Mechanistic study of the gas-phase decomposition of methyl formate

The major products of the thermal decomposition of methyl formate in the gas phase are CH(3)OH, CH(2)O, and CO. Experimental studies have proposed that the mechanism to describe these observations involves two key steps: (1) unimolecular decomposition of methyl formate to yield CH(3)OH + CO, followed by (2) thermal decomposition of methanol to yield CH(2)O + H(2). The present study shows that there exists an alternative mechanism that is energetically more favorable. The new mechanism involves two competing parallel unimolecular decomposition pathways to yield the observed major products.     

Theoretical study of the decomposition reactions in substituted nitrobenzenes

The influence of substituent nature and position on the unimolecular decomposition of nitroaromatic compounds was investigated using the density functional theory at a PBE0/6-31+G(d,p) level. As the starting point, the two main reaction paths for the decomposition of nitrobenzene were analyzed: the direct carbon nitrogen dissociation (C6H5 + NO2) and a two step mechanism leading to the formation of phenoxyl and nitro radicals (C6H5O + NO). The dissociation energy of the former reaction was calculated to be 7.5 kcal/mol lower than the activation energy of the second reaction. Then the Gibbs free energies were computed for 15 nitrobenzene derivatives characterized by different substituents (nitro, methyl, amino, carboxylic acid, and hydroxyl) in the ortho, meta, and para positions. In meta position, no significant changes appeared in the reaction energy profiles whereas ortho and para substitutions led to significant deviations in energies on the decomposition mechanisms due to the resonance effect of the nitro group without changing the competition between these mechanisms. In the case of para and meta substitutions, the carbon-nitro bond dissociation energy has been directly related to the Hammett constant as an indicator of the electron donor-acceptor effect of substituents.     

Mechanistic study on the thermal decomposition of polybenzoxazines: Effects of aliphatic amines

The thermal degradation of a series of polybenzoxazines based on bisphenol A and various aliphatic amines has been studied. Using the hyphenated techniques of thermogravimetric analysis-Fourier transform infrared spectroscopy (TGA-FTIR), and gas chromatography-mass spectrometry (GC-MS), the mechanisms of thermal decomposition have been proposed. It is also proposed that the Mannich base in polybenzoxazines plays a significant role in the thermal degradation of polybenzoxazines. The contribution of hydrogen bonding to the degradation mechanism of the Mannich base has been examined. The proposed mechanisms have also been supported through the thermal degradation study of benzoxazine model dimers. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1935–1946, 1998     

Mechanistic considerations of guanidine-catalyzed reactions

This feature article discusses the various mechanistic aspects of guanidine-catalyzed reactions. Guanidines are well known as strong organic bases; however, in the first section, three most common commercially available guanidines, TMG, TBD and MTBD, will be used to illustrate the use of guanidines as nucleophilic catalysts. In the second section, different modes of hydrogen bonding interactions of the conjugate acid of guanidine, the guanidinium, are discussed. Particularly interesting are the possibilities of mono-functional or bifunctional activation of a nucleophile and an electrophile by guanidinium.     

Mechanistic study of homogeneous reactions coupled with electrochemical oxidation of catechols

The electrochemical oxidation of catechols was described and has shown that these compounds can be oxidized to related o-benzoquinones. The electrochemically generated o-benzoquinones are quite reactive and can be attacked by a variety of nucleophiles under various mechanistic disciplines such as CE, EC, EC′, ECE, ECEC, ECEC ₂, ECECE, ECECEC, ECECECE and trimerization, in which E represents an electron transfer at the electrode surface, and C represents a homogeneous chemical reaction. The mechanistic pathways and final products are depending on some parameters such as electron withdrawing or donating properties of nucleophile, electrolysis medium (solvent, acidity or pH) and nature of catechol.     

