B3LYP and MP2 methods with the most popular basis set, 6-311++G(d,p) are applied to optimize the equilibrium conformers of 4-amino-3-pentene-2-thial. Furthermore, to have more reliable energies, the total electron energies of all forms are recomputed at the CBS-4M level of theory. A theoretical investigation of the equilibrium conformers clearly shows that various intramolecular hydrogen bonds (IHBs) such as N–H...S, S–H...N, S–H...π, C–H...N, and C–H...S are the most effective factors in the conformational preference of thialamine, thiolimine, and thialimine groups. Hence, the IHB strengths are evaluated in various resonance-assisted hydrogen bond systems by geometrical factors, topological parameters, and charge transfers corresponding to orbital interactions. Also, the solvent effect on the IHB strength is considered using Tomasi′s PCM. Our results in the gas phase reveal that the thialamine group has extra stability with respect to thiolimine and thialimine ones. The population analyses of all the possible conformers by the NBO method predict that the origin of this tautomeric preference is mainly due to more significant π electron delocalization in the framework of thialamine forms, especially πC=C → π C = S* and Lp(N) → π C = C* charge transfers. Moreover, the excited state properties of IHBs in these systems are investigated theoretically using the time-dependent DFT method.
