Determination of pK_a for dithiophosphinic acids using density functional theory

Aromatic dithiophosphinic acids have shown remarkable abilities for separating minor actinides from accompanying lanthanide elements. In particular, the bis(o-trifluoromethylphenyl)dithiophosphinic acid has displayed excellent separation properties, and this molecule also has an unexpectedly high pK_a. To investigate the intrinsic chemistry responsible for the separation and acidity behavior, and envelope of dithiophosphinic acid derivatives were investigated using Density Functional Theory. Symmetric aromatic dithiophosphinic acids of the form (XC₆H₄)₂P(=S)(SH), where X = H, o-CH₃, p-CH₃, p-Cl, p-F, o-CF₃, m-CF₃, and p-CF₃, and asymmetric aromatic acids of the form (X′C₆H₄)(X″C₆H₄)P(=S)(SH), where X′ = o-CF₃, X″ = m-CF₃; X′ = H, X″ = o-CF₃, have been investigated using B3LYP/6-311G(d,p) and 6-311++G(d,p) (Gaussian03). Solvation was included in the calculations using the CPCM continuum solvation method. Using the thermochemical data from vibrational frequency calculations, the pK_a was calculated for the acids, and compared to that of Cyanex-301. The unexpectedly high pK_a for bis(o-trifluoromethylphenyl)dithiophosphinic acid, when compared to the ortho-meta, meta-meta, and para-para isomers, is rationalized by electron repulsion between nearby fluorines and the sulfurs in the anion. This repulsion destabilizes the anion to a greater extent than the other isomers, thus raising the pK_a.