Ab initio studies on mechanisms of cycloaddition reaction between dichloro-vinylidene and ethylene

Mechanisms of cycloaddition reaction between singlet dichloro-vinylidene (R1) and ethylene (R2) have been investigated with MP2 and B3LYP /6-31G* methods, including geometry optimization, vibrational analysis, and energy for the involved stationary points on the potential energy surface. CCSD(T)/6-31G* single point calculations are also applied to the geometries from both methods. CCSD(T) relative energies for the stationary points predicted by MP2 and B3LYP are in a very good agreement. Three reaction pathways are located. Along the first one, one intermediate (INT1) is firstly generated, which then rearranges into a three-membered ring compound (P1) via a small barrier of 5.4 kJ/mol; the other two paths share the other intermediate (INT2), which isomerizes into two four-membered ring compounds (P2 and P3) via a chlorine and a hydrogen transfer, respectively. The barriers of the latter two paths are significantly higher by approximately 25 kJ/mol than that of the former (27.2 and 28.8 vs 5.4 kJ/mol), the major reaction is therefore the formation of P1.     

Basis set effects on the energy and hardness profiles of the hydrogen fluoride dimer

In earlier work, the present authors have shown that hardness profiles are less dependent on the level of calculation than energy profiles for potential energy surfaces (PESs) having pathological behaviors. At variance with energy profiles, hardness profiles always show the correct number of stationary points. This characteristic has been used to indicate the existence of spurious stationary points on the PESs. In the present work, we apply this methodology to the hydrogen fluoride dimer, a classical difficult case for the density functional theory methods.     

Theoretical study on the reaction mechanism of VO2+ with propyne in gas phase

Possible molecular mechanisms of the gas-phase ion/molecule reaction of VO2+ in its lowest singlet and triplet states (1A1/3A' ') with propyne have been investigated theoretically by density functional theory (DFT) methods. The geometries, energetic values, and bonding features of all stationary and intersystem crossing points involved in the five different reaction pathways (paths 1-5), in both high-spin (triplet) and low-spin (singlet) surfaces, are reported and analyzed. The oxidation reaction starts by a hydrogen transfer from propyne molecule to the vanadyl complex, followed by oxygen migration to the hydrocarbon moiety. A hydride transfer process to the vanadium atom opens four different reaction courses, paths 1-4, while path 5 arises from a hydrogen transfer process to the hydroxyl group. Five crossing points between high- and low-spin states are found: one of them takes place before the first branching point, while the others occur along path 1. Four different exit channels are found: elimination of hydrogen molecule to yield propynaldehyde and VO+ (1Sigma/3Sigma); formation of propynaldehyde and the moiety V-(OH2)+; and two elimination processes of water molecule to yield cationic products, Prod-fc+ and Prod-dc+ where the vanadium atom adopts a four- and di-coordinate structure, respectively.     

CH3S自由基H迁移异构化及脱H2反应的直接动力学研究

采用密度泛函方法(MPW1PW91)在6-311G(d,p)基组水平上研究了CH₃S自由基H迁移反应CH₃S→CH₂SH (R1), 脱H₂反应CH₃S→HCS＋H₂ (R2)以及脱H₂产物HCS异构化反应HCS→CSH (R3)的微观动力学机理. 在QCISD(t)/6- 311＋＋G(d,p)//MPW1PW91/6-311G(d,p)＋ZPE水平上进行了单点能校正. 利用经典过渡态理论(TST)与变分过渡态理论(CVT)分别计算了各反应在200～2000 K温度区间内的速率常数k^TST和k^CVT, 同时获得了经小曲率隧道效应模型(SCT)校正后的速率常数k^CVT/SCT. 结果表明, 反应 R1, R2 和R3的势垒△E^≠分别为160.69, 266.61和241.63 kJ/mol, R1为反应的主通道. 低温下CH₃S比CH₂SH稳定, 高温时CH₂SH比CH₃S更稳定. 另外, 速率常数计算结果显示, 量子力学隧道效应在低温段对速率常数的计算有显著影响, 而变分效应在计算温度段内对速率常数的影响可以忽略.     

