Perturbational Molecular Orbital PMO Approach

The group transfer reactions can be easily explained by perturbation molecular orbital (PMO) approach. If both the R groups migrate without inversion, in a... 

In a recent book Dewar (1969) has given a general account of the perturbation molecular orbital (PMO) approach and its relevance in discusrions on physical-organic chemistry. Of particular interest is one application which provides a general theory of pericyclic reactions (Dewar, 1969, 1971), The PMO treatment in this context has but one central concept, namely the potential aromaticity of pericyclic transition states (see p. 61), and may be generalized into a single definitive rule ... 

One of the most used approaches for predicting homoaromaticity has been the perturbational molecular orbital (PMO) theory of Dewar (1969) as developed by Haddon (1975). This method is based on perturbations in the Hiickel MO theory based on reducing the resonance integral (/3) of one bond. This bond represents the homoaromatic linkage. The main advantage of this method is its simplicity. PMO theory predicted many potential homoaromatic species and gave rise to several experimental investigations. 

Antiaromaticity [1] is the phenomenon of destabilization of certain molecules by interelectronic interactions, that is, it is the opposite of aromaticity [2], The SHM indicates that when the n-system of butadiene is closed the energy rises, i.e. that cyclobutadiene is antiaromatic with reference to butadiene. In a related approach, the perturbation molecular orbital (PMO) method of Dewar predicts that union of a C3 and a Ci unit to form cyclobutadiene is less favorable than union to form butadiene [3]. 

It is clear that there are quite a few possible theoretical approaches to the formulation of a comprehensive model for the adsorption processes discussed above. The most fundamental one would be based on the perturbation molecular orbital (PMO) theory of chemical reactivity [730,731] in which the wave functions of the products are approximated using the wave functions of the reactants. A key issue in the use of Klopman s PMO theory is the relative importance of the two terms in the expression for the total energy change of the system, Afpen. which is taken to be a good index of reactivity, [732,733] ... 

In this chapter the molecular orbital theory covered in Chapter 1 is applied to the problem of aromaticity. The HMO approach is shown to be somewhat limited in its usefulness. However the PMO (perturbational molecular orbital) method, which uses Hiickel orbitals, does lead to satisfactory criteria of... 

Another aspect of qualitative application of MO theory is the analysis of interactions of the orbitals in reacting molecules. As molecules approach one another and reaction proceeds, there is a mutual perturbation of the orbitals. This process continues until the reaction is complete and the new produet (or intermediate in a multistep reaction) is formed. PMO theory incorporates the eoneept of frontier orbital control. This concept proposes that the most important interactions will be between a particular pair of orbitals. " These orbitals are the highest filled orbital of one reactant (the HOMO, highest occupied molecular orbital) and the lowest unfilled (LUMO, lowest unoccupied molecular orbital) orbital of the other reactant. The basis for concentrating attention on these two orbitals is that they will be the closest in energy of the interacting orbitals. A basic postulate of PMO... 

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