Bifurcating reactions: distribution of products from energy distribution in a shared reactive mode

Bifurcating reactions yield two different products emerging from one single transition state and are therefore archetypal examples of reactions that cannot be described within the framework of the traditional Eyring''s transition state theory (TST). With the growing number and importance of these reactions in organic and biosynthetic chemistry, there is also an increasing demand for a theoretical tool that would allow for the accurate quantification of reaction outcome at low cost. Here, we introduce such an approach that fulfils these criteria, by evaluating bifurcation selectivity through the energy distribution within the reactive mode of the key transition state. The presented method yields an excellent agreement with experimentally reported product ratios and predicts the correct selectivity for 89% of nearly 50 various cases, covering pericyclic reactions, rearrangements, fragmentations and metal-catalyzed processes as well as a series of trifurcating reactions. With 71% of product ratios determined within the error of less than 20%, we also found that the methodology outperforms three other tested protocols introduced recently in the literature. Given its predictive power, the procedure makes reaction design feasible even in the presence of complex non-TST chemical steps.

Reactive Mode Composition Factor (RMCF) analysis is a powerful tool to forecast the product distribution of bifurcating reactions through analysis of the kinetic energy distribution within the first transition state traversed by the reacting system.     

Product state distribution in chemical reactions: Vibrational temperature and rotational distributions

The distributions of vibrational, rotational and translational energies in the products of atom-diatomic molecule reactions are studied on the basis of their “temperature parameters”. The validity of the vibrational temperature concept, which may characterize the variaous distributions, is examined from different points of view. The analysis is done on the basis of available data from chemiluminescence, chemical laser and molecular beams measurements and from classical trajectory calculations. All the reactions investigated are exothermic with exothermicities ranging between ≈ 17 kcal/mole and ≈ 85 kcal/mole. In most products a high degree of population inversion is found. An attempt is made to predict the complete vibrotational state distribution using the vibrational temperature only. This attempt is equivalent to the assumption that apart from the vibration, the other degrees of freedom have reached a (microcanonical) equilibrium. The agreement with experimental results is good.     

Consistency of a set of chemical reactions

It is a useful feature of many general chemical kinetics programs that the user's reaction scheme is tested as far as possible before a calculation is started. Attempts are made to answer the question: How far can a reaction scheme be tested for consistency, with respect to mass conservation, without knowing the molecular weights of the species involved? It is found that this problem can be transformed so that it is solvable by the standard methods used in linear programming.     

Peripheral chemical reactions

Forward scattering of the products of direct exoergic atom transfer reactions is proposed as an indication for a peripheral attraction: a reaction in which an atom at the periphery of the reactants is abstracted. Model computations for the O(¹D) + N₂O reaction are used to illustrate the proposed mechanism. The opacity function has a peak at higher impact parameters which is correlated with the forward scattering. Potential energy surfaces which do not manifest peripheral attraction lead to backwards scattering of the products.     

Valency changes in chemical reactions

An analysis of valence changes in selected organic gas phase reactions, calculated on SINDO1 potential surfaces, is performed. It is shown that transition states and intermediates have diradical character if singlet and triplets states are nearly degenerate. As a consequence, normal valencies are expected in Woodward-Hoffman allowed reactions and reduced valencies in forbidden reactions. The formalism is also applied to cationic reactions, but the effects of valency changes are less pronounced.     

On chemical reactions in the laser-induced breakdown of a liquid

It is shown experimentally that a laser-induced breakdown of a liquid is accompanied by chemical reactions initiated by radicals and excited species formed in the spark. It is found that, in water, the laser-induced breakdown is accompanied by the dissociation of water and dissolved nitrogen molecules with the formation of HNO₂ and HNO₃, while, in a FeSO₄ aqueous solution, by the Fe²⁺ → Fe³⁺ oxidation reaction. It is assumed that the mechanism of the process is analogous to that of the action of ionizing radiations and the chemical action of ultrasonically induced cavitation (it is proposed that this mechanism of chemical action of a laser-induced spark proposed be termed indirect). Energy yields of these reactions are found to be of the same order of magnitude as for sonochemical redox reactions. It is shown that the laser-induced breakdown of an aqueous solution of maleic acid is accompanied by its stereoisomerization into fumaric acid, a process catalyzed by small amounts of an alkyl bromide. It is established that, for the formation of fumaric acid in a laser-induced spark, the energy yield is about five orders of magnitude higher than that typical of the above-mentioned redox reactions in the laser-induced spark.     

