A Comprehensive Theoretical Study of Amide Resonance, Intramolecular Hydrogen Bonding, and π-Electron Delocalization in Diformyl and Dithioformyl Amine

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 ??-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, ??-electron delocalization of chelated rings was measured using various indices, such as q, ??', ??, and HOMA index. Finally, the orders of hydrogen bond strength and ??-electron delocalization, obtained by some of the indices, support the resonance assisted hydrogen bond theory.