Solvent effects on the molecular stability, intramolecular hydrogen bond, and π-electron delocalization in the simple RAHB systems with different donors and acceptors: a quantum chemical study

Abstract

In the present study, solvent effects on the molecular stability, intramolecular hydrogen bond (IMHB), and π-electron delocalization (π-ED) in some of the simple resonance-assisted hydrogen bond (RAHB) systems with different donors and acceptors are investigated. In this regard, all of the H-bonded structures (enol, thiol, and selenol) in the gas phase and presence of water/DMSO solvents with polarizable continuum (PCM)/self-consistent isodensity PCM methods at the M06-2X/6-311G++(d,p) level of theory are optimized. Relative energies clearly indicate that the enol conformers are more stable than the thiol and selenol ones in the gas phase while in the presence of polar solvents, the thiol structures are more stable than the other members. In the RAHB systems, the IMHB, π-ED, and tautomerization process are important factors in determining the stability of the H-bonded conformers. A survey of tautomeric equilibriums clearly shows that the tautomerization/activation energy of thio ⇋ thiol equilibrium is greater/smaller than the other equilibriums. These results reveal that the thiol tautomer is more preferred than the enol and selenol ones, from kinetic and thermodynamic points of view. Estimation of the IMHB by different descriptors emphasizes the presence of a stronger IMHB in the enol conformers. Moreover, the π-ED of enol tautomers are greater than those of the thiol and selenol ones. The results of IMHB and π-ED descriptors are compatible/incompatible with the stability order of the gas phase/solvents. Consequently, one can conclude that in the gas phase, the IMHB and π-ED are superior factors to determine the stability of tautomers. But in the presence of polar solvents, the tautomerization process is a dominant factor.