H/D Isotopic and Temperature Effects in the Polarized IR Spectra of Hydrogen-Bond Cyclic Trimers in the Crystal Lattices of Acetone Oxime and 3,5-Dimethylpyrazole

Polarized IR spectra of hydrogen-bonded acetone oxime and 3,5-dimethylpyrazole crystals were measured at 293 and 77 K in the νX–H and νX–D band frequency ranges. These crystals contain molecular trimers in their lattices. The individual crystal spectral properties remain in a close relation with the electronic structure of the two different molecular systems. We show that a vibronic coupling mechanism involving the hydrogen-bond protons and the electrons on the π-electronic systems in the molecules determines the way in which the vibrational exciton coupling between the hydrogen bonds in the trimers occurs. A strong coupling in 3,5-dimethylpyrazole trimers prefers a “tail-to-head”-type Davydov coupling widespread via the π-electrons. A weak through-space exciton coupling in acetone oxime trimers involves three adjacent hydrogen bonds in each cycle. The relative contribution of each exciton coupling mechanism in the trimer spectra generation is temperature and the molecular electronic structure-dependent. ...