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 H12 ₃45 2-methylimidazole and of its H1D2 ₃45, D1H2 ₃45 and D12 ₃45 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 γ _N-H⋯N proton bending "out of plane" vibration overtone and the ν _N-H proton stretching vibration.