The hydrogen bond IR spectra of indole-3-carboxaldehyde and 3-acetylindole crystals - The reason behind their similarity

► We explain why the IR spectra of crystalline samples of In3CA and 3AcIn are similar. ► It was shown that H-bond dimers of the Ci and C2v symmetry yield similar spectra. ► It results from the “side-to-side” interactions of H-bonds in two crystal systems. ► A non-random distribution of H- and D-atoms in the two lattices was found.