Understanding the interaction mechanism of carbazole/anthracene with N,N-dimethylformamide: NMR study substantiated carbazole separation

Carbazole and anthracene, two aromatic hydrocarbon components contained in coal tar, are used as essential organic intermediates to synthesize various carbazole derivatives and anthraquinones. N,N-Dimethylformamide (DMF) is a commonly used solvent to extract carbazole from crude mixtures of carbazole and anthracene. However, the interaction between carbazole/anthracene and DMF in the extraction process is still to be fully understood at the molecular level. In this work, the intermolecular interaction of carbazole/anthracene with DMF was investigated using various NMR techniques, including ¹H NMR titration, variable temperature NMR spectroscopy (VT-NMR), Nuclear Overhauser Effect Spectroscopy (NOESY), and diffusion-ordered spectroscopy (DOSY). The observed ¹H chemical shift changes of carbazole indicated strong intermolecular hydrogen bonds between carbazole and DMF, which was further supported by the decrease in the molecular self-diffusion coefficients (D) of both carbazole and DMF according to DOSY measurements. Moreover, NOESY experiments revealed that the distance between the aldehydic hydrogen of DMF and the N–H of carbazole was smaller than 5 Å. Accordingly, an intermolecular hydrogen bond between carbazole and DMF in the form of CO⋯H–N was proposed. This research increases our knowledge about the separation process of carbazole and anthracene and hence helps improve the methods.