DFT Study of α-Keggin-type Iso-polyoxotungstate Anions [HnW12O40](8-n)- (n =1-4): Can [H4W12O40]4- Exist?

The Keggin-type iso-polyoxotungstate (iso-POT) anions [HnW12O40](8-n)- 's, in which their central vacancies are occupied by protons, are attractive materials. It is of importance to reveal if the vacancies can be fully occupied by four protons. For further understanding the speciation of these iso-POT anions, relative stabilities and proton transfer reactions between H1[Hn-1W12O40](8-n)- and [HnW12O40](8-n)- were examined in detail by using the first-principles calculations (the nudged elastic band method, the synchronous transit-guided quasi-Newton method, the intrinsic reaction coordinate method, and frequency analysis calculations). Thermochemistry analysis of the proton transfer was also performed. [HnW12O40](8-n)- was energetically more stable than H1[Hn-1W12O40](8-n)-. This held for n = 4. According to the results of thermochemistry analysis, the rate constant and the Wigner correction were respectively 3.1 × 101 s-1 and 2.2 at T = 298.15 K for the proton transfer from H1[H3W12O40]4- to [H4W12O40]4-, indicating that [H4W12O40]4- can exist when H1[H3W12O40]4- is formed by protonating [H3W12O40]5-.