Microwave spectroscopy of partially and fully deuterated HCl dihydrate clusters

A detailed understanding of water's remarkable ability to solvate small molecules is a central theme in chemistry. In this work, we have identified 12 new partially and fully deuterated isotopologues of the hydrochloric acid dihydrate cluster, (H2O)2 · HCl, by chirped-pulse Fourier transform microwave spectroscopy. We also performed high-level ab initio calculations relevant to the structure and dynamics of the cluster. The observation of singly substituted isotopologues at each unique hydrogen position enables a more detailed experimental characterization of the geometry than was possible previously. In addition, an analysis of splittings in the spectrum that are caused by population of more than one vibration-tunneling level enables a detailed characterization of hydrogen bond-breaking bifurcation tunneling motions that occur in the complex. Not only do our results confirm theoretical predictions about hydrogen atoms that are permuted by the tunneling motions-they also provide experimental evidence for concerted large-amplitude motions of other atoms. Finally, an analysis of the chlorine nuclear quadrupole coupling tensor suggests that the HCl bonding character might be influenced to a small extent by isotopic substitution of atoms involved in the Cl-H⋯O hydrogen bond due to zero-point vibrational effects.