Orbitals in Polyatomic Molecules

The molecular orbital theory of polyatomic molecules follows the same principles as those outlined for diatomic molecules, but the molecular orbitals spread over all the atoms in the molecule. An electron pair in a bonding orbital helps to bind together the whole molecule, not just an individual pair of atoms. The energies of molecular orbitals in polyatomic molecules can be studied experimentally by using ultraviolet and visible spectroscopy (see Major Technique 2, following this chapter). 

One way to view interactions between atomic orbitals in polyatomic molecules and ions is to consider separately the orbitals on a central atom and the orbitals on surrounding atoms. In this approach, the orbitals on surrounding atoms, considered a group, are labeled group orbitals.8... 

As in diatomics (Section 4.3), the molecular orbitals in polyatomic molecules may be expressed as linear combinations of atomic orbitals (AOs) centered on the nuclei. A minimal basis set of AOs contains all of the AOs that are occupied in the separated atoms [1, 2]. In the bent ozone molecule, for example, the separated O atoms have the ground state configuration (ls) (2s) (2p). The minimal basis set for ground-state O3 therefore consists of the Is, 2s, and three 2p orbitals centered on each of the oxygen nuclei (Fig. 7.1). The orientations selected for the 2p AOs in the basis set are of course arbitrary (aside from the constraint that basis AOs centered on any nucleus must be linearly independ-... 

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