Polymorphism, Hydrogen Bond Properties, and Vibrational Structure of 1H-Pyrrolo[3,2-h]Quinoline Dimers

Two forms of cyclic, doubly hydrogen-bonded dimers are discovered for crystalline 1H-pyrrolo[3,2-h]quinoline, a bifunctional molecule possessing both hydrogen bond donor and acceptor groups. One of the forms is planar, the other is twisted. Analysis of IR and Raman spectra, combined with DFT calculations, allows one to assign the observed vibrations and to single out vibrational transitions which can serve as markers of hydrogen bond formation and dimer structure. Raman spectra measured for samples submitted to high pressure indicate a transition from the planar towards the twisted structure. Formation of intermolecular hydrogen bonds leads to a large increase of the Raman intensity of the NH stretching band: it can be readily observed for the dimer, but is absent in the monomer spectrum. 1. Introduction In studies of the intermolecular hydrogen bond (HB), an important class of model compounds consists of molecules which can form both H-bonded dimers and complexes with water or alcohols [1]. Such molecules are usually characterized by the simultaneous presence of HB donor and acceptor groups. Whether the strength of the intermolecular HBs is greater for dimers or complexes depends on the relative positions of the donor and acceptor in the molecular frame. Interestingly, different structures and stoichiometries are often encountered for the same molecule. A well-known example is 7-azaindole (7AI, Figure 1), which forms doubly hydrogen bonded dimers in solution [2], while the X-ray data reveal a tetrameric structure in the crystalline state [3]. Different stoichiometries and structures are possible for the complexes of 7AI with methanol and water: 1？:？1, 1？:？2, and 1？:？3 species have been detected [4–9]. Figure 1: Various motifs of intermolecular HB formed by 7-azaindole. (a) Dimers and tetramers, (b) complexes with water. The crystal structure of multiply H-bonded dimers/oligomers seems to be determined by the interplay of H-bonding and longer range intermolecular interactions. For instance, 1-azacarbazole (1AC), a molecule closely related to 7AI, exists in the crystal in the form of planar, doubly hydrogen bonded dimers [10] (Figure 2). While there is no doubt that 1AC also forms dimers in solutions, various possible structures have been discussed [11–14]. Figure 2: The structure of dimers of 1-azacarbazole in the solid phase. 1H-pyrrolo[3,2-h]quinoline (PQ, Scheme 1) can be considered a counterpart to 7AI with regard to intermolecular HB characteristics. The NH group of PQ (HB-donor) and the pyridine nitrogen (HB-acceptor) are positioned three bonds