Strained hydrogen bonding in imidazole trimer: a combined infrared, Raman, and theory study

In this work, a careful analysis of anharmonic couplings in NH and some CH stretch modes of imidazole is carried out. This includes IR and Raman spectra of the isolated molecule and aggregates up to the trimer, together with two different theoretical approaches to the calculation of anharmonic shifts and absolute band positions. The imidazole dimer is vibrationally characterized for the first time in vacuum isolation under supersonic jet conditions, showing substantial shifts from previous helium droplet experiments and evidence for Fermi resonance for the hydrogen-bonded NH stretch. The most stable imidazole trimer structure is unambiguously shown to be cyclic with three non-equivalent, highly strained hydrogen bonds. This contrasts the helium droplet observation of a chain trimer involving two unstrained hydrogen bonds. These experimental conclusions are strongly corroborated by theory, including vibrational perturbation theory and anharmonic normal mode analysis. Systematic error compensation in some of these methods is emphasized. Intramolecular anharmonic coupling constants from perturbation theory are validated by Raman hot band jet spectroscopy of the monomer. Imidazole aggregation is shown to provide valuable benchmarking opportunities for electronic structure and in particular for anharmonic vibrational methods, covering the field of strong and strongly distorted hydrogen bonding.