Timothy Hughbanks authored molecular-orbital teaching notes that explain bonding in diatomic molecules and s–p mixing. Orbitals can interact only when their symmetry permits it, and the resulting energy shifts depend on their energetic separation and overlap.
The example shows why the ordering of molecular orbitals cannot always be obtained by placing unmixed atomic levels beside one another.[ref]
Texas A&M University institutional profile: artsci.tamu.edu/chemistry/contact/profiles/timothy-hughbanks.html.
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