Science & Technologies Volume VII, 2017, Number 3: NATURAL AND MATHEMATICAL SCIENCE 170 NATURAL BOND POPULATION ANALYSIS OF DIBROMOBIS(BENZIMIDAZOLE)Cu (II) COMPLEX Rumyana Yankova, Donika Stoicheva, Iva Aleksandrova Department of Inorganic and Analytical Chemistry, Assen Zlatarov University, 8010 Bourgas, Bulgaria, r_iankova@yahoo.com ABSTRACT The heterocycles are good ligands due to the presence of possible coordination sites. The complexes of transition metal salts with benzimidazole derivatives are important biological molecules. The structure of dibromobis(benzimidazole)Cu (II) complex was calculated by Density Functional Theory (DFT) method, with Becke’s three parameter exchange-functional of Lee, Yang and Paar and 6-31G(d,p) basis set. The electronic properties and the bond orders of the molecule were calculated. The natural bond orbitals calculations were performed in order to understand second-order interactions between the filled and vacant orbitals, which is a measure of the intramolecular delocalization or hyper conjugation. The strongest electron donation become from the lone pair orbitals on the nitrogen atoms, LP(1) N to the antibonding acceptor orbitals LP*(6;8;9) Cu and from the lone pair orbitals on the bromine atoms, LP(1;4) Br to the antibonding acceptor orbitals LP*(6;7) Cu. Key words: dibromobis(benzimidazole)Cu (II) complex, Density functional theory, NBO analysis INTRODUCTION The pharmacological properties and biological activity of the benzimidazole derivatives [1-4], have led researchers to investigate the coordination behavior of benzimidazole derivatives with transition metal ions. To predict properties of metal complexes with benzimidazole and choose their application, theoretical study seems to be necessary. The use of density functional theory makes an important contribution to promotion of theoretical calculations. A great number of metal complexes with benzimidazole are investigated with DFT method and are determined their geometry, electronic structure and chemical reactivity [5-9]. The molecular structural parameters of dibromobis(benzimidazole)Cu (II) complex and its chemical reactivity are calculated with DFT-B3LYP method using 6-31G(d,p) basis set [10]. Quantum chemical descriptors such as global hardness and softness, chemical potential, electronegativity and electrophilicity index, HOMO–LUMO energy gap and HOMO/LUMO energy are studied. Molecular electrostatic potential analysis is shown that the most negative charge is localized around bromine atoms, while a large electropositive potential is observed in the area of the benzimidazole ligands. The vibrational frequencies of the fundamental modes of the compound are precisely assigned, analyzed and the theoretical results are compared with the experimental ones. The objective of the present work is to investigate the nature of bonding in the dibromobis(benzimidazole)Cu (II) complex, by using natural bond orbital (NBO) analysis. The results from NBO calculations can provide the detailed insight into the electronic structure of molecule. EXPERIMENTAL The dibromobis(benzimidazole)Cu (II) complex was synthesized according to the method outlined by M. Bukowska-Strzyzewska and A. Tosik [11]. Identification of [Cu(benzimidazole) 2 Br 2 ] was carried out by means of FT-IR spectroscopy. The infrared spectra were recorded on a Nicolet iS 50 Thermo Scientific FT-IR spectrophotometer in the frequency region of 4000 – 400 сm –1 using nujol. The thermos-gravimetric analysis was performed on an apparatus STA 449 F3 JUPITER (Netzsch) for TG-DTG/DSC at heating rate 10°C min –1 from room temperature to 1000°C under flowing air (20 cm 3 min –1 ).