Ab initio rate constants from hyperspherical quantum scattering: application to H+C2H6 and H+CH3OH

The dynamics and kinetics of the abstraction reactions of H atoms with ethane and methanol have been studied using a quantum mechanical procedure. Bonds being broken and formed are treated with explicit hyperspherical quantum dynamics. The ab initio potential energy surfaces for these reactions have been developed from a minimal number of grid points (average of 48 points) and are given by analytical functionals. All the degrees of freedom except the breaking and forming bonds are optimized using the second order perturbation theory method with a correlation consistent polarized valence triple zeta basis set. Single point energies are calculated on the optimized geometries with the coupled cluster theory and the same basis set. The reaction of H with C2H6 is endothermic by 1.5 kcal/mol and has a vibrationally adiabatic barrier of 12 kcal/mol. The reaction of H with CH3OH presents two reactive channels: the methoxy and the hydroxymethyl channels. The former is endothermic by 0.24 kcal/mol and has a vibrationally adiabatic barrier of 13.29 kcal/mol, the latter reaction is exothermic by 7.87 kcal/mol and has a vibrationally adiabatic barrier of 8.56 kcal/mol. We report state-to-state and state-selected cross sections together with state-to-state rate constants for the title reactions. Thermal rate constants for these reactions exhibit large quantum tunneling effects when compared to conventional transition state theory results. For H+CH3OH, it is found that the CH2OH product is the dominant channel, and that the CH3O channel contributes just 2% at 500 K. For both reactions, rate constants are in good agreement with some measurements.     

Marcus电子转移理论的科学性探讨

利用科学原理对Marcus电子转移理论的科学性进行了考察, 结果表明Marcus电子转移理论违背了能量守恒定律.     

较快反应的热动力学天空

根据热导式热量计的工作原理,提出了热导式热量计的界面模型,建立了感温滞后条件下热量计的理论方程,从而提出了一种较快反应的热动力学研究法---失真热谱曲线修正法,并利用该法成功地研究了几个较快的准一级和等浓度二级反应的热动力学。     

Relaxation hindrance in nonadiabatic cage reactions

In-cage reactions are considered in the model of two intersecting parabolic terms different in curvature and lifetime. Transitions between the terms are assumed (contact approximation) possible only at the crossing point that is reached due to diffusion along the terms. The non-exponential kinetics of nonactivated reactions and the rate constants of activated reactions following the exponential law have been calculated within the model proposed. The results obtained allow one to connect the nonadiabatic theory (weak interaction) with the Kramers adiabatic theory (strong interaction). It has been found that at strong friction in the nonadiabatic theory as well as in the Kramers theory, the reactions are hindered by relaxation, i.e. the limiting stage is either the reagent activation (diffusion along the first term), or the product relaxation (diffusion along the second term).     

Computational tests of models for kinetic parameters of unimolecular reactions of organophosphorus and organosulfur compounds

A computational study of the kinetics of isomerization and elimination reactions of organophosphorus and organosulfur reactions is presented with a view to characterizing the predictive capabilities of widely applied techniques for processes that pertain to the destruction of chemical warfare agents. A set of 22 reactions has been studied, and the results have been compared to experimentally derived data. The BMK functional and the MG3S basis set have been used to compute minimum energy paths. Corrections have been added from CBS-QB3, CASSCF, and CASMP2 calculations. Thermal rate constants at experimental temperatures have been calculated with canonical variational transition state theory and small-curvature tunneling theory. The quality of these results may depend on recrossing of the variational transition state, the amount of radical or diradical character found in the minimum energy paths, or the accuracy of barrier heights.     

Theoretical studies on reductive etherification reactions between aromatic aldehydes and alcohols

The mechanism of reductive etherification reactions between aromatic aldehydes and alcohols has been investigated with the DFT(B3LYP)/6-31G method. One or two BH(3) molecules have been used to simulate the role of the catalyst and reducing agent. The solvent effects of the title reactions have been studied by the PCM model. It is found that the reactions between aromatic aldehydes and primary, secondary, and tertiary alcohols can proceed more easily in a polar solvent such as acetonitrile. The results provide evidence in theory to broaden the applications of reductive etherification reactions for the optimization of the radiochemical synthesis process of (18)F-labeled ether radiotracers.     

