Abstract
The structures of the mono- and the dihalogenated N-unsubstituted 2-aminobenzamides were characterized by means of the spectroscopic (1H-NMR, UV-Vis, FT-IR, and FT-Raman) and X-ray crystallographic techniques complemented with a density functional theory (DFT) method. The hindered rotation of the C(O)–NH2 single bond resulted in non-equivalence of the amide protons and therefore two distinct resonances of different chemical shift values in the 1H-NMR spectra of these compounds were observed. 2-Amino-5-bromobenzamide (ABB) as a model confirmed the presence of strong intramolecular hydrogen bonds between oxygen and the amine hydrogen. However, intramolecular hydrogen bonding between the carbonyl oxygen and the amine protons was not observed in the solution phase due to a rapid exchange of these two protons with the solvent and fast rotation of the Ar–NH2 single bond. XRD also revealed the ability of the amide unit of these compounds to function as a hydrogen bond donor and acceptor simultaneously to form strong intermolecular hydrogen bonding between oxygen of one molecule and the NH moiety of the amine or amide group of the other molecule and between the amine nitrogen and the amide hydrogen of different molecules. DFT calculations using the B3LYP/6-311++G(d,p) basis set revealed that the conformer (A) with oxygen and 2-amine on the same side predominates possibly due to the formation of a six-membered intramolecular ring, which is assisted by hydrogen bonding as observed in the single crystal XRD structure.
Highlights
The amide moiety is abundant in biologically-relevant molecules and its propensity for hydrogen bonding plays an important role in the spatial structure of proteins, nucleic acids and biological membranes as well as in the interaction of bioactive compounds with receptors [1,2]
The non-equivalence of the amide protons in the solution phase is due to a hindered rotation around the CO-NH2 single bond
Intramolecular hydrogen bonding between the carbonyl oxygen and the amine proton was not observed in the solution phase due to a rapid exchange of the two amine hydrogens with the solvent
Summary
The amide moiety is abundant in biologically-relevant molecules and its propensity for hydrogen bonding plays an important role in the spatial structure of proteins, nucleic acids and biological membranes as well as in the interaction of bioactive compounds with receptors [1,2]. 2solutionAminobenzamide, and in the solid state [18] Based on this observation, the authors concluded that a on the other hand, is considered to lack intramolecular hydrogen bonds and this substituent on the to nitrogen atom of of the plays an important role in[18]. Observed for the corresponding N-aryl formation of intramolecular bonds for substituted anthranilamides in solution and in the solid state [18] Based on this observation, the Despite the enormous pharmacological [11,12,13,15,16,17] and chemical [19,20,21,22,23,24] interest in the authors concluded that a substituent on the nitrogen atom of the arylamine moiety plays an. Means with of spectroscopic (NMR, UV-Vis, IR, Raman) methods and single X-ray crystallography in combination with density functional theory methods
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