Evolution of vibrational bands upon gradual protonation/deprotonation of arsinic acid H2As(O)OH in media of different polarity

This computational work is devoted to the investigation (MP2/def2-TZVP) of the geometry and IR parameters of arsinic acid H2AsOOH and its hydrogen-bonded complexes under vacuum and in media with different polarity. The medium effects were accounted for in two ways: (1) implicitly, using the IEFPCM model, varying the dielectric permittivity (epsilon) and (2) explicitly, by considering hydrogen-bonded complexes of H2As(O)OH with various hydrogen bond donors (41 complexes) or acceptors (38 complexes), imitating a gradual transition to the As(OH)(2)(+) or AsO2- moiety, respectively. It was shown that the transition from vacuum to a medium with epsilon > 1 causes the As(O)OH fragment to lose its flatness. The solvent polar medium introduces significant changes in the geometry and IR spectral parameters of hydrogen-bonded complexes too: as the polarity of a medium increases, weak hydrogen bonds become weaker, and strong and medium hydrogen bonds become stronger; in the case of a complex with two hydrogen bonds cooperativity effects are observed. In almost all cases the driving force of these changes appears to be preferential solvation of charge-separated structures. In the limiting case of complete deprotonation (or conversely complete protonation) the vibrational frequencies of nu(As )and nu(As-O) turn into nu(As-O)(asym) and nu(As-O)(sym), respectively. In the intermediate cases the distance between nu(AsO) and nu(As-O) is sensitive to both implicit solvation and explicit solvation and the systematic changes of this distance can be used for estimation of the degree of proton transfer within the hydrogen bond.

Automated summary

This study computationally investigated the geometry and IR parameters of arsinic acid (H2AsOOH) and its hydrogen-bonded complexes in different media. The effects of the medium on the complexes were examined both implicitly using the IEFPCM model and explicitly by considering hydrogen-bonded complexes with various hydrogen bond donors or acceptors. The results showed that the transition from vacuum to a polar medium caused changes in the geometry and IR parameters of the complexes, with weak hydrogen bonds becoming weaker and strong and medium hydrogen bonds becoming stronger. The preferential solvation of charge-separated structures was identified as the driving force behind these changes.