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The spectral properties of two different hydrogen-bonded crystalline systems, 1,2,4-triazole and 3-methyl-2-oxindole, containing molecular chains in their lattices, were examined by polarized IR spectroscopy, aided by the calculations utilizing the "strong-coupling" model. The experimental and theoretical approaches have shown that the individual crystal spectral properties in IR remain in a close relation with the electronic structure of the individual molecular systems. A vibronic coupling mechanism involving the hydrogen bond protons and the electrons occupying the π-electronic orbitals in the molecules determines the way in which the vibrational exciton coupling between the hydrogen bonds in the crystals occurs. For the associating systems, which molecules contain large delocalized π-electronic systems coupled directly with H-bonds, strong exciton interactions involving the vibrationally excited hydrogen bonds in the chains prefer a "tail-to-head"-type Davydov-coupling widespread via the π-electrons. A weak through-space exciton coupling involves two closely-spaced hydrogen bonds belonging to two different adjacent chains in the case, when large π-electronic systems in the molecules are absent. The relative contribution of each exciton coupling mechanism in the chain system spectra generation is temperature-dependent. The two competing individual Davydov-coupling mechanism are responsible for the appearance in the polarized spectra of temperature-dependent Davydov-splitting effects differentiating the spectral properties of the two crystalline systems. The H/D isotopic ''self-organization'' phenomenon, depending on a non-random distribution of protons and deuterons in the crystal hydrogen bridges was also analyzed., (Copyright © 2013 Elsevier B.V. All rights reserved.)

This article focuses on the problem of remarkably strong changes in the fine structure patterns of the ν(N-H) and ν(N-D) bands attributed to the hydrogen and deuterium bonds accompanying the phase transition, which occurs between two polymorphic forms of oxindole. The lattices of these two different crystals contain hydrogen-bonded cyclic dimers differ in their geometry parameters. The source of these differences in the polymorph spectral properties results from the geometry relations concerning the dimers constituting the lattice structural units. In the case of the "alpha" phase, the hydrogen bond lengths of the dimers differ by 0.18 Å. This leads to the "off-resonance exciton coupling" weakly involving the dimer hydrogen bonds. For the "beta" phase, with practically symmetric dimers in the lattice, the spectra become typical for centrosymmetric hydrogen bond systems due to the full resonance of the proton or deuteron vibrations.

The title compound, C(10)H(9)NO, contains an acetyl group that is nearly coplanar with the indole ring system, with an angle between the planes of the heterocyclic ring and the acetyl group of 1.75 (17) degrees . The planes of the benzene and pyrrole rings in the indole system make a dihedral angle of 2.05 (11) degrees . Each molecule in the unit cell is linked through N-H...O hydrogen bonds to two other molecules, forming hydrogen-bonded chains in the [101] direction with graph set C(6). The significance of this study lies in the analysis of the interactions occurring via hydrogen bonds in this structure, as well as in the comparison drawn between the molecular structure of the title compound and those of several other indole derivatives possessing a 3-carbonyl group. The correlation between the IR spectrum of this compound and the structural data is also discussed.