The Electronic Structure of Polyatomic Molecules

The electronic structure of polyatomic molecules can be described approximately with LCAOMOs that involve only two atomic orbitals if appropriate hybrid orbitals are used. 

Qualitative descriptions of the electronic states of molecules can be obtained by using general criteria for forming good bonding LCAOMOs. 

Group theory can be used to obtain useful information about molecular orbitals of polyatomic molecules. 

Various semiempirical and ab initio techniques exist for carrying out molecular quantum mechanical calculations, and software packages are available to carry out these techniques. 

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Many qualitative and semiempirical studies followed Mulliken s analysis [6], but it was not until 1960 that the first ab initio calculation on CH2 was published [7] by J. M. Foster and S. F. Boys in the famous April 1960 issue of the Reviews of Modem Physics which contained the papers from a conference on Molecular Quantum Mechanics that was held at the University of Colorado in June of 1959. This is the same Boys who in 1950 introduced the idea of using oscillator functions or as we know them now, gaussian fionctions as basis functions for calculating the electronic structure of polyatomic molecules. 

Thus far we have discussed the chemical bonding in polyatomic molecules in terms of the VB model, or more crudely in terms of Lewis structures. These two treatments are related in that they focus on chemical bonding in terms of the sharing of electron pairs by adjacent atoms. In many cases, however, a more sophisticated approach based on molecular orbital concepts is needed to accurately picture the electronic structure of polyatomic molecules—even on a quahtative level. 

The extension of Mulliken s program to polyatomic molecules started with a series of 14 papers titled "The Electronic Structure of Polyatomic Molecules and Valence," which spanned the years 1932-1935. The series appeared in Physical Review and, after the fourth paper, in the newly created Journal of Chemical Physics, which some claimed was founded just to house Mulliken s papers (Platt 1966, 746) In this way, foundational papers for the subdisdpline of quantum chemistry written by two of its founders were included in the first and subsequent volumes of the Journal of Chemical Physics, which thereafter functioned as a privileged outlet for the new subdiscipline. 

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