Experimental and computational study on dimers of 5-halo-1H-indole-2-carboxylic acids and their microbiological activity

Molecular structures and vibrational spectra of 5-fluoro-1H-indole-2-carboxylic acid (5FI2CAH2), 5‑chloro-1H-indole-2-carboxylic acid (5ClI2CAH2) and 5‑bromo-1H-indole-2-carboxylic acid (5BrI2CAH2) have been investigated. Single crystal X-ray diffraction for 5FI2CAH2 has revealed that a pair of 5FI2CAH2 molecules forms a cyclic dimer connected by dual linear O−H⋯O hydrogen bonds, while the NH group of the pyrrole ring does not participate in hydrogen bond. Theoretical studies were performed for the four possible dimers of 5XI2CAH2 (X = F, Cl, Br) using the B3LYP and MP2 methods with the 6–31++G(d,p) basis set. The calculated dimers included various possible combinations of O−H⋯O and N−H⋯O intermolecular interactions. The optimized structural parameters and the calculated vibrational spectra of the studied compounds show a good agreement with the experiment. The in vitro antibacterial activity against Gram-negative (E. coli, K. pneumoniae and P. aeruginosa) and Gram-positive (B. subtilis, L. monocytogenes and S. aureus) bacteria and antifungal activity against A. niger and C. albicans of 5XI2CAH2 have been investigated. All 5XI2CAH2 molecules have revealed particularly high activity against L. monocytogenes which is one of the most common of foodborne pathogens.