The planarity of heteroatom analogues of benzene: Energy component analysis and the planarization of hexasilabenzene

Abstract

There are various nonplanar heteroatom analogues of benzene—cyclic 6π electron systems—and among them, hexasilabenzene (Si6H6) is well known as a typical example. To determine the factors that control their planarity, quantum chemical calculations and an energy component analysis were performed. The results show that the energy components mainly controlling the planarity of benzene and hexasilabenzene are different. For hexasilabenzene, electron repulsion energy was found to be significantly important for the planarity. The application of the pseudo Jahn–Teller effect and the Carter–Goddard–Malrieu–Trinquier model for the interpretation of the planarity of the benzene analogues was also investigated. Furthermore, based on the quantitative results, it was revealed that the planarization of hexasilabenzene is realized by introducing substituents with π‐accepting ability, such as the boryl group, that bring about a reduction of the π‐electron repulsion on the silicon skeleton. © 2018 Wiley Periodicals, Inc.

Planarity of ethylene/linear polyene analogues focused on π‐electron holding ability of the components

“…For both the cases, the nonplanarity increases as the calculational levels become highly accurate except for the B3LYP level. This is in contrast to the tendency of the planarity of benzene analogues observed in our previous study . On the other hand, the planarity increases, as a basis set becomes big and flexible at every calculational level in the same manner as in the case of heavy benzenes.…”

“…The energy component mainly controlling the planarity is different between ethylene and the heavier analogues. Interestingly, the same was also seen in the relation of benzene and heavy benzenes as shown in our previous study, in which it was suggested by the RD model that electron repulsions play a dominant role in the planarity of heavy benzenes. As shown in Figure , EE and NN decrease as the structure becomes nonplanar for every molecule.…”

“…It is common knowledge that the trans‐bent structure of heavy alkenes has been explained sufficiently by the CGMT model. However, the idea of fragmentation cannot be applicable for every system, for instance, benzene analogues as noted in the previous study . Here, another explanation by our RD model has been applied for the present issue about the planarity of the linear π‐conjugated systems composed of group 14 elements.…”

“…Because electron repulsions were found to be important for the planarity of heavy ethylenes, the introduction of the substituents having π‐accepting ability is expected to be effective to make them planar as seen in the heavy benzenes reported in our previous study . Therefore, the effect of some π‐electron‐accepting substituents for the planarity of ethylene analogues was investigated.…”

“…[10][11][12][13][14] For heavy ethylenes like disilene or heavy benzenes like hexasilabenzene (Si 6 H 6 ), the mechanism of adopting the nonplanar geometry has been discussed by the pseudo Jahn-Teller effect [15] and the Carter-Goddard-Malrieu-Trinquier (CGMT) model [16][17][18][19] so far. In addition to these discussion, the repulsion dominant (RD) model [20] focusing on electron repulsions was proposed in our previous research, where the cause of the nonplanarization of high-periodic benzene analogues and promising methods to control the planarity such as substituent effects were suggested. In the present study, the planarity of linear unsaturated compounds has been investigated to deepen the RD model for the comprehensive understanding of the planarity of the π-conjugated systems including heavier group 14 hetero elements.…”