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

Abstract

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.

Hydrogen-Bonded Cyclic Dimers at Large Compression: The Case of 1H-pyrrolo[3,2-h]quinoline and 2-(2′-pyridyl)pyrrole

“…Both contain hydrogenbonded cyclic dimers of PQ molecules, which are bent for the C2 polymorph and planar for P2 1 /c (Figure 2). The ambient-pressure spectra of the two polymorphs are similar with the largest difference found for two relatively intense peaks located at 1062 and 1074 cm -1 [31]. For the P21/c structure the lower energy peak is less intense, while the opposite is found for The ambient-pressure spectra of the two polymorphs are similar with the largest difference found for two relatively intense peaks located at 1062 and 1074 cm −1 [31].…”

“…Adding to this spectroscopic signature is the difference in the calculated frequency of this vibration in the anti (3678 cm -1 ) and syn (3637 cm -1 ) conformers of PP. The considerable strength of the N-H•••N interaction can be linked to the short N•••N distance in both compounds (2.99 Å in PQ [31], 2.79 Å in PP [33]).…”

“…We note that, in both PP and PQ, the angle along the intramolecular N-H•••N interaction (circa 90 • ) can be viewed as too small to consider it a hydrogen bond. However, presence of a considerable intramolecular interaction between these atoms is supported by the redshift of the N-H stretching mode in isolated molecules: 3530 cm −1 in pyrrole [30] compared to 3456 (−74) cm −1 in PQ [31], and 3483 (−47) cm −1 in PP [32]. Adding to this spectroscopic signature is the difference in the calculated frequency of this vibration in the anti (3678 cm −1 ) and syn (3637 cm −1 ) conformers of PP.…”

“…Two crystal structures have been reported for PQ at ambient conditions-one of C2 symmetry (Z = 8) [34], the other with P2 1 /c symmetry (Z = 4) [31]. Both contain hydrogenbonded cyclic dimers of PQ molecules, which are bent for the C2 polymorph and planar for P2 1 /c (Figure 2).…”

Spectroscopic investigations of the new anhydrous proton‐conducting compound of pyrazole with oxalate acid

“…In studies of the intermolecular hydrogen bond (HB), an important class is compounds that consist of proton donors and proton acceptor groups and can form H‐bonded dimers . For such dimers, the strength of intermolecular HBs depends on the relative positions of the donor and the acceptor in the molecule . Carboxylic acid salts are a good group of compounds in which one can study a variety of supramolecular structures, including dimers, catemer chains, and rings .…”