Reactivity theory

From 1933 several theoretical approaches to the problem of the chemical reactivity of planar conjugated molecules began to appear, mainly by the Huckel molecular orbital theory. These were roughly divided into two groups The one was called the static approach 86,87-40) and the other, the localization approach 41,42). in 1952, another method which was referred to as the frontier-electron method, was proposed 4S) and was conventionally grouped 44) together with other related methods 46,46) as the delocalization approach . 

The first paper of the frontier-electron theory pointed out that the electrophilic aromatic substitution in aromatic hydrocarbons should take place at the position of the greatest density of electrons in the higher occupied (HO) molecular orbital (MO). The second paper disclosed that the nucleophilic replacement should occur at the carbon atom where the lowest umccnpied (LU) MO exhibited the maximum density of extension. These particular MO s were called frontier MO s . In homol3dic replacements, both HO and LU.were shown to serve as the frontier MO s. In these papers the partial density of 2 electron, in the HO (or LU) MO, at a certain carbon atom was simply interpreted by the square of the atomic orbital (AO) coefficient in these particular MO s which were represented by a linear combination (LC) of 2 AO s in the frame of the Huckel approximation. These partial densities were named frontier-electron densities . 

The explanation of these findings was at that time never self-evident. In contrast to the other reactivity theories, which then existed and had already been wdl-establi ed theoretically, the infant frontier-electron theory was ort of solid physical ground, having suggested a possibility of the involvement of a new principle relating to the nature of chemical reactions. 

In the same year as that of the proposal of the frontier-electron theory, the theory of charrge-transfer foru was developed by Mulliken with regard to the molecular complex formation between an electron donor and an acceptor 47). In this connection he proposed the overlap and orientation principle 48) in which only the overlap interaction between the HO MO of the donor and the LU MO of the acceptor is considered. 

The behaviour of the frontier electrons was also attributed to a certain type of electron delocalization between the reactant and the reagent A concept of pseudo -orbital was introduced by setting up a simplified model, and the electron delocalization between the w-electron system of aromatic nuclei and the pseudo-orbital was considered to be essential to aromatic substitutions. The pseudo-orbital was assiuned to be built up out of the hydrogen atom AO attached to the carbon atom at the reaction center and the AO of the reagent species, and to be occupied by zero, one, and two electrons in electrophilic, radical, and nucleophilic reactions. A theoretical quantity called superdelocalizability w is derived from this model. This quantity will be discussed in detail later in Chap. 6. 

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Chemical Reactivity Theory A Density Functional View... 

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