Electronic structure studies on deprotonation of dithiophosphinic acids in water clusters

We report herein a computational study of proton transfer reactions between dithiophosphinic acids (HAs) and water clusters using B3LYP and MP2 methods. The ground-state and transition-state structures of HA-(H₂O) _n (n = 1, 2, 3) cluster complexes have been calculated. The influence of water molecules on energy barrier heights of proton transfer reactions has been examined in the gas phase and solution for bis[o-(trifluoromethyl)phenyl]- and bis(2,4,4-trimethylpentyl)dithiophosphinic acids (HA1 and HA2, respectively). Gas-phase calculations indicate that electron-withdrawing substituents and trifluoromethyl groups in the ortho position favor deprotonation of HAl when three water molecules are included in the cluster. This suggests that at least three water molecules are necessary to solvate the abstracted proton in the presence of the anion. In the case of HA2, the electron-donating groups favor the reverse proton transfer reaction, namely, protonation of dithiophosphinate anion. Bulk solvent effects have been modeled for aqueous and organic media with the CPCM model. The calculated results show that polar solvents can lower the activation energy for less energetically stable transition states that have more localized charges.