Computational Study on the Hetero-Diels-Alder Reactions between Phosphonodithioformate and Butadienes

The mechanism, catalytic effect and substituent effect of the hetero-Diels-Alder reactions between phosphonodithioformate and butadienes have been investigated theoretically using density functional theory at the B3LYP/6-31G（d） level. The results show that all of these reactions proceed in a concerted but asynchronous way. In some reactions the formation of C-S bond is prior to that of C-C and opposite result is found in other reactions. The BF3 catalyst and trimethylsilyloxy group may lower the activation barriers by changing the energies of FMOs for reactant molecules. With the BF3-catalyzed reactions, the complete regioselectivity observed experimentally has well been reproduced by theoretical calculation and these results originate probably from blue-shifting C-H...F hydrogen bond interaction in some transition states.     

Catalytic phosphorus(V)-mediated nucleophilic substitution reactions: development of a catalytic Appel reaction

Catalytic phosphorus(V)-mediated chlorination and bromination reactions of alcohols have been developed. The new reactions constitute a catalytic version of the classical Appel halogenation reaction. In these new reactions oxalyl chloride is used as a consumable stoichiometric reagent to generate the halophosphonium salts responsible for halogenation from catalytic phosphine oxides. Thus, phosphine oxides have been transformed from stoichiometric waste products into catalysts and a new concept for catalytic phosphorus-based activation and nucleophilic substitution of alcohols has been validated. The present study has focused on a full exploration of the scope and limitations of phosphine oxide catalyzed chlorination reactions as well as the development of the analogous bromination reactions. Further mechanistic studies, including density functional theory calculations on proposed intermediates of the catalytic cycle, are consistent with a catalytic cycle involving halo- and alkoxyphosphonium salts as intermediates.     

Particular features of photochemical transformations of molecules initiated by continuous external impact

Photochemical transformations induced by long-term continuous exposure to light have been studied. Using the developed theory and methods for calculating photochemical processes, computer experiments have been performed and their results have been analyzed for a large number of model systems and particular reactions. The excitation process has been considered in an explicit form, and the population of energy levels in excited states by absorption of light quanta has been taken into account. It has been shown that continuous optical excitation qualitatively changes the situation in comparison with pulsed excitation: the quantum yield is not zero even at very low quantum beat frequencies. For fast reactions, the kinetics of the process is exponential; in the case of slow reactions, concentration oscillations (on the order of 25% or more) arise, which are clearly manifested in the stationary state of the molecular system. The choice of the optimal photoirradiation time is an important factor in experimental design aimed to obtain a desired amount of the product. Molecular modeling makes it possible to a priori evaluate this quantity.     

Molecular emission from primary and secondary processes in some alkali (Li,Na, K) and halogen chemiluminescence reactions

Emission spectra have been measured from reactions of Li, Na and K with halogens and halogenated molecules. A regular band structure which is typical of transitions from the loosely bound covalent to strongly bound ionic states, has been reproduced for the potassium reactions and registered and identified for NaBr and NaCl. In accordance with theory, the lithium reactions and NaI showed no alkali halide emission. The underlying mechanisms are discussed.     

The constancy of the concentration of an inhibitor preventing the ignition of gases

Mechanisms of consumption of inhibitors in gas mixtures in different combustion modes—ignition prevention and suppression of flame propagation and detonation—have been revealed. It has been found that no more than a few hundredths of a percent of the initial reagents, including the inhibitor, are consumed for the prevention of ignition. In suppression of flame propagation and detonation, the inhibitor is consumed only in chain termination reactions. Oxygen is additionally consumed only in reactions with the products of incomplete oxidation of the inhibitor. The results have been interpreted in the framework of the theory of nonisothermal chain processes.     

Polymerization in nonuniform latex particles. III. Kinetics of grafting in emulsion polymerization

The concept of nonuniform distribution of free radicals in polymerizing latex particles has been incorporated into the development of a kinetic model for grafting reactions. This theory permits prediction of grafting efficiency as a function of reaction conditions. It can also be used for evaluation of rate constants for grafting reactions. Experimental data for emulsion polymerization of styrene in the presence of polybutadiene seed latex have been used to assess the proposed grafting theory. The predominant grafting reaction appears to be the attack of growing polystyrene chains on the allyl hydrogen atoms of polybutadiene. The results further reinforce the hypothesis that the entering oligomeric free radicals do not distribute uniformly within the particle volume.     

