Computational studies on gas phase polyborate anions

The borate anions [B(OH)4]−, [B2O(OH)5]−, [B3O3(OH)4]−, [B3O3(OH)5]2−, [B3O3(OH)6]3−, [B4O5(OH)4]2−, [B5O6(OH)4]−, and [B7O9(OH)5]2− (2 isomers) and the neutral orthoboric and metaboric acids, B(OH)3 and B3O3(OH)3, have been structurally optimised in the gas phase at the B3LYP/6-311++G(d,p) level. Energetic data, combined with analogous data for ‘building blocks’ H2O and [OH]−, has enabled their relative gas phase stabilities (all exothermic) to be determined using an isodesmic approach as: [B5O6(OH)4]− > [B3O3(OH)4]− > [B(OH)4]− > [B7O9(OH)5]2− > [B4O5(OH)4]2− > [B3O3(OH)5]2−. The two isomers of [B7O9(OH)5]2− have similar total energies although the ‘ribbon’ isomer is calculated to be more stable by only 10.0 kJ mol−1. QTAIM analyses have been undertaken on all computed structures, and QTAIM charges for H, O and B atoms have been calculated. It is concluded that H-bond interactions dominate the solid-state energetics of non-metal cation polyborate salts.