-2.5 eV. The results also show that the channel of the reactive noncharge transfer with the largest cross section is the dominant one. A further investigation of the v-dependent behavior of the probabilities for the three channels revealed an interesting dominant trend for the reactive charge transfer and the nonreactive charge transfer at vibrational excitation v = 4 of H(2). In addition, the comparison between the centrifugal sudden (CS) and exact calculations showed the importance of the Coriolis coupling for the reactive system. The computed quantum cross sections are also compared with the experimental measurement results.     

Quasiclassical trajectory study of the postquenching dynamics of OH A 2Σ+ by H2/D2 on a global potential energy surface

We report full-dimensional, electronically adiabatic potential energy surfaces (PESs) for the ground state (1A(')) and excited state (2A(')) of OH(3). The PESs are permutationally invariant fits to roughly 23,000 electronic energies (MRCI + Q/aVTZ). Classical trajectory calculations of the postquenching dynamics of OH A (2)Σ(+) are carried out on the 1A(') PES for H(2) and D(2), at previously identified conical intersections (CoIs) [B. C. Hoffman and D. R. Yarkony, J. Chem. Phys. 113, 10091 (2000)]. The initial momenta are sampled fully and partially microcanonically, corresponding to "adiabatic" and "diabatic" models of the dynamics, respectively. Branching ratios of reactive to nonreactive channels from separate C(2v), C(∞v), and C(s) symmetries of CoIs are calculated, as are final rovibrational state distributions of OH and H(2) products. The rovibrational distributions of the OH and D(2) products, the D/H-atom translational energy distribution are calculated and compared to experimental ones. Agreement for these observable quantities is good. The branching between reactive and nonreactive quenching is sensitive to the momenta sampling; very good agreement with experiment is obtained using the diabatic sampling but not with the adiabatic sampling. The vibrational state distributions of H(2)O and HOD (although not measured by experiment) are also presented.     

Angular momentum decoupling approximations in the quantum dynamics of reactive systems

Two methods for implementing angular momentum decoupling approximations in quantum mechanical reactive scattering examined. Applications of both reactive and nonreactive H + H₂ collisions indicate that for the most intense individual reactive transitions and for all degeneracy-averaged ones, these decoupling methods (especially the “proper” decoupling method) yield results in good agreement with those of fully-coupled calculations. However, for the less intense reactive transitions and all individual nonreactive transitions, very large errors can result from use of these approximate methods.     

Transition state theory for laser-driven reactions

Recent developments in transition state theory brought about by dynamical systems theory are extended to time-dependent systems such as laser-driven reactions. Using time-dependent normal form theory, the authors construct a reaction coordinate with regular dynamics inside the transition region. The conservation of the associated action enables one to extract time-dependent invariant manifolds that act as separatrices between reactive and nonreactive trajectories and thus make it possible to predict the ultimate fate of a trajectory. They illustrate the power of our approach on a driven Henon-Heiles system, which serves as a simple example of a reactive system with several open channels. The present generalization of transition state theory to driven systems will allow one to study processes such as the control of chemical reactions through laser pulses.     

Quantum wave packet study of the H+ + D2 reaction on diabatic potential energy surfaces

The exact three-dimensional nonadiabatic quantum dynamics calculations were carried out for the title reaction by a time-dependent wave packet approach based on a newly constructed diabatic potential energy surface (Kamisaka et al. J. Chem. Phys. 2002, 116, 654). Three processes including those of reactive charge transfer, nonreactive charge transfer, and reactive noncharge transfer were investigated to determine the initial state-resolved probabilities and reactive cross sections. The results show that a large number of resonances can be observed in the calculated probabilities due to the deep well on adiabatic ground surface and the dominant process is the reactive noncharge-transfer process. Some interesting dynamical features such as v-dependent and j-dependent behaviors of the probabilities are also revealed. In addition, a good agreement has been achieved in the comparison between the calculated quantum cross sections from the ground rovibrational initial state and the experimental measurement data.     

Theory and simulation on the kinetics of protein-ligand binding coupled to conformational change

Conformational change during protein-ligand binding may significantly affect both the binding mechanism and the rate constant. Most earlier theories and simulations treated conformational change as stochastic gating with transition rates between reactive and nonreactive conformations uncoupled to ligand binding. Recently, we introduced a dual-transition-rates model in which the transition rates between reactive and nonreactive conformations depend on the protein-ligand distance [H.-X. Zhou, Biophys. J. 98, L15 (2010)]. Analytical results of that model showed that the apparent binding mechanism switches from conformational selection to induced fit, when the rates of conformational transitions increase from being much slower than the diffusional approach of the protein-ligand pair to being much faster. The conformational-selection limit (k(CS)) and the induced-fit limit (k(IF)) provide lower and upper bounds, respectively, for the binding rate constant. Here we introduce a general model in which the energy surface of the protein in conformational space is coupled to ligand binding, and present a method for calculating the binding rate constant from Brownian dynamics simulations. Analytical and simulation results show that, for an energy surface that switches from favoring the nonreactive conformation while the ligand is away to favoring the reactive conformation while the ligand is near, k(CS) and k(IF) become close and, thus, provide tight bounds to the binding rate constant. This finding has significant mechanistic implications and presents routes for obtaining good estimates of the rate constant at low cost.     

Identifying reactive trajectories using a moving transition state

A time-dependent no-recrossing dividing surface is shown to lead to a new criterion for identifying reactive trajectories well before they are evolved to infinite time. Numerical dynamics simulations of a dissipative anharmonic two-dimensional system confirm the efficiency of this approach. The results are compared to the standard fixed transition state dividing surface that is well-known to suffer from recrossings and therefore requires trajectories to be evolved over a long time interval before they can reliably be classified as reactive or nonreactive. The moving dividing surface can be used to identify reactive trajectories in harmonic or moderately anharmonic systems with considerably lower numerical effort or even without any simulation at all.