Chemical bond: two-atom molecules and orbital symmetry

MO energy level scheme of two-atom homonuclear molecules

To determine the energetic position of the molecular orbitals relative to one another (Fig. 1), the following considerations are useful:

  • Of the binding σ-components is the A.G-Summary combination, due to the high ns component and the associated high electron density between the nuclei, energetically the more stable and represents the energetically lowest level of all molecular orbitals.
  • The binding or antibonding π orbitals B.2u/B.3u respectively. B.3g/B.2g are each energetically equivalent; the level of the bonding π orbitals is lower than that of the antibonding π orbitals.
  • The location of the A.G-Difference combination, relative to the binding π-orbitals, varies and can only be determined by exact calculations of the molecule or with the help of photoelectron spectroscopy. A simple quantum mechanical calculation, on the other hand, does not correctly represent the order of the orbital energies (Fig. 2). With an exact calculation one finds that only if there is a significant difference between the ns and np orbital energies (in the case of molecules made of elements at the end of a period, e.g. O, F), the two binding σ orbitals are lower in energy than the binding π orbitals. at C.2 and N2 on the other hand, the binding π orbitals lie energetically between the two binding σ orbitals.
Definition: degree of attachment
After distributing the electrons to the molecular orbitals, according to Hund's rule and the Pauli principle, the degree of bonding in two-atom molecules can be calculated as the difference, multiplied by ½, between the number of electrons in bonding molecular orbitals and the number of electrons in antibonding molecular orbitals.