Persistence of Paratropic Ring Currents in Nonplanar, Tub-Shaped Geometries of 1,3,5,7-Cyclooctatetraene

Abstract

Surprisingly, the anti-aromatic paratropic ring current of the planar D 4h transition state of 1,3,5,7-cyclooctatetraene (COT) survives almost all the way to the equilibrium tub-shaped D 2d COT structure (plane-to-plane distance, d ) 0.76 Å); the current vanishes at ca. 80% of the geometric change and ca. 90% of the energy change from planar to tub geometry. 1,3,5,7-Cyclooctatetraene (COT) is a paradigm anti-aromatic 4n π electron system. In an equilateral planar D 8h geometry, COT is predicted by simple Hückel theory to have a triplet ground state, but the real ground state has a closed-shell singlet electronic configuration and a nonplanar tub-shaped equilibrium geometry of D 2d symmetry. 1 On the original definition of an anti-aromatic system as a "cyclic conjugated system [that] has its π energy higher than that of a suitable reference compound, which is not cyclically delocalized", 2 it is reasonable to suppose that the inherent anti-aromaticity of the 8π electron cycle contributes to the nonplanarity of the equilibrium structure. However, calculations suggest that the π resonance energy is negligible in planar conformations of COT 3 and the larger [4n]-annulenes, 4 indicating that the π electrons have no role in driving COT to nonplanarity. An interpretation consistent with ideas of both strain release 4,5 and π-distortivity 6 would be that the COT system moves from the equilateral D 8h to the bond-alternated D 4h structure by a first-order Jahn-Teller (π-electronic) distortion and that further distortion to the out-of-plane D 2d geometry is driven primarily by release of angle strain, as C-C-C bond angles are thereby reduced from the 135°angle of the planar octagon toward the ideal sp 2 angle of 120°. From a combination of gas-phase electron-diffraction and single-crystal X-ray mea-

Recent Studies on the Aromaticity and Antiaromaticity of Planar Cyclooctatetraene

“…2 ppm upfield shifted from that of parent COT (δ 5.68) [65]. CTOCD-DZ calculations supported the presence of a paratropic ring current in 5 [52]. Here, we also calculated NICS(0) value (GIAO/HF/6-311+G**//B3LYP/6-31G**) and HOMO-LUMO gap (B3LYP/6-31G**) of 3a, 4a, and 5 ( Figure 5), and only 5 showed a relatively large positive NICS value in accord with the observed large upfield shift of the ring protons.…”

“…CTOCD-DZ clearly demonstrated that the paratropic ring current in D 4h COT is dominated by the HOMO-LUMO transition [50,51], meaning the paratropicity corresponds to circulation of only two-electrons in the frontier orbitals. Furthermore, the paratropic ring currents in tub-shaped D 2d COT were also investigated by means of CTOCD-DZ [52]. In the study, d (Å) value is defined as the distance between the planes of the upper and bottom four carbons of tub-shaped COT ring.…”

Persistent Currents in One Dimension: The Counterpart of Leggett’s Theorem

“…The presence of paramagnetism or orbital ferromagnetism [20] in cyclooctatetraene is still a matter of debate [18,21]. A ferromagnetic instability can occur in small paramagnetic rings provided their inductance is large enough.…”

“…We therefore have X 3 × 10 −3 , far from the ferromagnetic instability. Moreover, Jahn-Teller distortions in cyclotetraene have been predicted [21] which would reduce the curvatures.…”

The homotropenylium cation: a system with a pinched π ring current

“…This hampers an unambiguous analysis of the current density in terms of selection rules for purely rotationally or translationally allowed excitations between occupied-to-unoccupied delocalised canonical molecular orbitals (CMOs). 9,10 Here we choose a reference plane based on the near co-planarity of the main conjugated system 12,19,20 and take the common plane of centres C1, C3, C5 and C7 (See Fig. 2), the normal to which implies a direction of the magnetic field for any choice of the distance R (C1 … C7).…”