Pathways and Reactions

Halgren T A and Lipscomb W N 1977 The synchronous transit method for determining reaction pathways and locating molecular transition states Chem. Phys. Lett. 49 225... 

At the outset of the development of each module for one of the above mechanistic classes of reactions, a thorough analysis of the literature was performed. On that basis, the developer came up with an evaluation framework that was used to make decisions between various reaction pathways and mechanistic possibilities. 

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. 

This accurate measurement of the ratio of abundances of isotopes is used for geological dating, estimation of the ages of antiquities, testing athletes for the use of banned steroids, examining fine details of chemical reaction pathways, and so on. These uses are discussed in this book under various headings concerned with isotope ratio mass spectrometry (see Chapters 7, 14, 15, 16, 17, 47, and 48). 

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). 

CAD /CAM techniques have provided the framework for using the computer as a tool in the drawing and analysis of chemical stmctures and, more recently, in the use of chemical stmctures to design reaction pathways and new products. The essential elements in these appHcations of CAD/CAM are that the possible stmctures are relatively deterministic and that allowable changes in stmcture through reaction are governed by thermodynamic, stoichiometric, and steric constraints. 

The reactivity of isoxazole in the presence of light, heat or electron impact has been well studied and the various transformations analyzed in terms of reaction pathways and of the potential intermediates. These studies have also been extended to a large variety of substituted derivatives (79AHC(25)147). 

Recently, Chandler and coworkers [46,47] revisited this idea and developed an elegant and promising methodology for the computation of reaction pathways and transition rates in molecular systems. 

From that highest energy point, called the peak, search conjugate to the direction of the reaction pathway and find the lowest energy point. 

The animations in this problem have been artificially constrained to reveal electronic changes. They do not show realistic geometries for these reaction pathways, and should not be used to estimate reaction barriers. 

The Synchronous-Transit Method for determining Reaction Pathways and Locating Molecular Transition States Thomas A. Halgren and William N. Lipscomb Chemical Physics Letters 49 (1977) 225-232... 

In the fifty or so years since the discovery of a-metalated epoxides, our understanding of their reactivity has advanced to such a level that their use in routine organic synthesis is now possible. Many research groups continue to examine their unusual reaction pathways and to develop these into synthetically useful processes. In contrast, the chemistry of a-metalated aziridines is still in its infancy and there are undoubtedly many interesting facets of their nature still to be explored and applied in organic synthesis. 

Thus, the Streckcr synthesis and the Ugi reaction characteristically parallel each other in their reaction pathways and in the influence of the stereodiseriminating factors. 

While diene metathesis or diyne metathesis are driven by the loss of a (volatile) alkene or alkyne by-product, enyne metathesis (Fig. 2) cannot benefit from this contributing feature to the AS term of the reaction, since the event is entirely atom economic. Instead, the reaction is driven by the formation of conjugated dienes, which ensures that once these dienes have been formed, the process is no longer a reversible one. Enyne metathesis can also be considered as an alkylidene migration reaction, because the alkylidene unit migrates from the alkene part to one of the alkyne carbons. The mechanism of enyne metathesis is not well described, as two possible complexation sites (alkene or alkyne) exist for the ruthenium carbene, leading to different reaction pathways, and the situation is further complicated when the reaction is conducted under an atmosphere of ethylene. Despite its enormous potential to form mul-... 

B (a) The homogeneous catalyst changes the reaction pathway, and therefore changes the rate law. (b) Since a catalyst does not change the thermodynamics of the reaction, a homogeneous catalyst does not change the equilibrium constant. 

It is the basic task of electrochemical kinetics to establish the functional relations between the rate of an electrochemical reaction at a given electrode and the various external control parameters the electrode potential, the reactant concentrations, the temperature, and so on. From an analysis of these relations, certain conclusions are drawn as to the reaction mechanism prevailing at a given electrode (the reaction pathway and the nature of the slow step). 

Rearrangements Rearrangements of substrates always have atom economies of 100% Some rearrangement reactions involve rearrangements of intermediates along their reaction pathways and so their corresponding atom economies are less than 100%... 

The presence of solution at a metal surface, as has been discussed, can significantly influence the pathways and energetics of a variety of catalytic reactions, especially electrocatalytic reactions that have the additional complexity of electrode potential. We describe here how the presence of a solution and an electrochemical potential influence the reaction pathways and the reaction mechanism for methanol dehydrogenation over ideal single-crystal surfaces. 

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. 

Wagner et al. have investigated a series of catalysts, their reaction pathways, and reaction products ... 

TaniaPhos active catalyst discussion As shown by Salzer (2) such complexes with half sandwich stracture result in the catalyst cycle into a hydride species where the pentadienyl moiety can be hydrogenolyticaUy liberated (2, 6). This was verified in the case of BINAP complexes (2, diss. Podewils, Geyser). In accordance to this fact and other mechanistic aspects from Noyori s work (3, 5) it is likely that the pre-catalyst species undergoes the same reaction pathway and that the reactive part of the pre-catalyst, the pentadienyl moiety, will be liberated under hydrogenolytic conditions as shown below in Scheme 23.9 ... 

We will now discuss some very recent applications of the soft El ionization method for product detection in CMB experiments. We will first deal with two polyatomic reactions of ground state oxygen atoms with unsaturated hydrocarbons (acetylene and ethylene) these reactions are characterized by multiple reaction pathways and are of great relevance, besides being from a fundamental point of view, in combustion and atmospheric chemistry. 

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