Structure and dynamics of concentrated hydrochloric acid solutions. A first principles molecular dynamics study

Abstract

We have performed Car–Parrinello molecular dynamics simulations of highly concentrated DCl-solutions between 22 wt.% and 48 wt.%, both at ambient and elevated temperatures. According to the 3D-environment of the D₃O⁺ ion it has three heavy atoms (O,Cl) at the natural hydrogen bonded sites. The fourth is found at ∼10% probability below the central oxygen, or at ∼90% probability in a more diffuse area above the hydronium plane, the probability peaking between the DOD-angles. Total vibrational spectra of the systems, their difference from those of bulk water, and selected individual species are presented. The continuum absorption between 1200 and 2400 cm⁻¹ is shown to originate from both D₃O⁺ and D₅O₂⁺ structures. Up to concentrations of 22 wt.% all DCl molecules dissociate to produce Cl⁻ and hydronium ions. Above this concentration [ClDCl]⁻ ions were formed in analogy to hydrofluoric acid. In contrast to DF no Cl⋯D⋯OD₂ proton sharing pairs were detected, but DCl were almost exclusively solvated by Cl⁻. The dynamics of hydrogen bonds and the diffusion of atoms and charge defects, as well as, relaxation times of individual species are discussed. The overall barriers for deuterium jumps (PT) are very small; 4–5 kJ mol⁻¹ between oxygen atoms on average, and the systems are best described by a strongly coupled hydrogen bond network, rather than individual solvated hydronium species. Conductivity was estimated by following the charge defects. The values for molar conductivities qualitatively agree with experiments. The [Cl⁻(ClD)_n] structures participate in the PT processes yielding a jump mechanism also for the negative charge transport.