Reaction Pathways and Mechanisms

Computational studies investigate reaction mechanisms and pathways by constructing potential energy profiles. This involves exploring reaction thermodynamics and kinetics, by examining reactants and products as well as the transition states geometries and activation energy barriers. Like those seen in structure prediction, most current studies implement effective core potentials and density functional theory to perform calculations.However, ECPs can be paired with MP2 to account for electron correlation thus far, this approach has only been used for smaller chemical systems. Eurthermore, solvation methods such as the polarizable continuum model can be employed to examine 

LANL2DZ PM3) was used to optimize structures, to reduce computational time and to reduce the memory requirements for the calculation. The use of B3LYP with ECPs facilitated the incorporation of both electron correlation and relativistic effects. 

In addition, the stretching vibrations from the DFT frequency analysis agreed with the experimental values. 

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The challenges for computational chernislry are to characteri/e and predict the structure and stability of chemical systems, to estimate energy differences between different states, and to explain reaction pathways and mechanisms at the atomic level. Meeting these challenges could eliminate tinie-consiini mg experiments. 

The chemistry at the electrified aqueous/metal interface is quite fascinating, as its structure, properties, and dynamics can significantly influence reaction energetics, dictate the kinetics that control catalytic selectivity, and open up novel reaction pathways and mechanisms. 

This review analyzed the chemistry involved, thermodynamics, catalysts used, reaction pathways and mechanisms of various reforming techniques reported for the conversion of ethanol into H2-rich gas. The known reforming processes are broadly classified into three categories, namely steam reforming of ethanol (SRE), partial oxidation of ethanol (POE) and oxidative steam reforming (OSR)/autothermal reforming of ethanol. All these reactions are thermodynamically favorable even at lower temperatures, above 200 °C. 

The majority of studies on the TiOz photocatalysis of organophosphorus compounds rely primarily on the disappearance of the initial substrate coupled with the monitoring of the mineralization products. Because of the complex mixtures formed in the TiOz photocatalysis of these compounds, there is limited information on the reaction pathways and mechanisms involved in the degradation processes. A recent report by Konstantinou et al. identified a number of intermediate products in the degradation of dichlofenthion and bromophos methyl by TiOz... 

The present review summarizes contemporary views of the problems, achievements, and prospects involved in the deep desulfurization of gas oils, including identification and reactivity of sulfur species in the feed, the reaction pathways and mechanisms, activity and selectivity of the conventional catalysts, and concerns of fluorescence color production. Process schemes and guidelines for the development of the next-generation catalysts for improved deep desulfurization technology based on these discussions are also proposed. The structure and nature of the active sites of current catalysts will not be extensively covered in this review, because several excellent reviews have been published on these subjects within the past two years (1-3). 

As discussed in Section III, when the sulfur content is lowered from 0.20 to 0.05%, the chemistry of HDS of gas oils is essentially the chemistry of alkyl-substituted dibenzothiophenes. Though gas oils initially contain mostly alkyl-substituted benzothiophenes, these are completely removed by the time 0.20% S is achieved. Thus, this review will deal predominantly with the reaction pathways involved in the HDS of alkyl-substituted dibenzothiophenes. There are many excellent reviews on reaction pathways of the more reactive sulfur species such as thiophenes and benzothiophenes (2, 5, 8, 23, 24), and the reader is referred to those reviews for information on the reaction pathways and mechanisms of HDS for the more reactive... 

Chemical kinetics and thermochemistry are important components in reacting flow simulations. Reaction mechanisms for combustion systems typically involve scores of chemical species and hundreds of reactions. The reaction rates (kinetics) govern how fast the combustion proceeds, while the thermochemistry governs heat release. In many cases the analyst can use a reaction mechanism that has been developed and tested by others. In other situations a particular chemical system may not have been studied before, and through coordinated experiments and simulation the goal is to determine the key reaction pathways and mechanism. Spanning this spectrum in reactive flow modeling is the need for some familiarity with topics from physical chemistry to understand the inputs to the simulation, as well as the calculated results. 

Marteii AE (1982) Reaction pathways and mechanisms of pyridoxai cataiysis. Advances in Enzymology and Related Areas of Molecular Biology 53,163-99. 

Watson BA, Klein MT, Harding RH. Catalytic cracking of alkylbenzenes Modeling the reaction pathways and mechanisms. Appl Catal A-Gen 1997 160 13-39. 

In conclusion, an extensive literature is available on the reaction networks that are thought to operate in HD,S of various types of thiophenic molecules besides the great advances that have been made in direct studies on molybdenum sulfides and related catalysts, this is another area in which organometallic chemistry has made an impressive contribution to HD,S catalysis, as a number of reaction pathways and mechanisms for the hydrogenation and hydrogenolysis of thiophenes on metal complexes in solution has been well established with the aid of a variety of physical techniques. 

In this study, the reaction was characterized using a combination of in-situ kinetic probes, heat flow from reaction calorimetry and measurement of the hydrogen uptake rate, in addition to the commonly-used method of analysis of samples taken from the reactor. Heat flow from the reaction calorimetry and measurement of rate of hydrogen uptake are intrinsically superior kinetic tools in that they both provide rate data directly and in a quasi-continuous fashion. Consequently, they are capable of producing clear and detailed kinetic pictures which offers hints on reaction pathways and mechanism (4,5). It will be shown that thermodynamic information regarding each step in the hydrogenation reaction network may also be obtained directly from a combination of the heat flow and the hydrogen uptake data. 

Burrows, H.D., Canie, L., Santaballa, J.A. and Steenken, S. (2002) Reaction pathways and mechanisms of photodegradation of pesticides. Journal of Photochemistry and Photobiology B Biology 67, 71-108. 

Correlations of concentrations are measures of the statistical dependence of the concentration of one species on that of one or more of the other species in the system. Such correlations can be determined from measurements of time series of concentrations collected around a stationary state (nonequilibrium or equilibrium). We shall show that from concentration correlations it is possible to construct a skeletal diagram of the reaction system that gives a graphical measure of strong control and regulatory stmcture in reaction networks, gives some information on connectivity, leads to information on the reaction pathway and mechanism, and may simplify the analysis of such networks by identifying possible, nearly separable subsystems [1]. 

In this chapter we present an experimental test case of the deduction of a reaction pathway and mechanism by means of correlation metric construction from time-series measurements of the concentrations of chemical species [1], We choose as the system an enzymatic reaction network, the initial steps of glycolysis (fig. 8.1). Glycolysis is central in intermediary metabolism and has a high degree of regulation [2]. The reaction pathway has been well studied and thus it is a good test for the theory. Further, the reaction mechanism of this part of glycolysis has been modeled extensively [3]. 

Such studies have led to a better understanding of the structural flexibility of various chemical groupings and, in some cases, to a description of chemical reaction pathways and mechanisms. For example, the geometry of the peptide moiety, as observed in many crystal structures of amides, differs from that of an isolated molecule in the gas phase the pattern of changes is consistent with an increase in C-N and an increase in C =0 double bond character as intermolecular N-H "0=C H-bonding becomes stronger (incipient protonation of the C=0 group) [43]. 

Major areas of electrochemical measurements with CPEs (in order of appearance) (1) Electrode reaction pathways and mechanisms of electroactive organic compounds (see also Figure 11.3a-d) (II) Pharmaceutical and clinical analysis (III) Solid-phase voltammetry with electroactive CPEs (IV) Analysis of (i) inorganic ions and molecules, (ii) organic compounds -i- environmental pollutants, and (iii) analysis of biologically important compounds (BICs) (V) Voltammetry in vivo (also known as brain electrochemistry). 

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