Electron Theories and the a—it Separation

The justification of jr-electron theories has been repeatedly questioned during recent years indeed, it has become almost fashionable to emphasize the shortcomings of the cr—it separation and the non-validity of the theories based upon it. These are, in fact, approximations and cannot be expected to lead to unconditionally reliable conclusions. However, the numerical results that have provoked the criticisms in question are not a necessary consequence of the a—it separation and the related approximations. Therefore, we shall begin by restating and clarifying the basic concepts on which the whole question of the a—it separation rests. We shall consider the conditions under which the electrons of a molecule can be classified into a and it electrons and indicate what should be understood be a—7i separation and what are the limitations of this approximation. We shall show that the most important part of th e a—n interaction is usually taken into accound within the a—n separation scheme and, finally, discuss whether the a—it interaction has a significant effect on the theoretical predictions made for the physical properties of unsaturated molecules (ionization potentials, electronic spectra, charge densities and dipole moments etc.). 

To preserve the rigor of certain arguments, a number of quantum-mechanical formulas are useful they will be introduced, when required, with the necessary explanations of notations. 

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