Bond breaking problem

Due to the inherent multireference nature of bond-breaking problems, it seems rather unlikely that single-determinant CC methods of any sort will ever provide a satisfactory solution to these problems. It seems much more sensible to use MR approaches to study problems of this sort when they are relevant for a particular chemical application. For small molecules, MRCI offers a suitable option, while CASSCF calculations corrected by perturbation theory are more generally useful for larger systems. 

The Car-Parrinello quantum molecular dynamics technique, introduced by Car and Parrinello in 1985 [1], has been applied to a variety of problems, mainly in physics. The apparent efficiency of the technique, and the fact that it combines a description at the quantum mechanical level with explicit molecular dynamics, suggests that this technique might be ideally suited to study chemical reactions. The bond breaking and formation phenomena characteristic of chemical reactions require a quantum mechanical description, and these phenomena inherently involve molecular dynamics. In 1994 it was shown for the first time that this technique may indeed be applied efficiently to the study of, in that particular application catalytic, chemical reactions [2]. We will discuss the results from this and related studies we have performed. 

Neglect of electrons means that molecular mechanics methods cannot treat chemical problems where electronic effects predominate. For example, they cannot describe processes which involve bond formation or bond breaking. Molecular properties which depend on subtle electronic details are also not reproducible by molecular mechanics methods. 

Free radical initiators play an important role in many chemical reactions (see also Chapter 17, Problem 5). For example, combustion of gasoline is assisted by compounds such as tetraethyl lead, heating of which results in bond breaking and generation of ethyl radical. 

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... 

It is also worth emphasizing that recent theoretical work on photoinduced stepwise and concerted electron transfer/bond-breaking reactions opens the route to a more systematic combination than before of the electrochemical and photochemical approaches to the same problems. 

When addressing problems in computational chemistry, the choice of computational scheme depends on the applicability of the method (i.e. the types of atoms and/or molecules, and the type of property, that can be treated satisfactorily) and the size of the system to be investigated. In biochemical applications the method of choice - if we are interested in the dynamics and effects of temperature on an entire protein with, say, 10,000 atoms - will be to run a classical molecular dynamics (MD) simulation. The key problem then becomes that of choosing a relevant force field in which the different atomic interactions are described. If, on the other hand, we are interested in electronic and/or spectroscopic properties or explicit bond breaking and bond formation in an enzymatic active site, we must resort to a quantum chemical methodology in which electrons are treated explicitly. These phenomena are usually highly localized, and thus only involve a small number of chemical groups compared with the complete macromolecule. 

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. 

The timing of bond making and bond breaking in direct substitutions at a sulfonyl group presents the same problems as it did in substitutions at sulfinyl OS=0) and sulfenyl ( S) sulfur. Are such reactions concerted (196a) or are they stepwise (196b) with an intermediate [77] present on the reaction... 

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