Shapes of Polyatomic Molecules Walsh Diagrams

In this section we will describe how the niles and concepts of QMOT enable one to rationalize andpredictmolecular shapes. The earliestsystematictreatmentofthis problem was givenby Walsh, whose approachhas been extended by others, particularly Gimarc [6] 

We begin by considering the symmetric triatomic class of molecules HAH, where A is any atom. Such molecules can be linear or bent. Walsh s approach predicts which are linear, which are bent, and sometimes which of two bent molecules is more bent. 

On the right side of Fig. 14-7, the symmetry orbitals t and Ai are shown to be slightly split. This reflects the greater stability of the in-phase, or bonding, combination. However, the splitting is slight because the hydrogen atoms are quite far apart (so that atom A can fit between them). These two levels sandwich the separated-atom limit of —5 a.u. or —13.6eV. 

The AOs of atom A are sketched on the left. Their energies are arranged so that the 2p energies are about the same as the g, Ai symmetry orbital energies on the right. (For example, AO energies used for nitrogen in EHMO calculations are 2s —25 eV, 2p — 13 eV.) For the linear molecule, we can label the s and p AOs on atom A as a or 7T and g or u. 

To generate the MO energy-level pattern from the interactions between these AOs on A and g, , we use the foUowing rules from perturbation theory (see Chapter 12)  

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Section 14-7 Shapes of Polyatomic Molecules Walsh Diagrams... 

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