Hamilton-Jacobi equation for the least-action/least-time dynamical path based on fast marching method

Classical dynamics can be described with Newton's equation of motion or, totally equivalently, using the Hamilton-Jacobi equation. Here, the possibility of using the Hamilton-Jacobi equation to describe chemical reaction dynamics is explored. This requires an efficient computational approach for constructing the physically and chemically relevant solutions to the Hamilton-Jacobi equation; here we solve Hamilton-Jacobi equations on a Cartesian grid using Sethian's fast marching method. Using this method, we can--starting from an arbitrary initial conformation--find reaction paths that minimize the action or the time. The method is demonstrated by computing the mechanism for two different systems: a model system with four different stationary configurations and the H+H(2)-->H(2)+H reaction. Least-time paths (termed brachistochrones in classical mechanics) seem to be a suitable chioce for the reaction coordinate, allowing one to determine the key intermediates and final product of a chemical reaction. For conservative systems the Hamilton-Jacobi equation does not depend on the time, so this approach may be useful for simulating systems where important motions occur on a variety of different time scales.     

Efficient statistical mapping of energy surfaces of nanoclusters and molecules

A statistical technique to efficiently map out the energy surfaces of nanoclusters and molecules is described. Global energy minimizations are performed to reach of the catchment basins of the lowest energy stationary points. Saddle points are located by using a large value of the iterative energy change as the stopping criterion of a final local relaxation. Minima are derived from saddle points by simply tightening the stopping criterion and continuing the relaxation. A statistical approximation to the widths of the paths in phase space between saddle points and minima is obtained. Application is made to argon clusters of 7 and 38 atoms.     

Simulating rare events in equilibrium or nonequilibrium stochastic systems

We present three algorithms for calculating rate constants and sampling transition paths for rare events in simulations with stochastic dynamics. The methods do not require a priori knowledge of the phase-space density and are suitable for equilibrium or nonequilibrium systems in stationary state. All the methods use a series of interfaces in phase space, between the initial and final states, to generate transition paths as chains of connected partial paths, in a ratchetlike manner. No assumptions are made about the distribution of paths at the interfaces. The three methods differ in the way that the transition path ensemble is generated. We apply the algorithms to kinetic Monte Carlo simulations of a genetic switch and to Langevin dynamics simulations of intermittently driven polymer translocation through a pore. We find that the three methods are all of comparable efficiency, and that all the methods are much more efficient than brute-force simulation.     

Catchment region partitioning of energy hypersurfaces,I

The space of internal coordinates of a molecular system is partitioned into catchment regions of various critical points of the energy hypersurface. The partitioning is based on an ordering of steepest descent paths into equivalence classes. The properties of these catchment regions and their boundaries are analyzed and the concepts of chemical structure, reaction path and reaction mechanism are discussed within the framework of the Born-Oppenheimer and energy hypersurface approximations. Relations between catchment regions and the chemically important reactive domains of energy hypersurfaces, as well as models for branching of reaction mechanisms, caused by instability domainsD , 1, are investigated.     

Mapping the triplet potential energy surface of 1-methyl-8-nitronaphthalene

Spin-unrestricted calculations and time-dependent DFT were used to characterize structure and reactivity of 1-methyl-8-nitronaphthalene (1) in the triplet state. Four hybrid models (B3LYP, PBE0, MPW1K, BHLYP) with significantly different amount of the exact exchange were employed. The triplet potential energy surface of 1 was mapped by using the UB3LYP and UMPW1K techniques. Both hybrid models provided qualitatively consistent pictures for the potential energy landscape. Thirty-one stationary points, of which 15 were minima, were found at the UB3LYP level of theory. Three minima corresponding to the nitro form of 1 were located on the triplet surface; just one was found for the singlet ground state. Two reaction paths leading from 1 either to a nitrite-type intermediate (2) or to the aci-form (3) were characterized. For both paths, reaction products were of diradical nature. The lower activation energy was obtained for the triplet-state tautomerization affording 3. The ground state of triplet multiplicity was predicted for two isomers of the aci-form. The triplet diradical 3 is expected to react through the thermal population of a close-lying singlet excited state. The results are discussed in relation to mechanisms of photoinduced rearrangements of peri-substituted nitronaphthalenes that can be used to develop novel photolabile protecting groups.     

