Molecular Charge Distributions and Chemical Bonding. V. Molecular Excitation, Ionization, and Electron Attachment

The detailed changes in the electronic charge distributions upon electronic excitation, ionization, and electron attachment are studied for selected diatomic molecules. The processes considered involve the following systems and states: O2(X 3Σg−, a 1Δg, b 1Σg+), O2+(X 2Πg), O2−(2Πg); BeH and MgH (X 2Σ+, A 2Πr); OH and SH (X 2Πi, A 2Σ+), OH+ and SH+ (X 3Σ−), and OH− and SH− (X 1Σ+). It is shown that metastable excitation in the oxygen molecule produces only a very small change in the electronic charge distribution of the system. This suggests an important role for the spin density in determining the differential behavior of these systems. The A 2Πr ← X 2Σ+ and A 2Σ+ ← X 2Πi excitations for the (BeH; MgH) and (OH; SH) pairs are examined and compared to the “active” electron approximation. The changes in binding of the system associated with these various excitations, ionization, or electron attachments is examined in terms of the partial forces exerted by the various molecular orbitals. In particular, the sharp conversion to a pronounced “ionic” molecule is evident in the BeH and MgH excitations. The A 2Πr states of BeH and MgH are as ionic as the LiH and NaH systems in their ground states. It is demonstrated that the binding ability or properties of a given orbital shows a simple graduated trend in a sequence (AB+; AB; AB−) which is not strongly dependent on the particular state symmetry of the system. The binding role assigned to a particular molecular orbital in a molecule is shown to depend on the particular state and charge situation. The implications of these results towards providing a role for the excited states and molecular ions in the theory of chemical bonding are briefly discussed.