Klopman-Salem theory

Topsom, 1976) and to treat them separately. In this review we will be concerned solely with polar or electronic substituent effects. Although it is possible to define a number of different electronic effects (field effects, CT-inductive effects, jt-inductive effects, Jt-field effects, resonance effects), it is customary to use a dual substituent parameter scale, in which one parameter describes the polarity of a substituent and the other the charge transfer (resonance) (Topsom, 1976). In terms of molecular orbital theory, particularly in the form of perturbation theory, this corresponds to a separate evaluation of charge (inductive) and overlap (resonance) effects. This is reflected in the Klopman-Salem theory (Devaquet and Salem, 1969 Klop-man, 1968 Salem, 1968) and in our theory (Sustmann and Binsch, 1971, 1972 Sustmann and Vahrenholt, 1973). A related treatment of substituent effects has been proposed by Godfrey (Duerden and Godfrey, 1980). 

Frontier Orbital theory supplies an additional assumption to this calculation. It considers only the interactions between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). These orbitals have the smallest energy separation, leading to a small denominator in the Klopman-Salem equation. The Frontier orbitals are generally diffuse, so the numerator in the equation has large terms. 

Molecular Orbital (HOMO) of one component and the Lowest Unoccupied Molecular Orbital (LUMO) of the other. Within the scope of FMO theory, the reactivity of two reaction partners towards each other is described quantitatively by the Klopman-Salem-Fukui relationship (equation i)60 62 ... 

A detailed examination of the reactions of ambident nucleophiles has shown that hard-soft acid-base theory and the Klopman-Salem model based on charge- and orbital-control of a reaction are not able to predict the results found in many reactions involving ambident nucleophiles. The analysis of the reactions of many ambident nucleophiles with a variety of substrates indicates that Marcus theory, which derives... 

We cannot, then, expect this approach to understanding chemical reactivity to explain everything. Most attempts to check the validity of frontier orbital theory computationally indicate that the sum of all the interactions of the filled with the unfilled orbitals swamp the contribution from the frontier orbitals alone. Even though the frontier orbitals make a weighted contribution to the third term of the Salem-Klopman equation, they do not account quantitatively for the many features of chemical reactions for which they seem to provide such an uncannily compelling explanation. Organic chemists, with a theory that they can handle easily, have fallen on frontier orbital theory with relief, and comfort themselves with the suspicion that something deep in the patterns of molecular orbitals must be reflected in the frontier orbitals in some disproportionate way. 

Another way of looking at the same problem uses the Salem-Klopman equation that we saw in the last chapter, and hence follows from the concept of HS AB, instead of preceding it, and is based on theory rather than experiment. Using only the HOMO of a nucleophile and the LUMO of an electrophile, Klopman simplified Equation 3.13 to Equation 4.2 ... 

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