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 (H3AsS3, H2AsS3−, HAsS32−, AsS33−, H3AsS4, H2AsS4−, HAsS42−, AsS43−, H3AsO3, H2AsO3−, HAsO32−, AsO33−, H3AsO4, H2AsO4−, HAsO42−, and AsO43−) are presented. pKas 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 H3PO4 and H3AsO4 series’ of deprotonations with a linear extrapolation showing r2 values of 0.97 for the CBS-QB3 method with a single explicit water molecule in the CPCM. Though the unfitted pKas of H3AsO4 (pKa1 = 4.6, pKa2 = 17.7, and pKa3 = 28.6) compare unfavorably with experimental values of pKa1 = 2.3, pKa2 = 7, pKa3 = 13, by the linear fit: 0.42907 × pKa − 0.23, the predicted pKas become pKa1 = 1.7, pKa2 = 7.3, and pKa3 = 12.1, which correspond well. The H3PO4 calculation shows a similar trend; pKa1 = 4.9, pKa2 = 19.9, and pKa3 = 29.8 becomes pKa1 = 1.9, pKa2 = 8.3, pKa3 = 12.6. Experimentally, H3PO4 has pKa1 = 2.2, pKa2 = 7.1, and pKa3 = 12.3. With the same extrapolation, we predict H2SO3 to have pKa1 = 1.1 and pKa2 = 8.1, which compare favorably with the experimental values of pKa1 = 1.9 and pKa2 = 7.2.