Successive protonation of Lindqvist hexaniobate, [Nb₆O₁₉]^8-: electronic properties and structural distortions

Lindqvist hexaniobate, [Nb₆O₁₉]^8-, is an intriguing type of polyoxoniobate presenting a significant negative charge, high symmetry, robust structure and applications in photocatalysis. In this work, the [Nb₆O₁₉]^8- polyanion was submitted to an investigation of consecutive protonation in water, seeking for the most stable structure in each step. Initially, the preferred protonation sites lie in more distant oxygen atoms from each other, for example, in H₂[Nb₆O₁₉]⁶ the protons added are separated by 4.54 Å in the final structure. However, as the successive protonation proceeds, two distinct groups of protons are formed, being clustered at opposite sides of the structure. The geometrical distortions become more significant when an external oxygen site is protonated, with relevant perturbations reaching the structure core. The main ultraviolet/visible excitations along protonation are still associated with an electronic charge transfer phenomenon occurring predominantly from bridging oxygens (O_b) to niobium (Nb) atoms such as those noticed before in the non-protonated structure. Most of the main electronic transitions with photocatalytic interest, with excitation wavelengths between 240 and 200 nm, present a total charge transfer amount near 0.4 e.