Theoretical study of the isomerization mechanism of azobenzene and disubstituted azobenzene derivatives

A series of azobenzenes was studied using ab initio methods to determine the substituent effects on the isomerization pathways. Energy barriers were determined from three-dimensional potential energy surfaces of the ground and electronically excited states. In the ground state (S(0)), the inversion pathway was found to be preferred. Our results show that electron donating substituents increase the isomerization barrier along the inversion pathway, whereas electron withdrawing substituents decrease it. The inversion pathway of the first excited state (S(1)) showed trans --> cis barriers with no curve crossing between S(0) and S(1). In contrast, a conical intersection was found between the ground and first excited states along the rotation pathway for each of the azobenzenes studied. No barriers were found in this pathway, and we therefore postulate that after n --> pi (S(1) <-- S(0)) excitation, the rotation mechanism dominates. Upon pi --> pi (S(2) <-- S(0)) excitation, there may be sufficient energy to open an additional pathway (concerted-inversion) as proposed by Diau. Our potential energy surface explains the experimentally observed difference in trans-to-cis quantum yields between S(1) and S(2) excitations. The concerted inversion channel is not available to the remaining azobenzenes, and so they must employ the rotation pathway for both n --> pi and pi --> pi excitations.     

The study of decomposition reactions through the exchange of thermal energy

In many decomposition reactions, the reaction velocity can be described as a product of two functions: a temperature dependent part K(T) and the kinetic function f(1 – α), where T designates the temperature and α the fraction of reactant that has decomposed. The physical interpretation of these functions is discussed for both solid and homogeneous systems. A method is described by which f(1 – α) and K(T) can be determined from kinetic data. The mechanism of decomposition can subsequently be identified which should be consistent with the derived kinetic parameters. The method has been applied to analyze the kinetics of the thermal decomposition of nitromethane. © 1994 John Wiley & Sons, Inc.     

Mechanistic insight into the chemiluminescent decomposition of firefly dioxetanone

The peroxide decomposition that generates the excited-state carbonyl compound is the key step in most organic chemiluminescence, and chemically initiated electron exchange luminescence (CIEEL) has been widely accepted for decades as the general mechanism for this decomposition. The firefly dioxetanone, which is a peroxide, is the intermediate in firefly bioluminescence, and its decomposition is the most important step leading to the emission of visible light by a firefly. However, the firefly dioxetanone decomposition mechanism has never been explored at a reliable theoretical level, because the decomposition process includes biradical, charge-transfer (CT) and several nearly degenerate states. Herein, we have investigated the thermolysis of firefly dioxetanone in its neutral (FDOH) and anionic (FDO(-)) forms using second-order multiconfigurational perturbation theories in combination with the ground-state intrinsic reaction coordinate calculated via the combined hybrid functional with Coulomb attenuated exchange-correlation, and considered the solvent effect on the ground-state reaction path using the combined hybrid functional with Coulomb attenuated exchange-correlation. The calculated results indicate that the chemiluminescent decomposition of FDOH or FDO(-) does not take place via the CIEEL mechanism. An entropic trap was found to lead to an excited-state carbonyl compound for FDOH, and a gradually reversible CT initiated luminescence (GRCTIL) was proposed as a new mechanism for the decomposition of FDO(-).     

Mechanistic studies of hydrazide-catalyzed enantioselective Diels-Alder reactions

The mechanism of the enantioselective, hydrazide-catalyzed Diels-Alder cycloaddition was investigated in detail. Both the formation of the reactive iminium species and the hydrolysis of the product iminium intermediates were found to be extremely rapid, leaving the cycloaddition as the kinetically significant step. Mechanistic studies using NMR showed that a retro-Diels-Alder reaction occurred during the catalytic cycle, suggesting a thermodynamic component to the reaction.     

H2ClCS单分子分解反应机理的理论研究

用量子化学B3LYP/6 - 311+G(d,p)方法优化了H2ClCS单分子分解反应驻点物种的几何构型,并在相同水平上通过频率计算和内禀反应坐标(IRC)分析对过渡态结构及连接性进行了验证.用QCISD(T)/6-311++G(d,p)方法计算各物种的单点能,并对总能量进行了零点能校正.利用经典过渡态理论(TST)与...     

Mechanistic study of trans⇆cis isomerization of the substituted azobenzene moiety bound on a liquid‐crystalline polymer

A mechanistic study of the trans?cis isomerization of the azobenzene moiety in a side‐chain liquid‐crystal polymer system was carried out with six liquid‐crystalline polymethacrylates in which different electron‐withdrawing substituents were attached to the para‐positions of the azobenzene chromophores. Compared to the non‐nitro‐substituted azo polymers, the nitro‐substituted azo polymers exhibited two quite different behaviors: an extraordinarily high reaction rate of the thermal cis–trans isomerization and an unexpected composition of cis–trans isomers obtained from the photochemical trans–cis isomerization process. A potential energy profile for the isomerization process was established on basis of the structures of the proposed transition states and was employed to elucidate the reaction mechanism. The results confirmed that the nitro‐substituted azo polymer system proceeded via a rotation mechanism in either direction of the trans?cis isomerization reaction, whereas the non‐nitro‐substituted species were more likely to follow an inversion mechanism. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2296–2307, 2001     

Application of programmed temperature decomposition to the study of solid decomposition reactions taking place through the prout and tompkins mechanism

A method has been developed to allow kinetic analysis of the Prout and Tompkins mechanism of solid decomposition reactions using a single TG diagram obtained in a linear heating program. On the other hand, provided that the nickel formate decomposition follows such a mechanism, this reaction has been used for checking the kinetic equation obtained.     

Mechanistic alteration in micellar peptide esterolysis reactions

Buffer or CTACl-catalyzed basic cleavages of $\text{Z}$-$\text{D}$- or $\text{L}$-AA-$\text{L}$-Pro $\text{p}$-nitrophenyl esters proceed mainly via intramolecular cyclization to diketopiperazines. These reactions are more facile with $\text{DL}$ than with $\text{LL}$ substrates.     

Mechanistic approaches to palladium-catalyzed alkene difunctionalization reactions

Alkene difunctionalization, the addition of two functional groups across a double bond, exemplifies a class of reactions with significant synthetic potential. This emerging area examines recent developments of palladium-catalyzed difunctionalization reactions, with a focus on mechanistic strategies that allow for functionalization of a common palladium alkyl intermediate.     

Mechanistic modeling of the wall reactions in the pyrolysis of pentachloroethane

The thermal dehydrochlorination C₂HCl₅ → C₂Cl₄ + HCl has been studied in a static system between 565 and 645 K at pressures ranging from 5 to 21 torr. The course of the reaction was followed by measuring the pressure rise in the conditioned quartz reaction vessel and by analyzing the products by gas chromatography. The observed experimental results and data from the literature for flow systems can be explained quantitatively in terms of a radical reaction model involving heterogeneous chain initiation and termination steps. The rate constants have been deduced for reactions of Cl, Cl₂, and C₂HCl₅ over reactor walls covered with a pyrolytic carbon film and for reactions of adsorbed Cl atoms. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 322–330, 2002     

Mechanistic aspects of transition metal-catalyzed hydrogen transfer reactions

In this tutorial review recent mechanistic studies on transition metal-catalyzed hydrogen transfer reactions are discussed. A common feature of these reactions is that they involve metal hydrides, which may be monohydrides or dihydrides. An important question is whether the substrate coordinates to the metal (inner-sphere hydrogen transfer) or if there is a direct concerted transfer of hydrogen from the metal to substrate (outer-sphere hydrogen transfer). Both experimental and theoretical studies are reviewed.     

Mechanistic insight into fragmentation reactions of titanapinacolate complexes

Reactions between terminal alkynes or aromatic ketones and titanapinacolate complexes (DMSC)Ti(OCAr(2)CAr(2)O) (2, Ar = Ph, and 3, Ar = p-MeC(6)H(4); DMSC = 1,2-alternate dimethylsilyl-bridged p-tert-butylcalix[4]arene dianion) occur via rupture of the C-C bond of the titanacycle. Thus, reactions of 2 and 3 with terminal alkynes produce 2-oxatitanacyclopent-4-ene or 2-oxatitanacycloheptadiene complexes along with free Ar(2)CO. These compounds have been characterized spectroscopically and by X-ray crystallography. Because metallapinacolate intermediates have been implicated in important C-C bond-forming reactions, such as pinacol coupling and McMurry chemistry, the mechanism of the fragmentation reactions was studied. Analysis of the kinetics of the reaction of (DMSC)Ti[OC(p-MeC(6)H(4))(2)C(p-MeC(6)H(4))(2)O] (3) with Bu(t)Ctbd1;CH revealed that the fragmentation reactions proceed via a preequilibrium mechanism, involving reversible dissociation of titanapinacolate complexes into (DMSC)Ti(eta(2)-OCAr(2)) species with release of a ketone molecule, followed by rate-limiting reaction of (DMSC)Ti(eta(2)-OCAr(2)) species with an alkyne or ketone molecule.