Abstract
The goal of this brief partly review paper is to summarize the results of the works published over the last few years regarding the origin of the out-of-plane distortions (puckering) of heterocyclic compounds. In all the papers devoted to this problem, it is shown that the instability of planar configurations of heterocyclic molecules leading to symmetry breaking and distortions is induced by the pseudo Jahn-Teller effect (PJTE). Special attention in this work is paid to the mechanism of suppression and enhancement of the PJTE distortions of heterocycles by oxidation, reduction, and chemical substitutions. It is demonstrated that oxidation of 1,4-dithiine containing compounds leads to suppression of the PJTE and to restoration of their planar nuclear configurations. An example of a dibenzo[1,2]dithiine molecule is used to demonstrate the mechanism of enhancement of the PJTE by reduction. It is shown that the reduction of the neutral C12H8S2 molecule up to the dianion (C12H8S2)2- enhances the PJTE, followed by the S-S bond cleavage and significant structural distortions of the system. The change of the PJTE by chemical substitutions, accompanied either by puckering or by planarization of heterocyclic compounds, is discussed using as examples 1,4-ditinine and its S-oxygenated derivatives.
Highlights
In all the papers devoted to this problem, it is shown that the instability of planar configurations of heterocyclic molecules leading to symmetry breaking and distortions is induced by the pseudo Jahn-Teller effect (PJTE)
All these compounds are unstable in the high-symmetry planar nuclear configuration of D2h symmetry, in the equilibrium configuration of C2v symmetry they are bent at the S-S axis
Analysis of the results of published works and the results obtained in this paper allows us to draw the following conclusions: 1) The instability of planar nuclear configurations of all the considered heterocyclic molecules and their out-of-plane distortions are due to the pseudo Jahn-Teller effect
Summary
The first group of compounds includes 1,4-dithiine molecules C4S2L4 [10] with a variety of the ligands (L = H, F, Cl, Br), S-oxygenated derivatives of 1,4-dithiine, two tricyclic systems (thianthrene [11] and C6S8 with two S-atoms in the 1,4-position of the central six-membered ring [13]), and 1,4-dihydropyrazine and its derivatives C4N2H4L2 [15] in which two imide hydrogen atoms are replaced with halogens (L = F, Cl, Br) All these compounds are unstable in the high-symmetry planar nuclear configuration of D2h symmetry, in the equilibrium configuration of C2v symmetry they are bent at the S-S axis. As in the previous cases, in these compounds the planar nuclear configuration of C2v symmetry is distorted along the out-of-plane b1 coordinate of instability that leads to equilibrium puckered structure of Cs symmetry. All the calculations were carried out using the GAUSSIAN 09 program package [23].The numerical values of the vibronic coupling constants were estimated by means of fitting the solutions of the secular equations to the ab initio calculated energy profiles
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