Probing the influence of solvent effects on the conformational behavior of 1,4-diazacyclohexane systems Anik Sen, Ajeet Singh, Bishwajit Ganguly ⇑ Central Salt & Marine Chemicals Research Institute (CSIR), Bhavnagar 364002, Gujarat, India article info Article history: Received 19 August 2010 Accepted 29 September 2010 Available online 23 October 2010 Keywords: DFT calculations Stereoelectronic effects Conformers Solvent Ab initio molecular dynamics (AIMD) calculations abstract Structural and energetic consequences of replacing one or more carbon atoms in small cyclic hydrocar- bons with electronegative atoms have been observed due to stereoelectronic anomeric and gauche effects. We have reported the influence of gauche effect on the conformational behavior of a 1,4-diaza- cyclohexane system using the DFT B3LYP/6-311 + G  level of theory. Factors which are found to control the conformational preferences of a 1,4-diazacyclohexane system include steric, dipolar repulsions and hyperconjugative interactions. The stability order of the 1,4-diazacyclohexane conformers was predicted to be similar in the gas phase and solvent continuum model (aqueous phase). The interactions of explicit water molecules with 1,4-diazacyclohexane conformers however predicted a different order of stability compared to the gas phase and continuum phase results. Ab initio molecular dynamics (AIMD) study showed the minor perturbation in the orientation of explicit water molecules compared to the DFT results and no dramatic changes was observed during the simulation with 1,4-diazacyclohexane conformers. Ó 2010 Elsevier B.V. All rights reserved. 1. Introduction Conformational analysis has always been an important topic of research in physical organic chemistry [1] as most of the properties like reactivity [2], spectroscopic behaviors etc. depends on the con- formation. Sachse was the first to recognize the existence of two kinds of CAH bonds for cyclohexane [3]. However, it is a matter of controversy, that, what mostly controls the properties or stabil- ities for even the simple systems and hence studies on conforma- tional analysis is still actively pursued. The stereoelectronic behavior of X–C–Y containing systems known as the anomeric effect and that of X–C–C–Y known as gauche effect, where X and Y are any electronegative atoms con- taining lone pairs have been studied widely [4–20]. The anomeric effect in an X–C–Y system is due to Xn p -r  C—Y two electron–two orbital interaction. The gauche effect was originally defined as the tendency for a molecule to adopt that structure which has the maximum number of synclinal (sc, gauche, 60°) interactions be- tween adjacent electron pairs and/or polar bonds in a molecular fragment X–C–C–Y, where X and Y are two electronegative substit- uents [19]. Both gauche and anomeric effects are absent when X and Y do not contain any interacting lone pair. In the recent studies interpretations of gauche effect are based on hyperconjugation ef- fect [16,18,21–23]. The hyperconjugation approach states a two electron/two orbital interaction which depends on the donor– acceptor ability of the orbitals, on the energy difference between them and on overlap symmetry [24]. Ethylenediamine (EDA) is one of the most studied structures for the conformational analysis for its vast number of structures avail- able. Van Alsenoy et al. [25] and Radom et al. [26] reported that EDA consists of 10 minimum energy structures. Electron diffrac- tion experiments in the gas phase have shown that nonprotonated EDA is predominantly (95%) in a gauche conformation with a dihe- dral angle u NCCN of 64° [27] and r CC and r NC bond lengths of 1.545 Å and 1.469 Å [26] respectively. Moreover microwave study [28] showed that in the dominant gauche form, one of the NH 2 groups may assume either two positions, depending on the hydrogen atom involved in the hydrogen bonding. A number of theoretical studies have also been performed in order to investigate the con- formational properties of EDA in the gas phase including empirical [27], semi-empirical calculations [29,30]. These studies shed light on the most stable structural conformers, their relative energies and electronic properties. The factors stabilizing the gauche con- formers are assumed to be both ‘gauche effect’ and intramolecular hydrogen bonding between two amine groups. Kazerouni and Hed- berg [31] estimated 0.20 kcal/mol stabilization through the gauche effect. Lee et al. [32] proposed 1.1 kcal/mol stabilization through the hydrogen bonding and a 2.4 kcal/mol destabilization for the lone pair repulsions. DFT calculations performed by Corte et al. [33] also showed that gauche conformers are the most stable con- formers in both gas and aqueous phase. The structural stabilities of 0022-2860/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2010.09.045 ⇑ Corresponding author. Tel.: +91 0278 2567760; fax: +91 278 2567562. E-mail address: ganguly@csmcri.org (B. Ganguly). Journal of Molecular Structure 984 (2010) 294–299 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc