分子动力学模拟飞秒激光对C60光裂解反应机理的影响

基于半经典分子动力学方法模拟超快激光诱导的C60光裂解反应.选择能量为2.0eV,半峰全宽(FWHM)分别为40和500fs的激光作用于C60分子,调节脉冲强度使其发生裂解反应,比较长短脉宽飞秒激光对C60裂解反应机理的影响.通过分析产物分布、原子平衡指数、温度以及吸收能量(包括动能、势能和电子能量),证实飞秒激光脉冲下C60的光裂解主要由电子激发态控制,非热力学效应在该反应中起重要作用.激光场的作用时间和强度均影响光裂解反应过程,而电子吸收能量饱和后光场强度的作用则变得不明显.     

Low‐Temperature Thermal Rate Constants for the Water Formation Reaction H2+OH from Rigorous Quantum Dynamics Calculations

Thermal rate constants for the prototypical water‐forming reaction H₂+OH→H+H₂O were obtained for temperatures between 150 K and 600 K by rigorous quantum dynamics calculations including all degrees of freedom. Results are reported for a recent, highly accurate neural network potential (NN1) and compared to results obtained on a previous, semi‐empirical potential (SE). The rate constants computed on both potentials significantly differ in their temperature dependence, and differences of over one order of magnitude in the rates were found. The rate constants computed for the NN1 potential compare very well to experimental work. Furthermore, the influence of overall rotation is discussed for the title reaction. While previous close‐coupling simulations were limited to thermal rate constants above room temperature, we report rate constants for temperatures as low as 250 K. The high‐level results reported here provide an accurate benchmark for the development of approximate methods for the calculation of thermal as well as microcanonical rate constants.     

H+CN体系中平行络合反应的动力学

利用分子反应动力学的经典轨迹法研究化学反应体系已经开展了很多工作,但是,研究长寿命络合物的几何异构体就显得有些困难,主要表现在该方法对几何异构体不容易判别．由于这些原因和计算机时大增,Davis[1]利用经典轨迹法和统计方法相结合,研究了H＋CN和H＋NC体系在固定方位时的长寿命络合物动力学行为,并且采用Murrel等[2]拟合光谱数据的势能函数,该势能函数在HCN和HNC平衡点的频率与实验值相差较大．本文利用经典轨迹法研究了H＋CN产生HCN和HNC平行络合物异构体的分子反应动力学行为,揭示了平行络合反应产生几何异构体的…     

分子反应动力学如何走向21世纪

分子反应动力学领域正在发展之中,还远未进入成熟期。有些课题,如涉及对化学反应性基本理解方面的飞秒化学、立体化学动力学、态一态化学、选模化学以及有关新反应类型方面的团簇化学动力学、自由基反应动力学、气一固表面反应动力学等皆处于它们发展的中间阶段。另外有些课题,如激光控制化学反应,则仅处于初创阶段。它们如何进入21世纪呢?我们建议将来应该研究如下课题:(1)飞秒化学及立体态一态动力学;(2)激光控制化学反应;(3)团簇形成规律、结构及动力学;(4)分子高振动激发态光谱、自由基光谱及动力学;(5)理论研究。本文对上述课题给予简要的讨论。     

Rate Coefficients and Kinetic Isotope Effects of Cl+XCl→XCl+Cl (X=H,D, Mu) Reactions from Ring Polymer Molecular Dynamics

The ring-polymer molecular dynamics (RPMD) was used to calculate the thermal rate coefficients and kinetic isotope effects of the heavy-light-heavy abstract reaction Cl+XCl\begin{document}$ \rightarrow $\end{document}XCl+Cl (X = H, D, Mu). For the Cl+HCl reaction, the excellent agreement between the RPMD and experimental values provides a strong proof for the accuracy of the RPMD theory. And the RPMD results are also consistent with results from other theoretical methods including improved-canonical-variational-theory and quantum dynamics. The most novel finding is that there is a double peak in Cl+MuCl reaction near the transition state, leaving a free energy well. It comes from the mode softening of the reaction system at the peak of the potential energy surface. Such an explicit free energy well suggests strongly there is an observable resonance. And for the Cl+DCl reaction, the RPMD rate coefficient again gives very accurate results compared with experimental values. The only exception is at the temperature of 312.5 K, results from RPMD and all other theoretical methods are close to each other but slightly lower than the experimental value, which indicates experimental or potential energy surface deficiency.     

Molecular Dynamics Simulation of the Formation of Polymer Networks

The random end linking of different amounts of trifunctional crosslinkers with 3 000 prepolymer linear chains, with length varying from 10 to 30 monomers, to form networks at different system number densities was dynamically simulated by the molecular dynamics method. Investigation of the crosslinking kinetics shows that, with a stoichiometric number of crosslinkers present, the time evolution of free ends fraction decays as a power law in time t^−3/4. This scaling behavior is different from the one in a dense polymer melt. Structural analyses of the resulting networks indicate that the imperfections of the network structure strongly depend on the initial synthesis conditions including the initial system number density and the ratio of crosslinkers to precursor polymer chain ends.

     

Free Radical Bulk Polymerization of Styrene: Simulation of Molecular Weight Distributions to High Conversion Using Experimentally Obtained Rate Coefficients

Previously obtained experimental conversion‐dependences of the propagation rate coefficient (k_p), the termination rate coefficient (k_t) and the initiator efficiency (f) for the free‐radical bulk polymerization of styrene at 70 °C have been used to simulate the full molecular weight distributions (MWD) to high conversion using the software package PREDICI, providing a robust test of the kinetic model adopted. Satisfactory agreement with the experimental MWD's (GPC) was obtained up to approximately 70% conversion. Beyond 70% conversion, the high MW shoulder that appears was correctly predicted, although the amount of such polymer was somewhat underestimated. This discrepancy is believed to probably have its origin in experimental error in the conversion‐dependences of k_p, k_t and f, in particular k_t, that were employed in the simulations, rather than indicate a more fundamental short‐coming of the model employed.

     

Molecular Dynamics Simulations of Polymer Translocations

Summary: Molecular dynamics simulation studies of the translocation of charged homopolymers of length, N, driven by an electric potential gradient through a channel have been performed. We find that the translocation time, τ, displays an inverse power dependence on the temperature of the simulation τ ∼ (T − T₀)^−7/4, which is in very good agreement with experimental results. In addition, the dependence of τ on the driving field strength and the velocity of translocation on the polymer length N have also been obtained. The results suggest that such minimalist models are useful in modelling biological processes and that the molecular dynamics method is a suitable approach for carrying out these simulations.

Snapshot of the polymer during the simulation.     

Pu（^7Fg）＋H2（X^1∑g^＋,0,0）的分子反应动力学

基于PuH_2分子基态(X~7A_1)的分析势能函数，用准经典的Monte-Carlo轨线法 对Pu(~7Fg)+H_2(X~1∑_g~+，0，0)的分子反应动力学过程进行了计算。结果表明 ：Pu(~7F_g)与H_2(X~1∑_g~+，0，0)碰撞是弹性碰撞。     

A Study of the Reaction Dynamics for the H+BrCH_3 System Using Quasi-classical Trajectory Theory

In the present paper, scattering probabilities and rate constants of different channels for the H + BrCH_3 reaction system have been calculated by means of quasiclassical trajectory (QCT) method. Several important kinetic effects such as vibrational enhancement, channel competition, vibrational adiabaticity, mass combination, coupling of angular momenta and the relation between the kinetic effects and the feature of the potential energy surface have been discussed. Based on these analyses, a direct-type rebonded mechanism for this reaction has been inferred and used to explain the nonsymmetric angular distribution of the products crossed-molecular beam experiment. The agreement of calculation with experimental results is satisfactory.     

光异构化反应速率的量子理论

光异构化反应是在光场存在下，分子吸收光子引起的单分子化学反应，包括通常的异构化与环合、开环反应．一些作者用量子化学方法及分子轨道相关图和态相关图等方法对这类反应进行过研究［1］．本文用多声子光跃迁理论［2］研究光异构化反应，导出反应速率及其在低温条件下的解析表达式，对所得结果进行了讨论．1理论方法与结果在光异构化反应中，分子的电子状态与构型都发生变化，而且电子态的变化是与构型的改变紧密耦合的．分子的构型用分子的振动波函数来描写．由于电子与原子质量相差悬殊，可以采用绝热近似处理这一电子－振动相互作用…     

H2 + CN(n=0,1)→H + HCN振动选态反应

用从头算方法获得了Ｈ２＋ＣＮ反应的内禀反应坐标（ＩＲＣ），沿着ＩＲＣ，计算了各垂直于ＩＲＣ的简正模所对应的频率（Ｗ）以及沿ＩＲＣ运动与垂直ＩＲＣ运动的简正模之间的耦合常数（ＢＫＦ），根据传统过渡态，变分过渡态理论和选态公式，计算了ｎＣＮ＝０及ｎＣＮ＝１时反应的速率常数，并得到了实验相一致的结果，还计算了ｎＣＨ＝１及ｎＣＮ＝１的Ｈ＋ＨＣＮ→Ｈ２＋ＣＮ反应速率常数，可供实验工作者参考。     

Roaming Dynamics of H+C\begin{document}$_{2}$\end{document}D\begin{document}$_{2}$\end{document} Reaction on Fundamental-Invariant Neural Network Potential Energy Surface

We performed extensive quasiclassical trajectory calculations for the H+C\begin{document}$_2$\end{document}D\begin{document}$_2$\end{document}\begin{document}$\rightarrow$\end{document}HD+C\begin{document}$_2$\end{document}D/D\begin{document}$_2$\end{document}+C\begin{document}$_2$\end{document}H reaction based on a recently developed, global and accurate potential energy surface by the fundamental-invariant neural network method. The direct abstraction pathway plays a minor role in the overall reactivity, which can be negligible as compared with the roaming pathways. The acetylene-facilitated roaming pathway dominates the reactivity, with very small contributions from the vinylidene-facilitated roaming. Although the roaming pathways proceed via the short-lived or long-lived complex forming process, the computed branching ratio of product HD to D\begin{document}$_2$\end{document} is not far away from 2:1, implying roaming dynamics for this reaction is mainly contributed from the long-lived complex-forming process. The resulting angular distributions for the two product channels are also quite different. These computational results give valuable insights into the significance and isotope effects of roaming dynamics in the biomolecular reactions.     

Mode Specificity Dynamics of Prototypical Multi-Channel H+CH3OH Reaction on Globally Accurate Potential Energy Surface

The H+CH\begin{document}$ _3 $\end{document}OH reaction, which plays an important role in combustion and the interstellar medium, presents a prototypical system with multiple channels. In this work, mode specific dynamics of different product channels is investigated theoretically on a recently developed reliable potential energy surface based on a large number of data points calculated at the level of UCCSD(T)-F12a/AVTZ. It has been demonstrated that vibrational excitations of the O\begin{document}$ - $\end{document}H stretching motion, the torsional motion, the C\begin{document}$ - $\end{document}H stretching vibrations, show different influences on the four product channels, H\begin{document}$ _2 $\end{document}+CH\begin{document}$ _3 $\end{document}O, H\begin{document}$ _2 $\end{document}+CH\begin{document}$ _2 $\end{document}OH, H\begin{document}$ _2 $\end{document}O+CH\begin{document}$ _3 $\end{document}, and H+CH\begin{document}$ _3 $\end{document}OH. This work is helpful for understanding the mode-specific dynamics and controlling the competition for complicated reactions with multiple product channels.     

HNCO+OH->H2O+NCO的反应机理

采用从头算分子轨道法 (UHF/6 31G 水平 ,并用MP4加以相关能校正 )研究了HNCO OHH2 O NCO反应机理 .同时用Morokuma数值法获得了反应途径即内禀反应坐标 (IRC) .沿着IRC ,运用反应途径哈密顿理论 ,获得反应途径动态学信息 .在此基础上 ,根据过渡态理论和相应隧道效应校正 ,计算了在不同温度下的反应速率常数 ,得到了和实验相一致的结果 .计算结果表明 ,此反应是一步直接型的抽提H反应 .     

Molecular Dynamics of PGA Bioabsorbable Polymer During Isothermal Cold Crystallization

An investigation was carried out on the molecular dynamics of poly(glycolide) (PGA) in its completely amorphous state and during isothermal cold crystallization. Experimental results were generated over a wide range of frequency and temperature by broad-band dielectric spectroscopy (DRS). The variation of the average relaxation time (defined as τ= ½πf_max where f_max is the frequency at maximum loss for the main α relaxation) has been studied during cold crystallization and the temperature dependence of this average relaxation time for completely amorphous and crystallized samples has been analyzed. This behaviour has been modelled by Havriliak-Negami and Vogel-Fulcher equations. The sensitiveness of the segmental dynamics to the degree of crystallinity has been analyzed, taking into account the relaxing segments and the amorphous layers between lamellae. Supporting evidence about the thermal behaviour of the polymers has been obtained with DSC. Complementarily, the evolution of the morphologies obtained during crystallization processes has been followed by optical microscopy.