Gas‐Phase Cation Affinity and Basicity Scales

Abstract

The study of the formation of complexes between cations and chemical species in the gas phase offers the unique possibility of measuring the intrinsic (i.e. free of solvent and counter ion influences) Lewis basicity of these species. Although the charge-transfer component of the interaction energy in cation adducts is normally smaller than the electrostatic and polarization components, gas-phase cations are generally considered as archetypal Lewis acids because the full positive charge of the naked cation allows strong binding with classical Lewis bases. In fact, almost any entity bearing some electron density, such as the rare gases, can form complexes with cations that may be observable in the gas phase.There is a large array of cations that have been studied in the gas phase for their interaction with Lewis bases (generally neutral organic molecules) and are still being actively investigated today. The most studied is the proton [1-6]. This special cation will not be dealt with in this chapter (however, see Section 1.3 in Chapter 1), since the proton and its transfer serve to define the Brönsted acids and bases, as opposed to Lewis acids and bases, which do not exchange protons. Moreover, although the proton and a cation with an empty orbital may be seen as identical from the point of view of the Lewis theory, the special 'electronic structure' of the proton (in fact the absence of any electron) makes the proton affinity and basicity scales completely different, qualitatively and quantitatively, from any other cation scales. A significant part of the studies of the interaction of gas-phase metal cations with organic molecules was devoted to their chemical reactivity: bond insertion, bond cleavage, elimination and so on. This aspect is not considered here, although some of these reactions may be used as indirect routes for the formation of cation/molecule adducts.Apart from the proton, experimental studies have focused primarily on the binding of alkali metal cations and transition metal monocations. Few studies have considered alkaline Lewis Basicity and Affinity Scales: Data and Measurement