Search

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 crystal structure of the title compound, C(10)H(12)O(4).H(2)O, consists of (3,4-dimethoxyphenyl)acetic acid and water molecules linked by O-H...O hydrogen bonds to form cyclic structures with graph-set motifs R(1)(2)(5) and R(4)(4)(12). These hydrogen-bond patterns result in a three-dimensional network with graph-set motifs R(4)(4)(20) and R(4)(4)(22), and the formation of larger macrocycles, respectively. The C-C bond lengths and the endocyclic angles of the benzene ring show a noticeable asymmetry, which is connected with the charge-transfer interaction of the carboxyl or methoxy groups and the benzene ring. The title compound is one of the simple carboxylic acid systems that form hydrates. Thus, the significance of this study lies in the analysis of the interactions in this structure and the aggregations occurring via hydrogen bonds in two crystalline forms of (3,4-dimethoxyphenyl)acetic acid, namely the present hydrate and the anhydrous form [Chopra, Choudhury & Guru Row (2003). Acta Cryst. E59, o433-o434]. The correlation between the IR spectrum of this compound and its structural data are also discussed.