2053 ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2019, Vol. 93, No. 10, pp. 2053–2060. © Pleiades Publishing, Ltd., 2019. A Comprehensive Theoretical Study of Amide Resonance, Intramolecular Hydrogen Bonding, and π-Electron Delocalization in Diformyl and Dithioformyl Amine Seyede Samira Hosseini a, *, Alireza Nowroozi b , Ebrahim Nakhaei a , and Abbas Heshmati Jannat Magham a a Department of Chemistry, Payame Noor University, Tehran, Iran b Department of Chemistry, Faculty of Science, University of Sistan and Baluchestan (USB), P.O. Box 98135-674, Zahedan, Iran *e-mail: s.hosseini@pnu.ac.ir Received September 7, 2018; revised January 6, 2019; accepted January 15, 2019 Abstract—In the present work, a comprehensive theoretical study of diformyl (DFA) and dithioformylamine (DTFA) were performed by various theoretical methods, HF, B3LYP, and MP2 with 6-311++G(3df,3pd) basis set, and the equilibrium conformations were determined. According to these calculations, tautomeric and conformeric preferences, intramolecular hydrogen bond (IMHB) and π-electron delocalization were investigated. Generally, DAI/DTAI tautomers have extra stability with respect to the AI/TAI ones. Also, among the AI/TAI series, the AI-11/TAI-13 conformer with/without IMHB is the most stable tautomer. Moreover, our computation results reveal that the formation of the intramolecular hydrogen bond is accepted as the origin of conformational preferences. The results of hydrogen bond descriptors, such as HB energy, geometrical, topological, spectroscopic and molecular orbital parameters are in agreement with the related energy values and show that the strength of intramolecular hydrogen bond in DFA is more than DTFA ones. On the other hands, π-electron delocalization of chelated rings was measured using various indices, such as q, λ', λ, and HOMA index. Finally, the orders of hydrogen bond strength and π-electron delocalization, obtained by some of the indices, support the resonance assisted hydrogen bond theory. Keywords: DFA, TDFA, intramolecular hydrogen bond, π-electron delocalization, resonance assisted hydro- gen bond and topological parameters DOI: 10.1134/S0036024419100091 INTRODUCTION The resonance assisted hydrogen bond (RAHB) theory claims that the IMHB and π-electron delocal- ization (π-ED) are interrelated [1–3]. In recent years, the RAHB model has become a widely accepted and applied to understand the nature of hydrogen bonding in a broad range of benchmark systems [4, 5]. How- ever, the RAHB idea has been the subject of various challenges, and Yanez et al. have published a series of papers and questioning the validity of RAHB idea [6– 8]. Malonaldehyde (MA), as the simplest member of RAHB systems, have been extensively investigated theoretically and experimentally [9–14]. The presence of amide groups in peptides, pro- teins, and biological molecules determines the impor- tance of this class of compounds in biochemistry, organic and medicinal chemistry [15–18]. Diformyl and dithioformyl amine (DFA and DTFA) are known as the simplest derivatives of amide compounds. The molecular structure of DFA and DTFA are similar to the MA, except that they have C=W (O, S), and C‒N=C instead of C=O and C–C=C function group. Proton transfer in the tautomeric equilibrium results in existence of cis and keto-enol form of these structures (Fig. 1) with different conformers. In some of this tautomers, the proton donor and proton accep- tor of hydrogen bond is connected by a conjugated framework, and the result of this association is a reso- nance assisted hydrogen bond (RAHB). In this regard, Spirko and coworkers in 2002 published a brief study [19] on the DFA molecules, but no other complete work on the DFA and DTFA yet been published. Since, small RAHB systems can be used as models for recognition of the behavior of complex biological mol- ecules. By considering the characteristics of DFA and DTFA, such as the variety of tautomers, amide reso- nance, different IMHB containing, and π-ED, we decided to implement a comprehensive theoretical study on the possible conformations of this structures with emphasis on the RAHB systems as follows: (I) identification of the equilibrium structures of DFA and DTFA conformers, STRUCTURE OF MATTER AND QUANTUM CHEMISTRY