KCS

We report the noble intermolecular proton-bonded aldehydes of polyaromatic hydrocarbons. The charge of the hydrogen involving in the O-H⁺-O bond is +0.51 au (i.e., more proton-like). The proton bond between aldehydes corresponds to the Zundel-like cationic hydrogen bond (where two O atoms share a proton in the midpoint of the short O-O distance of ~2.4 Å), which is quite different from the H-bonds associated with the carboxylic acids. At varying temperatures, the crystal structures are analyzed with synchrotron X-rays in the range of 100-370K, and their IR spectra between 500 and 4000 cm^-1 are measured and characterized in the range of 123-413K. We obtained clear Zundel-like proton signatures from the O-H⁺-O vibrational spectra (~1000 cm^-1 instead of the normal H-bond signature around ~3000 cm^-1), which is the first demonstration in crystals. The proton bonds are thermally very stable up to 393 K due to the π-π stacking interactions between aromatic rings. We also show how much the O-H⁺-O distance and the vertical π-stacking distances change with the increasing temperature.