Hard basicity of borate anion clusters

Abstract

The hardness and softness of basicity in the binary borate system were studied by the MNDO semi-empirical molecular orbital (MO) method with the cluster approximation. A new quantity “Sparkle Affinity” was defined as a measure of the hard basicity of borate anion clusters. The sparkle was a virtual chemical species whose behaviour was expected to be similar to the alkali or alkaline earth metal cation. The sparkle affinity was defined as the energetic gain when one sparkle was set near a borate anion cluster. The comparison was made among the delocalization energy, the proton affinity and the sparkle affinity from the viewpoint of basicity. The fourfold coordinated boron unit (BO 4 − ) was found to be a rather hard base and non-bridging oxygen (NBO) was found to be a rather soft base. A proton was found to be combined with one oxygen atom with high covalency, in contrast with alkali metal ions, the latter coordinating plural oxygen atoms with the coulombic force.