Connecting structure to infrared spectra of molecular and autodissociated HCl--water aggregates

The properties of perdeuterated HCl(H2O)n aggregates with n=1, 2, ..., 6 water molecules are studied by means of ab initio molecular dynamics simulations. The specific focus is on the phenomenon of autodissociation of the acid HCl as a function of the microsolvation environment size. The calculations provide a basis for characterization in terms of autodissociation energetics as well as in terms of the impact of thermal fluctuations on structure including proton fluxionality and in terms of anharmonic infrared vibrational spectra. Structure stabilization is dominated by strong hydrogen bonds resulting in distinct topologies, which, in turn, heavily influence acid dissociation. The latter is favored for the first time when n = 4. In that case, three hydrogen bonds can be donated toward the chlorine while at the same time a hydronium core is perfectly solvated according to the eigencomplex motif. Hydrogen-bonding interactions between DCl and its solvating molecules affect the dynamical behavior of the D-Cl bond significantly. This can be seen by the onset of fluxionality and an emerging tendency toward proton transfer for the larger clusters. Connecting IR spectra to structural information is possible by exploiting the following observations. Zwitterionic species show characteristic differences in the hydronium region, whereas the D-Cl stretching regime is useful to distinguish neutral aggregates. Furthermore, in the case of fluxional protons large-amplitude motion leads to characteristic band shifts and significant band broadening effects.