Molecular Orbital MO approach to acid-base reactions

2 Molecular Orbital (MO) approach to acid-base reactions Translated into the idiom of molecular orbital theory, the acid-base definitions should be read as follows  

The term species may mean a discrete molecule, a simple or complex ion or even a solid exhibiting non-molecularity in one or more dimensions (graphite as an example). Free atoms seldom act as Lewis acids and bases. They usually have one or more unpaired electrons and their reactions are more accurately classified as Ifee radical. The donor orbital is usually the highest occupied molecular orbital HOMO, and the acceptor orbital is usually the lowest unoccupied molecular orbital or LUMO. The molecular orbital definitions have a number of important consequences  

it is not necessary that the donor and acceptor orbitals be localizable on a single atom or between two atoms, as implied by Lewis dot structures. That is, the orbitals may be multi-centered even in a relatively localized representation. Thus donor-acceptor interactions involving delocahzed electron systems (Jt-ring) are naturally subsumed by the definitions. 

Second, the HOMO or donor orbital on a base is likely to be either bonding or non-bonding in character, the latter always being the case for mono-atomic species. The LUMO or acceptor orbital on an acid is likely to be either anti-bonding or non-bonding in character, the latter always being the case for mono-atomic species. 

all degrees of electron donation are possible, ranging fi-om essentially zero in the case of weak intermolecular forces and idealized ion associations to the complete transfer of one or more electrons from the donor to the acceptor. This continuity can be represented [10.2.1] by wave functions, were the degree of donation increases as the ratio (a/b).  

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