A Dissociative Quantum Mechanical/Molecular Mechanical Molecular Dynamics Simulation and Infrared Experiments Reveal Characteristics of the Strongly Hydrolytic Arsenic(III)

This work presents a hybrid ab initio quantum mechanical/molecular mechanical simulation at the RI-MP2 level of theory investigating the hydrolysis process of arsenic­(III), ultimately leading to arsenous acid (H₃AsO₃). A newly implemented dissociative water model has been applied to treat the interactions in the classical region, which is capable of describing non-neutral water species such as hydroxide and oxonium ions. Three stages of hydrolysis have been observed during the simulation and besides profound dynamical considerations, detailed insights into structural changes and atomic partial charge shifts are presented. In particular, the geometrical properties of H-bonds involved in each of the three proton transfer events and subsequent proton hopping reactions are discussed. A Laguerre tessellation analysis has been employed to estimate the molecular volume of H₃AsO₃. Estimations of pK_a values of the arsenic­(III)-aquo-complexes have been obtained at the G4 and CBS-Q//B3 levels of theory using a thermodynamic cycle, whereas rate constants for the final hydrolysis step have been determined via reaction path optimization and transition state theory. Newly recorded Fourier transform infrared (FT-IR) spectroscopy measurements have been compared to power spectra obtained from the simulation data, confirming its quality. The simulation findings, as well as results from computational spectroscopic calculations utilizing the PT2-VSCF methodology, proved valuable for the interpretation of the experimental FT-IR data, elucidating the particularities of the strongly observed IR Raman noncoincidence effect.