Synthesis, crystal structure and infrared spectra of new 6- and 7-propylamine-5,8-quinolinediones

We synthesized 6-chloro-7-propylamine-5,8-quinolinedione and 7-chloro-6-propylamine-5,8-quinolinedione and characterized these new compounds by X-ray diffraction and IR spectroscopy techniques supplemented by DFT calculations. Analyses were performed with the special attention to the structural changes caused by different positions of the propylamine chain in the studied 6- and 7-propylamine-5,8-quinolinediones. It was found that the title compounds differ significantly in crystal structures that strongly affects the formation of H-bonds. The detailed analysis of H-bonding in crystal structures of 6- and 7-substituents as well as in starting compound 6,7-dichloro-5,8-quinolinedione was performed. It was found that crystal structures were stabilized mainly by NH⋯O, CH⋯O and CH⋯N type hydrogen bonds. For 7-substituted 5,8-quinolinedione a strong twist of the propylamine chain against rings’ plane (near 90°) causing changes in formation of H-bonds was observed. The studied compounds were also analyzed due to their interesting carbonyl stretching bands in the infrared spectra. Considerable changes in the frequency separation of the carbonyl stretching bands for different positions of the propylamine chain were found. Correlation between frequency separation of the carbonyl bands and the nature and position of the 6- and 7-substituents may provide the opportunity to use the IR spectroscopy to study substitution reactions and regioisomerism in related compounds.