Quantenchemische untersuchungen zum mechanismus der elektrophilen substitution. I. Zur potentialhyperfläche des systems benzol/h+

Results of semiempirical calculations (CNDO/2-FK and MINDO/2 methods) for the σ-π complex problem on protonated benzene are given and compared with previous ones. The semiempirical methods were chosen according to the agreement of their results with new theoretical energy data (E_HF + E_korrel) concerning the classical–nonclassical problem of protonated ethylene. By these methods the corresponding part of the energy surface of the benzene/H⁺ system is simulated. The stationary points of this surface are found by a gradient method with complete optimization of the geometry. On the basis of this method we determined the energy profile of a reaction coordinate between the classical (σ-complex) and nonclassical (π-complex) cation. The so called strong π-complex is a saddle point between two σ-complex minima and can be interpreted as transition state of 1,2-proton shifts. Hypotheses for possible minimum energy paths of electrophilic attacks in the given region of the surface are discussed.     

Pyrazine in supercritical xenon: local number density defined by experiment and calculation

Toward our goal of using supercritical fluids to study solvent effects in physical and chemical phenomena, we develop a method to spatially define the solvent local number density at the solute in the highly compressible regime of a supercritical fluid. Experimentally, the red shift of the pyrazine n-pi* electronic transition was measured at high dilution in supercritical xenon as a function of pressure from 0 to approximately 24 MPa at two temperatures: one (293.2 K) close to the critical temperature and the other (333.2 K) remote. Computationally, several representative stationary points were located on the potential surfaces for pyrazine and 1, 2, 3, and 4 xenons at the MP2/6-311++G(d,p)/aug-cc-pVTZ-PP level. The vertical n-pi* ((1)B(3u)) transition energies were computed for these geometries using a TDDFT/B3LYP/DGDZVP method. The combination of experiment and quantum chemical computation allows prediction of supercritical xenon bulk densities at which the pyrazine primary solvation shell contains an average of 1, 2, 3, and 4 xenon molecules. These density predictions were achieved by graphical superposition of calculated shifts on the experimental shift versus density curves for 293.2 and 333.2 K. Predicted bulk densities are 0.50, 0.91, 1.85, and 2.50 g cm(-3) for average pyrazine primary solvation shell occupancy by 1, 2, 3, and 4 xenons at 293.2 K. Predicted bulk densities are 0.65, 1.20, 1.85, and 2.50 g cm(-3) for average pyrazine primary solvation shell occupancy by 1, 2, 3, and 4 xenons at 333.2 K. These predictions were evaluated with classical Lennard-Jones molecular dynamics simulations designed to replicate experimental conditions at the two temperatures. The average xenon number within 5.0 A of the pyrazine center-of-mass at the predicted densities is 1.3, 2.1, 3.0, and 4.0 at both simulation temperatures. Our three-component method-absorbance measurement, quantum chemical prediction, and evaluation of prediction with classical molecular dynamics simulation-therefore has a high degree of internal consistency for a system in which the intermolecular interactions are dominated by dispersion forces.     

Bifurcation of reaction pathways: the set of valley ridge inflection points of a simple three-dimensional potential energy surface

This paper serves for the better understanding of the branching phenomenon of reaction paths of potential energy hypersurfaces in more than two dimensions. We apply the recently proposed reduced gradient following (RGF) method for the analysis of potential energy hypersurfaces having valley-ridge inflection (VRI) points. VRI points indicate the region of possible reaction path bifurcation. The relation between RGF and the so-called global Newton search for stationary points (Branin method) is shown. Using a 3D polynomial test surface, a whole 1D manifold of VRI points is obtained. Its relation to RGF curves, steepest descent and gradient extremals is discussed as well as the relation of the VRI manifold to bifurcation points of these curves. Received: 8 July 1998 / Accepted: 24 August 1998 / Published online: 23 November 1998     

单嘧磺隆除草剂的晶体构象-活性构象转换的密度泛函理论研究

应用密度泛函理论在B3LYP/6-31G(d)水平上对新型除草剂单嘧磺隆绕脲桥部分两个C—N键的内旋转势能面进行计算, 然后对势能面上的驻点进行构型优化和过渡态搜索, 得到单嘧磺隆4种稳定构象和构象转换过程所涉及的8个过渡态结构. 研究结果表明, 单嘧磺隆晶体构象-活性构象转换过程中涉及4种稳定构象和8条转换途径, 脲桥部分NH基团与嘧啶环上N原子所形成的分子内氢键对于构象的稳定性及转换过程起着十分重要的作用. 应用极化连续介质溶剂模型(PCM)在B3LYP/6-31＋＋G(d, p)水平下进行溶剂化效应计算, 结果表明单嘧磺隆从晶体构象转换成活性构象主要是在水相中进行的.     

