A comprehensive theoretical study of conformational analysis, intramolecular hydrogen bond, π-electron delocalization, and tautomeric preferences in 2-selenoformyl-3-thioxo-propionaldehyde

In the present study, the conformational analysis, intramolecular hydrogen bond, π-electron delocalization, and tautomeric equilibriums in 2-selenoformyl-3-thioxo-propionaldehyde (STP) were investigated. First, conformational analysis of STP was performed, and our results have shown that the chelated thiol structures are more stable than the chelated enol and selenol ones; HB Thiol < HB Enol < HB Selenol. To justify this order, the affective factors on this stability such as tautomerism, hydrogen bonding, and π-electron delocalization were comprehensively investigated. Survey of tautomeric equilibriums in STP indicates that destabilization energy of keto⇋enol equilibrium (about 50 kJ/mol) is more than the seleno⇋selenol (about 22 kJ/mol) and then thio⇋thiol (about 20 kJ/mol) cases. Description of different hydrogen bonds by energetic, geometrical, topological, and molecular orbitals emphasizes on the presence of a stronger intramolecular hydrogen bond (IMHB) in the enol forms. Furthermore, evaluation of π-electron delocalization (π-ED) represents approximate superiority of enol conformers. The achieved results by HB and π-ED analyzes are in contrast with the mentioned stability order. Ultimately, investigation of above factors indicates that the tautomerism phenomenon plays a dominant role in determining of the conformational preferences.