Pseudo-Jahn−Teller Distortion from Planarity in Heterocyclic Seven- and Eight-Membered Ring Systems with Eight π Electrons

Abstract

To elucidate the nature of a pseudo-Jahn-Teller (JT) distortion from planarity, ab initio MCSCF calculations with the 6-31G(d,p) basis sets were carried out in the ground states of seven-and eight-membered unsaturated heterocycles with eight π electrons. The monocyclic systems examined were found to show a variety of structural changes. Azepine and oxepin undergo the pseudo-JT distortions from a planar C 2V to a boat C s conformation. In 1,5-diazocine, the pseudo-JT distortion takes place in two stages, the initial step being from a symmetric planar D 2h to a skew C 2h structure and the subsequent step from the skew C 2h to a tub C 2 structure. The 1,3,5,7-tetrazocine molecule undergoes pseudo-JT distortions from a symmetric planar D 4h to skew C 4h and crown-like D 2d structures through the respective in-plane and out-of-plane nuclear deformations. Moreover, the C 4h and D 2d structures are distorted into the same tub-shaped S 4 conformation. An inspection of the energy components comprised in the total energy reveals that the stability of nonplanar structures arises from a lowering of the electron-nuclear attraction energy. The energy variation is accounted for in terms of an electrostatic interaction and the proximity among the nuclei and electron clouds owing to a folding of the molecular skeleton. It is shown that the theoretical structural characteristics for azepine, oxepin, 1,5-diazocine, and 1,3,5,7-tetrazocine are in good agreement with available experimental facts.

Is Excited‐State Aromaticity a Driving Force for Planarization of Dibenzannelated 8π‐Electron Heterocycles?

“…Cyclic 8π‐electron molecules in their S 0 states normally adopt non‐planar structures that are non‐aromatic rather than antiaromatic. Cyclooctatetraene (COT) in S 0 adopts a tub‐shaped geometry, avoiding the angle strain at the planar D 4h symmetric structure, and also azepines, oxepines, and thiepines adopt puckered conformations . The resonance energies of the planar structures of azepine and oxepines obtained through extended Hückel MO theory and early Hartree‐Fock computations suggested antiaromatic character, later supported by NICS calculations .…”

“…Cyclooctatetraene (COT) in S 0 adopts a tub-shaped geometry, avoiding the angle strain at the planar D 4h symmetric structure, [12] and also azepines, oxepines, and thiepines adopt puckered conformations. [13][14][15][16][17][18][19] The resonance energies of the planar structures of azepine and oxepines obtained through extended Hückel MO theory and early Hartree-Fock computations suggested antiaromatic character, [20,21] later supported by NICS calculations. [14,22] However, the dibenz[b,f]annelated derivatives were found to have positive resonance energies associated with some aromatic character as their benzene rings keep their Hückel aromaticity in line with Glidewell-Lloyd's extension of Clar's rule.…”

Homoheteroaromaticity: the case study of azepine and dibenzazepineElectronic supplementary information (ESI) available: absolute chemical shieldings calculated at the B3LYP/6-311++G**//B3LYP/6-311++G** computational level for compounds 1b, 2b, 4b and 5b. See http://www.rsc.org/suppdata/ob/b3/b314742h/

“…4 Its most characteristic geometric parameters are the CC double/single bond alternation (0.913 ratio) and the four dihedral angles: C6C5C4C3 (0.0Њ), C5C4C3C2 (27.3Њ), C4C3C2N1 (Ϫ0.1Њ) and C3C2N1C7 The most interesting structures are those of 3a and 3b (see Fig. 1).…”

“…The minimum energy structure of 1a reported by Toyota et al is a boat of C s conformation. 4 Its most characteristic geometric parameters are the CC double/single bond alternation (0.913 ratio) and the four dihedral angles: C6C5C4C3 (0.0Њ), C5C4C3C2 (27.3Њ), C4C3C2N1 (Ϫ0.1Њ) and C3C2N1C7 Scheme 1 Cations that could be formed by protonation of 1H-azepine 1a. The most interesting structures are those of 3a and 3b (see Fig.…”

“…Toyota et al have carried out an MCSCF/6-31G(d,p) study of I and related heterocycles to elucidate the nature of a pseudo-Jahn-Teller distortion from planarity. 4 When R = H, the antiaromaticity of I could be relieved by tautomerization to the 3H-structure III. [5][6][7] In the present paper we want to discuss the antiaromaticity of I and its dibenzo derivative (5H-dibenz [b, f]azepine), 5,6 as well as the possible homoaromatic character of the cations obtained by protonation.…”