Acidities of Arsenic (III) and Arsenic (V) Thio- and Oxyacids in Aqueous Solution using the CBS-QB3/CPCM Method

摘要

A number of calculations of acidic As(III) and As(V) species formed with sulfur and oxygen (H_3AsS_3,H_1AsS_3~-,HAsS_3~(2-),AsS_3~(3-),H_3AsS_4,H_2AsS_4~(2-),AsS_4~(3-),H_3AsO_3~-,HAsO_3~(2-),AsO_3~(3-),H_3AsO_4,H_2AsO_4~-,HAsO_4~(2-),and AsO_4~(3-)are presented. pK_as 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_3PO_4 and H_3AsO_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_as of H_3AsO_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_as become pK_(a1)=1.7, pK_(a2)=7.3, and pK_(a3)=12.1,which correspond well. The H_3PO_4calculation 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_3PO_4has pK_(a1)=2.2, pK_(a2)=7.1, and pK_(a3)=12.3. With the same extrapolation, we predict H_2SO_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.