硫代双烯酮与异氰酸环加成反应的理论研究

采用从头算ＲＨＦ／６－３１Ｇ方法研究了硫代双烯酮与异氰酸之间两种可能的环加成反应的机理,并对反应各驻点进行了电子密度拓扑分析研究．结果表明,这两个生成不同四元杂环产物的平行反应均为非同步的协同反应,但两个反应进行的难易程度不同,形成硫氮杂环的反应更容易一些,而形成氮杂环反应的产物在热力学上更稳定一些     

Mechanism of hydrogen bond assisted cis-trans isomerization in the excited state of 2-naphthol

Stationary points on the ground and first two excited state potential energy surfaces of 2-naphthol and the hydrogen bonded complex 2-naphtholNH(3) have been located using an approximate method (time dependent density functional based tight-binding) based on time dependent density functional theory (TDDFT). For the obtained extrema, single point calculations at the full TDDFT level (PW916-311+G(**)) were performed for validation purposes. The energetical ordering of the cis and trans forms of 2-naphthol in the different electronic states is found to be in good agreement with available experimental data. Interestingly, also the optical spectrum is correctly predicted by TDDFT in contrast to calculations on unsubstituted acenes in the literature. Besides the stationary points, also the minimum energy paths connecting them are calculated using an adaptive nudged elastic band approach in order to estimate isomerization barriers. While the increase of barrier height going from the ground to the first excited state is already documented, the authors find as a new result a significant barrier reduction upon hydrogen bond formation in the S(2). The observed effect can contribute to the explanation of recent experiments on infrared laser induced cis-trans isomerization in 2-naphthol.     

Husimi representation for stationary states

This paper considers a Husimi representation of quantum mechanics in which the (stationary) state of a system or ensemble is described by a Husimi function and an observable is described by a phase space function or distribution such that the expectation value of the observable is given by an integral over phase space of the product of that function or distribution and the Husimi function. The density matrix, Wigner function, and Husimi function are considered to be alternative ways of describing the state of a system or ensemble, and methods of recovering the Wigner function or density matrix from the Husimi function are discussed. The classical limits of the Wigner and Husimi functions and of the relationship between them are considered. © 1993 John Wiley & Sons, Inc.     

锗烯X2Ge(X=H、CH3、F、Cl、Br)与乙烯环加成反应的量子化学研究

利用量子化学密度泛函理论的B3LYP方法,在6-311G**的水平上,对锗烯X2Ge(X=H、CH3、F、Cl、Br)与C2H4的环加成反应进行了计算研究.结果表明,锗烯的基态是单重态,取代基的电负性越强,单-三态的能量差越大;控制反应的因素是电子效应,而不是立体效应;取代基的电负性越强,反应的活化能越高,放热越少;该反应由两步组成,第一步生成中间配合物,是一个无势垒的放热过程,第二步经过渡态生成产物.     

The effect of hydration on the photo-deactivation pathways of 4-aminopyrimidine

The influence of water on the photo-deactivation process of 4-aminopyrimidine has been investigated by means of microsolvation and continuum solvation models. Two- and four-water models were used for the former purpose. Vertical excitations, stationary points on the first excited singlet surface, conical intersections (minima on the crossing seam) and reaction paths have been investigated at the state-averaged complete active space self-consistent field (CASSCF) and multistate CAS perturbation theory to second order (MS-CASPT2) levels of theory. A destabilizing effect of 0.2–0.3 eV has been observed for nπ∗ states while the ππ∗ state is almost unaffected. The two-water model gives already a good representation of hydration effects, especially when combined with the continuum model. A small enhancement of energy barriers (∼0.1 eV) is observed leading to the conclusion that the photodynamics of 4-aminopyrimidine should be affected only little by these solvent effects.     

一维中心势V(q)=Bq2+A/q2(A, B>0)中运动的粒子在量子相空间表示中的解

在Torres-Vega 和 Frederick(T-F)量子相空间理论的框架下, 求解了相空间中一维中心势场V(q)=Bq2+A/q2(A, B>0),中运动的单粒子的本征函数, 并对相空间中的概率密度函数进行了讨论.