微扰分子轨道法在有机化学中的应用

经Dewar, Hoffmann, Epiotis, Houk, Wolfe和Kost等理论化学家的努力,PMO理论已经成为研究影响分子构型、构象和(立体专一性和区域专一性)反应产物的结构因素的一个有效的手段。Epiotis的非键轨道作用理论已经使有机化学家认识到, 一个拥挤的构象或过渡态不一定是一个能量上不利的状态。π轨道作用的分析表明, 共轭基团间的作用在本质上是相斥的, 全平面构象不是共轭效应最大的构象。WSW-like法不仅为定量PMO法提供了一组完整的、具有正确集据数的轨道基组, 而且它还成功地将轨道作用能转化为原子作用能。     

A computational study of radical haloacetal cyclizations controlled by the acetal center

The stereochemical outcome of the radical haloacetal cyclization reaction (Ueno-Stork reaction) has been examined by ab initio and other molecular orbital techniques. It was found that the stereochemistry of 5-exo- and 6-exo trig cyclizations can be accurately predicted from calculations using moderate levels of theory (UHF/6-311G** or B3LYP/6-311G**). A simplified computational procedure, easily run on a standard desktop computer, has been developed that provides excellent predictive ability for the stereochemical outcome for the reactions in question. Interestingly, a novel twist transition state has been identified for the first time in 5-exo-trig radical cyclization reactions.     

化学反应过渡态的结构和动力学

化学反应过渡态决定了包括反应速率和微观反应动力学在内的化学反应的基本特性,而无论是从理论还是实验上研究和观测化学反应过渡态都是极具挑战性的课题.近年来,我国科学家们利用交叉分子束-里德堡氢原子飞行时间谱仪,结合高精度的量子动力学计算,对H＋H2和F＋H2这两个教科书式的典型反应体系进行了全量子态分辨的反应动力学研究,从中得出了关于这两个反应体系的过渡态的结构和动力学性质的结论性的研究成果.     

Graph-theoretic study of certain interstellar reactions

Certain interstellar reactions have been studied through the graph-theoretic method using a Fortran-77 program. A plausible mechanism has been proposed for the reactions. The criterion of the conversion of edge to loop and vice versa proposed by the Bratislava group [Theor. Chim. Acta 79 , 65 (1991)] has been given a reasonable chemical insight. In the reaction scheme the nucleophilic/electrophilic/carbene (singlet, triplet) or biradical loops have been generated by the fragmentation of reactant. Subsequently, Pearson's hard and soft acids bases (HSAB) theory and frontier orbital (FO) theory have been applied to explain the reaction mechanism. Few possible reactions of the interstellar and circumstellar molecules have been investigated with illustrations involving carbene intermediate and Woodward-Hoffmann rule. © 1997 John Wiley & Sons, Inc. Int J Quant Chem 62: 495–508, 1997     

PHOTOBINDING OF PSORALEN AND 8-METHOXYPSORALEN TO CALF THYMUS DNA

Abstract— The photobinding of psoralen and 8-methoxypsoralen to calf thymus DNA induced by 365 nm radiation has been measured for different concentrations of the furocoumarin and nucleotides. The results are consistent with the assumption that dark complexing of the furocoumarin to DNA is a pre-condition for photobinding, but do not exclude the possibility that the free furocoumarin participates in the reactions. The analysis with 'large target' diffusion theory indicates that photobinding should be inefficient for the free excited singlet state and competitive with reactions of the dark-complexed sensitizer for the free triplet state. The analysis indicates also that the diffusive reactions of singlet oxygen generated by the free furocoumarin can compete with photoadduct formation.     

Peculiarities and paradoxes of photoinduced electron transfer reactions

Many chemical reactions involve the electron transfer stage. The kinetics of photoinduced electron transfer reactions is commonly considered in terms of either the transition state theory as preliminary thermally activated reorganization of the medium and reactants (necessary for degeneracy of the electronic levels of the reactants and the products) or nonradiative quantum transitions, which do not require preliminary activation and are observed in the exoergic region. A new approach to the kinetics of such reactions that has been proposed recently considers a substantial reduction of the barrier in the contact reactant pair due to strong electronic interaction and takes into account the intermediate formation of a charge transfer complex. This approach has explained many well-known important features of electron transfer reactions that are inconsistent with the first two theories.     

单分子微正则系综振动选模反应速率常数的计算

以过渡状态理论为基础，研究了单分子振动选模反应的微正则系综速率常数的计算方法．在计算中考虑了量子力学隧道效应校正及振动对沿IRC运动的耦合作用校正．以反应C＝CH（F）→HC≡CF的氢迁移反应和C＝CF2→FC≡CF的氟迁移反应为例，研究了它们的面外振动选模反应的速率常数．结果表明，这两个反应在低能区有明显的选模性，在高能区选模性减弱．