Acidities of arsenic (III) and arsenic (V) thio- and oxyacids in aqueous solution using the CBS-QB3/CPCM method

A number of calculations of acidic As(III) and As(V) species formed with sulfur and oxygen (H(3)AsS(3), H(2)AsS(3)(-), HAsS(3)(2-), AsS(3)(3-), H(3)AsS(4), H(2)AsS(4)(-), HAsS(4)(2-), AsS(4)(3-), H(3)AsO(3), H(2)AsO(3)(-), HAsO(3)(2-), AsO(3)(3-), H(3)AsO(4), H(2)AsO(4)(-), HAsO(4)(2-), and AsO(4)(3-)) are presented. pK(a)s for successive deprotonations in both the gas phase and aqueous solution (using both explicit water molecules and a self-consistent reactive field conductive polarizable continuum model (SCRF CPCM) for solvation) are fitted to known experimental values for the H(3)PO(4) and H(3)AsO(4) series' of deprotonations with a linear extrapolation showing r(2) values of 0.97 for the CBS-QB3 method with a single explicit water molecule in the CPCM. Though the unfitted pK(a)s of H(3)AsO(4) (pK(a1) = 4.6, pK(a2) = 17.7, and pK(a3) = 28.6) compare unfavorably with experimental values of pK(a1) = 2.3, pK(a2) = 7, pK(a3) = 13, by the linear fit: 0.42907 x pK(a) - 0.23, the predicted pK(a)s become pK(a1) = 1.7, pK(a2) = 7.3, and pK(a3) = 12.1, which correspond well. The H(3)PO(4) calculation shows a similar trend; pK(a1) = 4.9, pK(a2) = 19.9, and pK(a3) = 29.8 becomes pK(a1) = 1.9, pK(a2) = 8.3, pK(a3) = 12.6. Experimentally, H(3)PO(4) has pK(a1) = 2.2, pK(a2) = 7.1, and pK(a3) = 12.3. With the same extrapolation, we predict H(2)SO(3) to have pK(a1) = 1.1 and pK(a2) = 8.1, which compare favorably with the experimental values of pK(a1) = 1.9 and pK(a2) = 7.2.