Gas phase Lewis acidity and basicity scales for boranes, phosphines and amines based on the formation of donor–acceptor complexes

Abstract

Gas phase Lewis acidity is analyzed in a set of substituted boranes by their hydride affinity, while Lewis basicity is estimated in a set of phosphines and amines through their proton affinity. Acidity and basicity trends are also evaluated from the bond energy of borane–NH3, phosphine–BH3/BF3 and amine–BH3/BF3 adducts, respectively. The relationship between the Lewis acidity/basicity estimations and other chemical reactivity descriptors is explored by the use of both global and local parameters. Intrinsic measures of electron-accepting/electron-releasing capability, like the vertical electron affinity and the vertical ionization energy, correlate with the acidity/basicity scales based on small basic/acidic counterparts, namely the hydride/proton affinity scales. The molecular electrostatic potential and the Fukui function provide detailed information on the charge deficiency/excess and electronic redistribution along the molecules, as well as a guide to identify the effect of the substituents on the acidity/basicity of these sets of molecules. We also discuss some difficulties in locating the true minima on the potential energy surface of some aromatic Lewis acids and bases, as well as of some of their acid–base complexes.