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

Abstract

In this work, the intermolecular interaction of carbazole/anthracene with DMF was investigated using various NMR techniques. More importantly, the intermolecular hydrogen bond between carbazole and DMF in the form of CO⋯H–N was proposed. 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 1 H NMR titration, variable temperature NMR spectroscopy (VT-NMR), Nuclear Overhauser Effect Spectroscopy (NOESY), and diffusion-ordered spectroscopy (DOSY). The observed 1 H 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 CO⋯H–N was proposed. This research increases our knowledge about the separation process of carbazole and anthracene and hence helps improve the methods.