Molecular Modes and Dynamics of HCl and DCl Guests of Gas Clathrate Hydrates

Recent years have yielded advances in the placement of unusual molecules as guests within clathrate hydrates (CHs) without severe distortion of the classic lattice structures. Reports describing systems for which observable but limited distortion does occur are available for methanol, ammonia, acetone, and small ether molecules. In these particular examples, the large-cage molecules often participate as non-classical guests H-bonded to the cage walls. Here, we expand the list of such components to include HCl/DCl and HBr as small-cage guests. Based on FTIR spectra of nanocrystalline CHs from two distinct preparative methods combined with critical insights derived from on-the-fly molecular dynamics and ab initio computational data, a coherent argument emerges that these strong acids serve as a source of molecular small-cage guests, ions, and orientational defects. Depending on the HCl/DCl content the ions, defects and molecular guests determine the CH structures, some of which form in sub-seconds via an all-vapor preparative method.