Symmetry breaking in the axial symmetrical configurations of enolic propanedial, propanedithial, and propanediselenal: pseudo Jahn–Teller effect versus the resonance-assisted hydrogen bond theory

The origin of the symmetry breaking in the axial symmetrical configurations of enolic propanedial (1), propanedithial (2), and propanediselenal (3) have been investigated by means of time-dependence density functional theory and natural bond orbital interpretations. The results obtained at the quantum chemistry composite (G2MP2, CBS-QB3), ab initio molecular orbital (MP2/6-311++G**), and hybrid density functional theory (B3LYP/6-311++G**) levels of theory showed that the hydrogen-centered synchronous axial symmetrical (C2v) configurations of compounds 1–3 possessing the maximum π-electron delocalization within the M1=C2–C3=C4–M5–H6 keto-enol groups are less stable than their corresponding plane symmetrical (Cs) forms. Importantly, the symmetry breaking in the C2v configurations of the enol forms of compounds 1–3 to their corresponding plane symmetrical Cs configurations is due to the pseudo Jahn–Teller effect (PJTE) by mixing the ground A1 and excited B2 electronic states resulting in a PJT (A1 + B2) ⊗ b2 problem. We may expect that by the decrease of the energy gaps between reference states in the C2v forms that are involved in the PJTE decrease from compound 1 to compound 3, the PJT stabilization energy (PJTSE) may increase but the results obtained showed that the corresponding PJTSEs decrease. This fact can be justified by the increase of the electron delocalizations from the nonbonding orbitals of the C=M moieties to the antibonding orbitals of the H–M bonds, which leads to an increase of the π-electron delocalization within the M1=C2–C3=C4–M5–H6 keto-enol groups. In confrontation between the impacts of the resonance-assisted hydrogen bond and PJTE in the structural and configurational properties of compounds 1–3, PJTE has an overwhelming contribution and causes the symmetry breaking of the C2v configurations to their corresponding Cs forms. The correlations between the structural parameters, synchronicity indices, natural charges, PJTSEs, electron delocalizations, and the hardness of compounds 1–3 have been investigated.