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

摘要

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 (C-2v) configurations of compounds 1-3 possessing themaximum pi-electron delocalization within the M-1=C-2-C-3=C-4-M-5-H-6 keto-enol groups are less stable than their corresponding plane symmetrical (C-s) forms. Importantly, the symmetry breaking in the C-2v configurations of the enol forms of compounds 1-3 to their corresponding plane symmetrical C-s configurations is due to the pseudo Jahn-Teller effect (PJTE) by mixing the ground A(1) and excited B-2 electronic states resulting in a PJT (A(1) + B-2) circle times b(2) problem. We may expect that by the decrease of the energy gaps between reference states in the C-2v 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 pi-electron delocalization within the M-1=C-2-C-3=C-4-M-5-H-6 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 C-2v configurations to their corresponding C-s forms. The correlations between the structural parameters, synchronicity indices, natural charges, PJTSEs, electron delocalizations, and the hardness of compounds 1-3 have been investigated.