Orbitals and Geometrical Perturbation

In this chapter, we will do basically two things—examine in some detaii the form of the molecular orbitals for an AH2 molecule and secondly find out howto predict the form and energetic consequences of moiecular orbitals when a molecule is distorted via geometric perturbation theory. Furthermore, we see the physical underpinnings of why some molecules distort away from the most symmetrical structures and a technique of how to predict the sense of distortion. 

Let us construct the MOs of a linear (Dooh) and bent (C2v) AH2 molecule shown in 7.1 and 7.2, respectiveiy, where the central atom A contributes four valence atomic orbitals s, x, y, and z. We will construct the MOs based upon the perturbation method of Chapter 3, and so it is convenient to construct AH2 from A and H - H units. The orbitals of H H are the in-phase, r+, and out-of-phase, r+, combinations of hydrogen s orbitais shown in 7.3, where the energy gap between r+ and r+ is smaii since the H - H distance in 7.1 and 7.2 is large in most cases of interest 

A number of reiative ordering patterns conceivable for the orbitals of A and H H are shown in 7.4. For exampie, the orbitals of H- H lie in between the s and p orbitais of A in 7.4b, which turns out to be relevant when A is carbon, phosphorus. 

Thomas A. Albright, Jeremy K. Burdett, and Myung-Hwan Whangbo. 

Correlation of some of the AH2 molecular orbitals in a linear (a), and bent (b) geometry. 

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