Independence of chemical reactions

Up to this time the existence of physically true dependences between elementary reactions has been a returning question. The goal of this paper is to show that the real elementary reactions of a mechanism are practically independent. Introducing the concept of the space-time volume of a reaction (the reciprocal of the reaction rate) it is shown that, except for extremely fast processes (explosions), the reactions form very dilute systems. This systems must be ideal in the sense that interactions between elementary processes are very rare. The system exhibits some analogies with a dilute gas or solution. The extents of interactions can be calculated and the limits of independence estimated in this way.

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Power law behavior in chemical reactions

Reactions between metals and chloride solutions have been shown to exhibit magnetic field fluctuations over a wide range of size and time scales. Power law behavior observed in these reactions is consistent with models said to exhibit self-organized criticality. Voltage fluctuations observed during the dissolution of magnesium and aluminum in copper chloride solution are qualitatively similar to the recorded magnetic signals. In this paper, distributions of voltage and magnetic peak sizes, noise spectra, and return times are compared for both reactions studied.     

化学反应的内禀反应坐标法VI. 甲胺脱氢反应的反应路径解析

本文用内禀反应坐标(IRC)法对甲胺的1,1-和1,2-脱氢反应途径进行了微观动力学解析。在RHF/4-31G水平上确定了反应的过渡态结构和势能变化曲线,得到了活化能、反应热、频率因子和活化熵等物理量。模式选择研究表明:1,1-和1,2-脱氢反应过程的反应坐标(IRC)分别与反应物中频率为1057和1348cm^-^1的两个变形振动模式相联结。在每条反应途径上都存在正则坐标间的振动耦合。本文的计算为激光诱导和模式选择化学反应的研究提供了重要信息和理论指导。     

Growing graphene sheets from reactions with methyl radicals: a quantum chemical study.

Hydrogen abstraction reactions by methyl radicals on the zigzag and armchair edges of perylene are studied by density functional theory (DFT) to explore various growth pathways that seem to be in line with experimental observations. The DFT approach is validated by comparing the results obtained from calculations with six different functionals with those obtained from correlated ab initio methods, thereby emphasizing the calculation of reaction barriers. A useful compromise between accuracy and computational cost is provided by DFT, and possible pathways are studied in detail at this level of calculation. Our computational study is carried out by ordering, as a first step, all of the isomers that arise from the abstraction of one or two H atoms from 1,12-dimethyl-1,12-dihydroperylene and 3,4-dimethyl-3,4-dihydroperylene with respect to their energies. Subsequently, only those pathways that connect low-energy isomers are investigated. The calculations reveal that the selected pathways are favored thermodynamically, and also that the reaction barriers are somewhat higher than the energy locally available for the respective reaction. Notably, in the case of 3,4-dimethyl-3,4-dihydroperylene, the first two reaction steps have no or only a very low reaction barrier. The final conclusion of our study is that a cascade of reactions is possible that leads to the growth of a graphene sheet on a graphite surface.     

Calculating chemical electron-polarization in triplet-molecule reactions

It is assumed that orientation fluctuations for molecules in a liquid form an uncorrelated Markov process, and an expression is derived in quadratures for the initial chemical polarization of the electrons formed by the triplet mechanism. The expression applies for any magnetic field strength and solvent viscosity. If the molecular rotations are slow and the magnetic field is weak, one can perform numerical calculations on the electron chemical polarization.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 2, pp. 203–206, March–April, 1986.