Molecular crystals

Protons in molecular crystals

Figure showing the simulation cell for the hydrogen chloride monohydrate crystals. Notice the alternating layers of Cl^- and H₃O⁺ ions. The figure also intends to illustrate the proposed disordered sites for the hydronium ions (reported in experimental crystal structures). This conjecture is based on an examination of the measured electron difference map, which is too small to conclude with certainty that disorder is truly present. Simulations based on Car-Parrinello and path integral Car-Parrinello techniques find that the hydronium ions are actually ordered.

The unit cell of the hydrogen chloride trihydrate crystal. Notice the filaments of hydrogen bonded water molecules, in particular the repeating patterns involving H₅O₂ ⁺. Path integral Car-Parrinello simulations indicate that the distribution function of the shared proton in H₅O₂ ⁺ is more strongly influenced by quantum effects than it is in the gas phase or in water, due to the large amplitude O--O vibrations in the crystal.

The following figure shows the contours of the two-dimensional distribution function of the O--O separation (R) and the asymmetric stretch of the proton in the O-H-O bond. The top and bottom correspond to the distributions which result when all nuclei are treated as classical point particles, while the bottom corresponds to a quantum treatment of nuclei within the path-integral paradigm. Both simulations were carried out at T=200K.

For comparison, the same contour plots for the H₅O₂ ⁺ complex in the gas phase are shown corresponding to temperatures of 150K:

and T=300K

Using new constant pressure path integral molecular dynamics techniques combined with Car-Parrinello, these and other crystals will be studied at various pressures to explore how pressure effects hydrogen bonding and proton conductivity.