Halogen bonding in the antibacterial 1,2,4-triazole-3-thione derivative – Spectroscopic properties, crystal structure and conformational analysis Barbara Miroslaw a,⇑ , Tomasz Plech b , Monika Wujec b a Department of Crystallography, Faculty of Chemistry, Maria Curie-Sklodowska University, Maria Curie-Sklodowska sq. 3, 20-031 Lublin, Poland b Department of Organic Chemistry, Faculty of Pharmacy, Medical University, Chodzki 4A, 20-093 Lublin, Poland highlights  Conformational analysis of 1,2,4- triazole-3-thione halogen derivatives.  Spectroscopic features of 1,2, 4-triazole-3-thione derivative in the solid state.  Halogen bonding in 1,2,4-triazole-3- thione derivatives. graphical abstract article info Article history: Received 30 October 2014 Received in revised form 21 November 2014 Accepted 25 November 2014 Available online 2 December 2014 Keywords: Conformational analysis Rotational disorder Molecular electrostatic potential Mercaptotriazoles Halogen bonding abstract The molecular structure of 4-(4-bromophenyl)-5-(3-chlorophenyl)-2,4-dihydro-3H-1,2,4-triazole-3-thi- one (TP-4) has been determined by the X-ray diffraction experiment and compared to the geometry calculated in the ground state by using HF and DFT methods. The compound crystallizes in the triclinic P-1 space group. To explain the observed rotational disorder of meta-chloro-substituted aromatic ring the conformational analysis was performed for TP-4 and the molecular energy profile has been obtained. The vibrational frequencies in the solid state were recorded and compared to the calculated in the ground state. The molecular electrostatic potential isosurfaces (MEPS) were calculated to confirm the role of halogen bonds in stabilizing the crystal structure. Ó 2014 Elsevier B.V. All rights reserved. Introduction Our previous studies have demonstrated that 4-(4-bromophenyl)- 5-(3-chlorophenyl)-2,4-dihydro-3H-1,2,4-triazole-3-thione (TP-4) (Fig. 1) exerts multidirectional pharmacological activity. One of the significant activities of the title compound is the ability to inhi- bit the growth of Gram-positive bacteria, including Staphylococcus aureus strains – responsible for life-threatening infections. More- over, it was observed that among the strains extremely susceptible to the activity of TP-4 were Bacillus subtilis and Bacillus cereus [1]. The significance of this fact is due to the morphogenetic similarity of the latter strain to the Bacillus anthracis (causing anthrax) [2]. It is therefore highly probable that a compound exhibiting antibacte- rial activity against B. cereus will also constitute an efficient drug http://dx.doi.org/10.1016/j.molstruc.2014.11.060 0022-2860/Ó 2014 Elsevier B.V. All rights reserved. ⇑ Corresponding author. Tel.: +48 81 5375582; fax: +48 815333348. E-mail addresses: barbara.miroslaw@poczta.umcs.lublin.pl (B. Miroslaw), to- maszplech@umlub.pl (T. Plech), monika.wujec@umlub.pl (M. Wujec). Journal of Molecular Structure 1083 (2015) 187–193 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc