Qualitative Model Perturbation Molecular Orbital Theory

Qualitative Model Perturbation Molecular Orbital Theory 

A particularly useful model for analyzing substituent effects uses perturbation molecular orbital (PMO) theory (7). Most simply, the theory states that the interaction of two orbitals, and J, results in the formation of two new orbitals, one of which is lowered in energy while the other is raised in energy compared to the interacting orbitals. For the case in which one orbital is occupied and one empty, the resultant interaction is stabilizing. 

We note here two effects, which will be of particular importance in our analysis, which modify orbital energies, hence the energy gap AE, and hence the stabilization energy SE. (i) The first effect concerns replacement of the 

An additional effect with which we shall have reason to be concerned and which can be usefully described in terms of PMO theory is the concept of in- 

For the substituted benzene systems studied here, the degree of both a and 7T charge transfer between the substituent and the ring are indicative of the magnitude of the substituent-ring interaction. The effect of a second substituent may be understood by analyzing how a and ir charge transfer between the first 

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B. Qualitative Model Perturbation Molecular Orbital Theory... 

Although sophisticated electronic structure methods may be able to accurately predict a molecular structure or the outcome of a chemical reaction, the results are often hard to rationalize. Generalizing the results to other similar systems therefore becomes difficult. Qualitative theories, on the other hand, are unable to provide accurate results but they may be useful for gaining insight, for example why a certain reaction is favoured over another. They also provide a link to many concepts used by experimentalists. Frontier molecular orbital theory considers the interaction of the orbitals of the reactants and attempts to predict relative reactivities by second-order perturbation theory. It may also be considered as a simplified version of the Fukui function, which considered how easily the total electron density can be distorted. The Woodward-Hoffmann rules allow a rationalization of the stereochemistry of certain types of reactions, while the more general qualitative orbital interaction model can often rationalize the preference for certain molecular structures over other possible arrangements. 

Various theoretical methods and approaches have been used to model properties and reactivities of metalloporphyrins. They range from the early use of qualitative molecular orbital diagrams (24,25), linear combination of atomic orbitals to yield molecular orbitals (LCAO-MO) calculations (26-30), molecular mechanics (31,32) and semi-empirical methods (33-35), and self-consistent field method (SCF) calculations (36-43) to the methods commonly used nowadays (molecular dynamic simulations (31,44,45), density functional theory (DFT) (35,46-49), Moller-Plesset perturbation theory ( ) (50-53), configuration interaction (Cl) (35,42,54-56), coupled cluster (CC) (57,58), and CASSCF/CASPT2 (59-63)). 

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