Toward less ambiguous vibrational spectroscopic notations for hydrogen-bonded water and methanol clusters

Abstract

The hydrogen bond (HB), a non-covalent interaction, leads to diverse structural motifs that dictate the physical properties of materials or biochemical processes. Infrared spectroscopy allows straightforward access to such structural motifs from laboratory experiments. These spectra indirectly reveal HBs through vibrational frequency shifts in a molecular cluster compared to the single molecules. Characterizing these shifts with descriptive vibrational notations is challenging due to the delocalized nature of intermolecular vibrations. Typically, vibrations of clusters are represented in terms of the respective individual molecules. This approach is somewhat debatable, mainly when notations are based on experience or visual interpretation of theoretical models, most notably the normal mode framework. While normal modes are straightforward to obtain, they often provide insufficient descriptions of delocalized vibrations. Here, the decomposition of normal modes into contributions from internal coordinates allows for both an illustrative framework and a quantitative basis for vibrational notations. In the present work, we apply such a decomposition scheme to various HB systems, assessing the plausibility of notations used in IR spectroscopy of molecular clusters. For water, methanol, and clusters thereof, we demonstrate the limitations of conventional notations and how normal mode decomposition schemes can provide a reasonable workaround.