Ions and Excited Electronic States

Abstract

The majority of molecules of interest in organic and inorganic chemistry have closed-shell ground states. In contrast, ions and excited states are often open-shell species and this is partly the reason for the difficulties in dealing with them. (In rare cases, such as the O₂ and NO molecules, even ground-state species may be open shell.) The consideration of excited states is of great importance in many areas, such as photochemistry, fluorescence, and gas-phase reactions. Solvated ions are involved in most classical chemical reactions in solution. Free ions in the gas phase have been shown to be important constituents in high-energy photoreactions and electric discharges, and are present in interstellar space and in the upper atmosphere. A further reason for the recent proliferation of quantum chemical calculations on excited and ionic states is the need to interpret spectroscopic data on neutral molecules (e.g., from electronic and photoelectron spectroscopy).