Inter-hydrogen bond coupling in crystals with molecular chains in their lattices investigated by polarized IR spectroscopy: 4-Bromo-3,5-dimethylpyrazole and 3,4-dimethoxyphenylacetic acid

We report the results of the experimental and theoretical studies of the polarized IR crystalline spectra of 4-bromo-3,5-dimethylpyrazole (4Br35DMPz) and 3,4-dimethoxyphenylacetic acid (34DMPAA) as well as the spectra of their deuterium-bonded analogues. The results of model calculations of the temperature impact exerted on to the band shapes measured in the X–H- and X–D bond stretching vibration regions, performed on the basis of the “ strong-coupling ” model, are also shown. These studies confirm a direct relationship between the spectral properties in IR and the electronic structure of the associating molecules in the crystals. Two different competing exciton coupling mechanisms involving hydrogen bonds, the “ tail-to-head ” (TH) in 4Br35DMPz and the “ side-to-side” coupling in 34DMPAA, were recognized. The molecular electronic structure determines the relative contribution of each individual vibrational exciton coupling mechanism in the spectra generation. It also strongly influences the temperature-induced evolution of the Davydov-splitting effects in the crystalline spectra. Dynamical co-operative interactions responsible for a non-random distribution of protons and deuterons in the crystal hydrogen bonds can also be identified in the investigated systems.