Density functional theory study of intermolecular interactions of cyclic tetrazole dimers Alireza Najafi Chermahini a, * , Aseyeh Ghaedi a , Abbas Teimouri b , Fariborz Momenbeik a , Hossein A. Dabbagh c a Department of Chemistry, Faculty of Science, Yasouj University, Yasouj, Iran b Payame Noor University, Isfahan, Iran c Department of Chemistry, Isfahan University of Technology, Isfahan, Iran article info Article history: Received 22 April 2008 Received in revised form 23 July 2008 Accepted 23 July 2008 Available online 31 July 2008 Keywords: Tetrazole Substituent effect Intermolecular hydrogen bonding DFT NBO Vibrational analysis abstract In this study the density functional theory at B3LYP/6-31++G(d,p) level was employed to evaluate the hydrogen bonding between tetrazole dimers in the gas phase. Four cyclic dimers including one 1H–1H and three 2H–2H dimers were considered. The N...H–N hydrogen bonding which studied in the self assembled structures has been estimated from stabilization energies. The calculated hydrogen-bonding energies of various tetrazole dimers showed a cooperative interaction in the cyclic ones. It was found that 1H–1H dimers are more stable and introduction of an electron-releasing group (ERG) into the tetrazole rings resulted in the formation of more stable hydrogen bonding. Harmonic frequencies of monomers and dimers were calculated and it was found that change in the complexation vibrations is sensitive to the strength of the N...H–N hydrogen bondings. Natural population analysis was performed to predict electrostatic interactions in the cyclic H-bonded complexes. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Tetrazole derivatives find a wide range of pharmaceutical appli- cations [1] where they act as stimulants or sedatives on the central nervous system. These compounds have antimicrobial, antiinflam- matory, antilipemic, and antiallergic activities cis-peptide bond mimics [2]. Moreover, such compounds are useful as oxidizers and effective agents for regulating plant growth and as explosives and rocket propellants [3]. In addition, tetrazole compounds have a significant role in medicinal chemical research [4]. In the gas phase, experimental results provide information about the exis- tence of two tautomeric forms for tetrazole and its derivatives [5–7]. On the other hand, theoretical calculations have shown that, in the gas phase, the 2H continues to be the lowest energy tauto- mer on the semiempirical, [8] ab initio HF, [9,10] and DFT levels [11]. Recently some publications describing the tautomerism of 5-substituted tetrazoles by ab initio method became available, for all the substituents at the position 5 of the ring the 2H-tauto- mer is the most stable in the gas phase and the difference in energy between 1H- and 2H-tautomers depends only slightly on the nat- ure of substituent R [12]. In the solid state, according to X-ray data and vibrational spectroscopy, the crystalline parent tetrazole and some of its derivatives in position 5, 1H-tautomers are dominant forms [13]. Recently our calculations have shown that for tetra- zoles with electron withdrawing groups in the gas phase and solu- tion 2-H forms are more stable [14]. Hydrogen bond is an important type of noncovalent interaction that is present in many chemical and biological systems [15].A sound knowledge of hydrogen bond is fundamental to understand chemical structures, enzyme catalysis, material properties, self- assembly phenomena, and functions of molecular and biological devices and machines. Therefore, considerable amount of experi- mental and theoretical researches has been conducted concerning the structural, spectroscopic, and energetic issues of diverse hydro- gen bonds [16–19]. The three types of H-bonding interactions which are most often discussed in the literature are weak, moder- ate, and strong [20]. In the X–H...Y that X is proton donor and Y is proton acceptor, the hydrogen bonding maybe evaluated geomet- rically by increase of X–H bond length and decrease of X...Y distance. In addition hydrogen-bond angle for strong hydrogen- bonds ranges from 170–180° [20]. 2. Computational procedures DFT calculations were performed using GAUSSIAN 03 package [21]. Density functional theory was used with Becke3-Lee-Yang- Parr (B3-LYP) exchange-correlation 6-31++G ** basis set [22,23]. 0166-1280/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2008.07.023 * Corresponding author. Tel./fax: +98 7412223048. E-mail addresses: najafi@mail.yu.ac.ir, najafy@gmail.com (A.N. Chermahini). Journal of Molecular Structure: THEOCHEM 867 (2008) 78–84 Contents lists available at ScienceDirect Journal of Molecular Structure: THEOCHEM journal homepage: www.elsevier.com/locate/theochem