Energetics and reaction pathways

D.4. Energetics and Reaction Pathways Metallic Edge States as Active Sites... 

Our goal has been to outline the general framework of DFT and to illustrate these principles with applications to geometries, electronic structures, energetics, and reaction pathways for transition... 

RRDE measurements with X-ray diffrac- cells. This gives tion results to investigate the detailed nature of the surface structures that are formed, particularly in coadsorption studies, for example, the influence of anion species on the UPD process. In Sect. 4.1.5, the oxygen reduction reaction (ORR) is used as a model electrochemical reaction to demonstrate the relation between the metal-O2 energetics and reaction pathway/kinetics as well as the importance of the local symmetry of surface atoms in determining the electrocatalytic properties of metal surfaces. 

Surface Interactions A Combined Quantum-Classical Molecular Dynamic Study of Energetics and Reaction Pathways. 

Parallel with these trends and related to them is the increase in chemical reactivity which is further enhanced by the increasing bond polarity and the increasing availability of low-lying vacant orbitals for energetically favourable reaction pathways. 

Thus, the parameters of acoustic intensity, temperature, ambient gas, and solvent choice have strong influences on sonochemical reactions. It is clear that one can fine tune the energetics of cavitation by the use of these variables and hence exercise control on the rates and reaction pathways followed by the associated chemistry. Specific examples will be discussed shortly. Clearly, the thermal conductivity of the ambient gas (e.g., a variable He/Ar atmosphere) and the overall solvent vapor pressure provides easy mechanisms for experimental control of the peak temperatures generated during the cavitational collapse. 

During the last decade knowledge of the ion chemistry of nitro compounds in the gas phase has increased significantly, partly due to the more widespread use of specialized techniques. Thus various ionization methods, in particular electron impact ionization and chemical ionization, have been used extensively. In addition, structure investigations as well as studies on fragmentation pathways have involved metastable ion dissociations, collision activation and neutralization/reionization studies, supplementary to studies carried out in order to disclose the associated reaction energetics and reaction dynamics. In general, the application of stable isotopes plays a crucial role in the in-depth elucidation of the reaction mechanisms. 

The lack of accurate experimental and ab initio information about the reaction pathways at the time limited the Sumpter and Thompson [70] studies. Since then quantum chemistry calculations have better defined the energetics and reaction coordinates for the decomposition of DMNA. Politzer et al. [79] studied DMNA using DFT. They predicted the N-N bond energy to be 43.8 kcal/mol, in excellent agreement with the experimental value of 43.3 kcal/mol [80]. Harris and Lammertsma [81] computed the... 

First-principle quantum chemical methods have advanced to the stage where they can now offer qualitative, as well as, quantitative predictions of structure and energetics for adsorbates on surfaces. Cluster and periodic density functional quantum chemical methods are used to analyze chemisorption and catalytic surface reactivity for a series of relevant commercial chemistries. DFT-predicted adsorption and overall reaction energies were found to be within 5 kcal/mol of the experimentally known values for all systems studied. Activation barriers were over-predicted but still within 10 kcal/mol. More specifically we examined the mechanisms and reaction pathways for hydrocarbon C-H bond activation, vinyl acetate synthesis, and ammonia oxidation. Extrinsic phenomena such as substituent effects, bimetallic promotion, and transient surface precursors, are found to alter adsorbate-surface bonding and surface reactivity. 

Another critical need identified in Database Needs for Modeling and Simulation of Plasma Processing (Database, 1996) is the measurement of thermodynamic data for species of interest in plasmas (radicals and ions). Such data provide benchmarks for comparison with calculated potential energy surfaces, allow energetically unfavorable reaction pathways to be identified, and supply information necessary to estimate unknown reaction rates by transition state theory. Such thermodynamic information is a critical tool in understanding deposition and etching processes and in evaluating the optimum conditions for plasma reactors (Kruis et al, 1992). 

The reaction mechanism and possible oxidation products for OH-initiated atmospheric reaction of 1,2-benzenedicarboxylic acid diethyl ester (DEP) were theoretically investigated by the DFT method. The geometries and frequencies of the reactants, intermediates, transition states, and products were calculated at the MPWBlK/6-3H-G(d,p) level, and the energetic parameters were further refined by the MPWBlK/6-311- -G(3df,2p) method. The present study shows that H abstractions from the Me and CH2 groups, as well as OH addition to the benzene ring, are energetically favourable reaction pathways for the reaction of DEP with OH radicals. ... 

Choosing appropriate reaction paths to study can be difficult and requires a considerable degree of chemical knowledge and intuition. An easier, and in some ways more direct, approach is to attempt to model chemical sensitisation/desensitisation effects on energetic molecules. Reaction pathways calculated with and without the sensitiser/desensitiser incorporated can then be compared and from the differences, the most likely mechanism deduced. This method has the advantage that computational time and effort is concentrated on a distinct problem which hopefully, will aid in understanding the decomposition mechanism of the pure explosive. This method has been used by Haskins and Cook (Refs 16-18) to investigate the initial steps in the decomposition of nitromethane. 

This is a question of reaction prediction. In fact, this is a deterministic system. If we knew the rules of chemistry completely, and understood chemical reactivity fully, we should be able to answer this question and to predict the outcome of a reaction. Thus, we might use quantum mechanical calculations for exploring the structure and energetics of various transition states in order to find out which reaction pathway is followed. This requires calculations of quite a high degree of sophistication. In addition, modeling the influence of solvents on... 

Aqueous ammonia and acryUc esters give tertiary amino esters, which form the corresponding amide upon ammonolysis (34). Modem methods of molecular quantum modelling have been appHed to the reaction pathway and energetics for several nucleophiles in these Michael additions (35,36). 

Reactions involve several enzymes, which have to follow in sequence for lactic acid and alcohol fermentation. This is known as the glucose catabolism pathway, with emphasis on energetic and energy carrier molecules such as ATP, ADP, NAD+ and NADH. In this pathway the six-carbon substrate yields two three-carbon intermediates, each of which passes through a sequence of reactions to the stable end product of pyruvic acid. 

A qualitatively different approach to probing multiple pathways is to interrogate the reaction intermediates directly, while they are following different pathways on the PES, using femtosecond time-resolved pump-probe spectroscopy [19]. In this case, the pump laser initiates the reaction, while the probe laser measures absorption, excites fluorescence, induces ionization, or creates some other observable that selectively probes each reaction pathway. For example, the ion states produced upon photoionization of a neutral species depend on the Franck-Condon overlap between the nuclear configuration of the neutral and the various ion states available. Photoelectron spectroscopy is a sensitive probe of the structural differences between neutrals and cations. If the structure and energetics of the ion states are well determined and sufficiently diverse in... 

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