Conformational study of the (z)-[(2-iminoethylidone)silyl]amine at the MP2, DFT and G2MP2 levels Heidar Raissi, Mehdi Yoosefian ⇑ , Samaneh Khoshkhou Chemistry Department, Birjand University, Birjand, Iran article info Article history: Received 30 August 2011 Received in revised form 2 December 2011 Accepted 3 December 2011 Available online 20 December 2011 Keywords: (z)-1-(2-Iminoethylidene) silane amine Molecular conformation AIM NBO HOMO and LUMO abstract (z)-[(2-Iminoethylidone)silyl]amine has been studied extensively as a model involved in intramolecular hydrogen bonding (HB) assisted by p-electrons resonance. In order to evaluate the stability and the nat- ure of intramolecular HB, all of its possible tautomeric structures (imine and amine) were probed at B3LYP/6-311++G ÃÃ , MP2/6-311++G ÃÃ , and G2MP2 levels of theory. The calculated geometrical parameters and conformational analysis in gas phase and water solution indicated that the (z)-1-(2-iminoethylidene) silane amine conformers of this compound were more stable than the others. This stability is mainly due to the formation of an NÁÁÁHÀN intramolecular HB and the position of Si atom in these conformers. The ‘‘atoms in molecules’’ theory of Bader was used to analyze critical points and to study the nature of hydrogen bond in these systems. Natural bond orbital (NBO) analysis was also performed for better understanding the nature of intramolecular interactions. The calculated highest occupied molecular orbi- tal (HOMO) and lowest unoccupied molecular orbital (LUMO) with frontier orbital gap are presented. The result showed that NÁÁÁHÀN intramolecular HB in the mentioned compound possesses a bonding character. Ó 2011 Elsevier B.V. All rights reserved. 1. Introduction HB is undoubtedly the most vital and interesting interaction force. It strongly affects the structure and properties of many par- ticipating molecules. For example, the anomalous properties of water, the basic secondary structural elements of polypeptides and proteins (such as a-helix and b-sheet) as well as the double helical structure of DNA and RNA all are consequences of HB inter- actions [1–3]. These interactions also play a crucial role in crystal engineering [4], enzymatic catalysis [5], molecular recognition [3], proton transfer reactions [6] and many other chemical and bio- chemical processes. In recent years quantum mechanical calcula- tions of HB have attracted the attention of theoretical chemists, physicists and biologists [3,7–9]. The importance of hydrogen bonded systems has long been known, although the understanding of its nature is not yet complete. Indeed, HB is responsible for remarkable changes in the properties of solids, liquids or large clusters [3,10]. However, most theoretical studies have concen- trated in relatively small hydrogen bonded systems in order to be able to perform high level quantum chemical calculations. There is a wide range of phenomena involving hydrogen bonded systems, which can be probed by quantum mechanical calculations. For example, chirality on the intermolecular interaction [11], coopera- tivity or non additivity factors [12] and solvatochromic shift in adsorption transition [13] have been consistently investigated in the domain of HB. In this work, (z)-1-(2-iminoethylidene)silane amine (IESA) was chosen, because it contains NÁÁÁHAN intramolecular HB and has a relatively simple structure. IESA is resembled to the 3-imino-prope- nylamine [14] except replacement of C by Si. Unlike the case of car- bon, which easily forms strong p bonds through sp 2 hybridization, the p bond in silicon compounds is fairly weak. Furthermore, our studies will also provide useful information on replacement effect of a CH group by a less electronegative SiH group leading to the nat- ure of the intramolecular HB strength. Theoretically, three classes of tautomers namely, imino ethyl silanimine (ISI), imino silyl ethylene amine (SEA) and imino ethylidene silane amine (ISA) have been demonstrated in Fig. 1. Our theoretical calculations confirm that the HB strength increases from ISA to ISI and SEA. The main goals of this work were (i) to determine the order of stability of the various IESA conformations, (ii) to predict the most stable structure in the gas phase and in solution, and (iii) to eval- uate the intramolecular HB strength in more stable conformers. Here, the exact values of the intramolecular HB energies were esti- mated by the Espinosa method [15]. 2. Analysis method All the computations in the present study were performed by Gaussian 03 series of programs [16]. The geometry optimization was carried out at B3LYP, MP2 methods with 6-311++G ÃÃ (160 2210-271X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.comptc.2011.12.002 ⇑ Corresponding author. Tel.: +98 5612502064; fax: +98 5612502065. E-mail address: myoosefian@yahoo.com (M. Yoosefian). Computational and Theoretical Chemistry 983 (2012) 1–6 Contents lists available at SciVerse ScienceDirect Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc