DFT (density functional theory) calculations on the structure and the protonation sites of the bicapped-Keggin-type heteropoly anion [H₄As₃Mo₁₂O₄₀]^- were performed using B3LYP method with LanL2MB basis sets. The calculation showed that, the formation of the two caps on the opposite Mo₄O₄ faces greatly influenced the electronic structure, thereafter the properties of this anion. The NBO analysis indicated that the negative charge density of the triply bridged oxygen atoms, participating in the formation of the caps, was greater than the density of the doubly bridged ones, thus protons might be preferably located on the triply bridged oxygen atoms as speculated by considering simply the electronic density. But detailed investigations through comparison of the stabilization energy after the cluster protonation predicted that, the most energetically favorable sites for the protonation of [H₄As₃Mo₁₂O₄₀]^- were located on four of eight edge-sharing doubly bridged oxygen atoms, rather than the triply bridged ones as indicated in the literature. The protonation of the Keggin-type heteropoly acids H₃PM₁₂O₄₀ (M＝Mo and W) was also investigated and compared with the results reported in literature. The acidic protons of H₃PM₁₂O₄₀ are bound to the bridged oxygen atoms for H₃PMo₁₂O₄₀, while for H₃PW₁₂O₄₀, they may be located on bridged oxygen atoms preferably, though they can also be located on terminal ones, and this conclusion is consistent with the literature.

Key words: heteropoly compound, bicapped α-Keggin structure, protonation, DFT, [H₄As₃Mo₁₂O₄₀]^-, H₃PMo₁₂O₄₀, H₃PW₁₂O₄₀