H/D isotopic recognition in hydrogen bonded systems: H/D isotopic self-organization effects in the IR spectra of the hydrogen bond in 2-methylimidazole crystals

Polarized IR spectra of H12345 2-methylimidazole and of its H1D2345, D1H2345 and D12345 deuterium derivative crystals are reported and interpreted within the limits of the “strong-coupling” theory. The spectra interpretation facilitated the recognition of the H/D isotopic “self-organization” phenomenon, which depends on a non-random distribution of protons and deuterons in the lattices of isotopically diluted crystal samples. The H/D isotopic “self-organization” mechanism engaged all four hydrogen bonds from each unit cell. These effects basically resulted from the dynamical co-operative interactions involving adjacent hydrogen bonds in each hydrogen bond chain. A weaker exciton coupling involved the closely spaced hydrogen bonds; each belonging to a different chain of associated 2-methylimidazole molecules. The high intensity of the narrow band at ca. 1880 cm−1 was interpreted as the result of coupling between the γN–H⋯N proton bending “out of plane” vibration overtone and the νN–H proton stretching vibration.

Graphical abstractThis paper presents the results of experimental and theoretical study of the polarized IR spectra of H12345 2-methylimidazole and its H1D2345, D1H2345 and D12345 deuterium derivative crystals.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The polarized IR spectra of 2-methylimidazole were quantitatively analyzed. ► The in- and inter-chain H/D isotopic self-organization effects were found. ► The self-organization mechanism involved all four hydrogen bonds from each unit cell.