Bond-breaking processes

BurJ81 Burdett, J. K., McLarnan, T. J. A study of the arsenic, black phosphorus and other structures derived from rock salt by bond-breaking processes. I. Structural enumeration. J. Chem. Phys. 75 (1981) 5764-5773. 

The approach discussed above can provide a qualitative description of the effect of external fields on bond-breaking processes. For example, consider the H2 molecule (HA — HB) in the presence of an Li+ ion 3 A away from HB on the A-B axis. To study this problem, we assume that there is no charge migration to the Li location (so that Pc = 0) and that fiAC = pBC = 0 since the Li+ ion is sufficiently far from HA and HB. In this case, we can write the H matrix as... 

The main features of the chemical bonding formed by electron pairs were captured in the early days of quantum mechanics by Heitler and London. Their model, which came to be known, as the valence bond (VB) model in its later versions, will serve as our basic tool for developing potential surfaces for molecules undergoing chemical reactions. Here we will review the basic concepts of VB theory and give examples of potential surfaces for bond-breaking processes. 

Barium, effectiveness as cofactor for, see also Enzyme cofactors phospholipase, 204 SNase, 200-204 Bond-breaking processes, 12 potential surfaces for, 13-14, 18-20 in solutions, 22,46-54... 

The Diels-Alder reaction is a pericyclic cycloaddition when bond-forming and bond-breaking processes are concerted in the six-membered transition state... 

As might be expected, the results from both theory and experiment suggest that the solution is more than a simple spectator, and can participate in the surface physicochemical processes in a number of important ways [Cao et al., 2005]. It is well established from physical organic chemistry that the presence of a protic or polar solvent can act to stabilize charged intermediates and transition states. Most C—H, O—H, C—O, and C—C bond breaking processes that occur at the vapor/metal interface are carried out homolytically, whereas, in the presence of aqueous media, the hetero-lytic pathways tend to become more prevalent. Aqueous systems also present the opportunity for rapid proton transfer through the solution phase, which opens up other options in terms of reaction and diffusion. 

Molecular dynamics simulations, with quantum-mechanically derived energy and forces, can provide valuable insights into the dynamics and structure of systems in which electronic excitations or bond breaking processes are important. In these cases, conventional techniques with classical analytical potentials, are not appropriate. Since the quantum mechanical calculation has to be performed many times, one at each time step, the choice of a computationally fast method is crucial. Moreover, the method should be able to simulate electronic excitations and breaking or forming of bonds, in order to provide a proper treatment of those properties for which classical potentials fail. 

Therefore, for a thermoneutral reaction, the intersection point between the bond order profiles for the bond making and the bond breaking processes coincides with the TS the reactivity of the two reacting atoms also equalizes at the TS, as can be seen from the intersection of their IT profiles. These intersection points of the associated bond orders and condensed FFs he toward the left (right) of the TS for an endothermic (exothermic) reaction, in agreement with the Hammond postulate. 

This class includes reactions in which the bond forming and bond breaking processes other consecutively so that one or more intermediates are involved. These intermediates may be highly stable and capable of isolation or they may be reactive species of only transient existence. When the intermediate is stable, it is convenient to regard the entire process as consisting of two or more consecutive concerted processes. But where the intermediates are unstable, the entire process is regarded as one reaction which proceeds in a stepwise manner. The energy curve for such a reaction is as shown (Fig. 3.2). 

The initial steps of the Kolbe reaction, the oldest organic electrochemical reaction, constitute a good illustration of the loss of an acid moiety upon oxidative electron transfer (Scheme 2.24). The issue of the stepwise versus concerted character of the electron transfer/bond-breaking process in this reaction is discussed in Chapter 3. 

Since bond making and breaking are two aspects of the same process (9), we have to consider just two basic types of reaction, in which bond-making and bond-breaking processes are either concerted (10a) or stepwise (10b,c). 

Concerted bond-forming/bond-breaking processes at tetrahedral carbon (the familiar SN2 reaction) are not easily studied by the crystal structure correlation method. The preferred approach of a nucleophile is sterically more encumbered than the approach to a singly or doubly bonded centre, and the transition states involved are generally of high energy. Intramolecular displacements, such as those described on pages 117-118, are a possible way round this problem, but no systematic study is available. 

Reaction of a sulfinyl sulfone with a nucleophile in the manner shown in (139) is, of course, an example of a nucleophilic substitution at sulfinyl sulfur. Reactions of this general type occur frequently and are of great importance in the chemistry of most kinds of sulfinic acid derivatives. At this point it would seem desirable to discuss what is known about such key aspects of their mechansim as stereochemistry and the timing of the bond-making and bond-breaking processes necessary in such a substitution. In doing this we will call upon results obtained from the study of such reactions using a variety of different types of sulfinic acid derivatives. 

In the case of stepwise electron-transfer bond-breaking processes, the kinetics of the electron transfer can be analysed according to the Marcus-Hush theory of outer sphere electron transfer. This is a first reason why we will start by recalling the bases and main outcomes of this theory. It will also serve as a starting point for attempting to analyse inner sphere processes. Alkyl and aryl halides will serve as the main experimental examples because they are common reactants in substitution reactions and because, at the same time, a large body of rate data, both electrochemical and chemical, are available. A few additional experimental examples will also be discussed. 

Several attempts have been made to analyse the captodative effect through rotational barriers in free radicals. This approach seems to be well suited as it is concerned directly with the radical, i.e. peculiarities associated with bond-breaking processes do not apply. However, in these cases also one has to be aware that any influence of a substituent on the barrier height for rotation is the result of its action in the ground state of the molecule and in the transition structure for rotation. Stabilization as well as destabilization of the two states could be involved. Each case has to be looked at individually and it is clear that this will provide a trend analysis rather than an absolute determination of the magnitude of substituent effects. In this respect the analysis of rotational barriers bears similar drawbacks to all of the other methods. 

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