Aromaticity and Antiaromaticity in the Low-Lying Electronic States of Cyclooctatetraene

Abstract

The levels of aromaticity of the most important geometries on the ground-state (S(0)), lowest triplet-state (T(1)), and first singlet excited-state (S(1)) potential energy surfaces (PESs) for cycloocta-1,3,5,7-tetraene (COT) are assessed using a wide range of magnetic criteria including nucleus-independent chemical shifts (NICSs), proton shieldings, and magnetic susceptibilities calculated using complete-active-space self-consistent-field (CASSCF) wave functions constructed from gauge-including atomic orbitals (GIAOs). It is shown that the ground state of D(8h) COT (transition state for the pi-bond-shift process on the S(0) PES) is markedly antiaromatic, even more so than the classical example of an antiaromatic system, the ground state of square cyclobutadiene. The CASSCF-GIAO magnetic properties of the ground state of D(4h) COT (transition state for the ring-inversion process on the S(0) PES) strongly suggest that it is much less antiaromatic than the ground state of D(8h) COT, whereas those of the ground state of D(2d) COT (local minimum on the S(0) PES) indicate that it is decidedly nonaromatic. The lowest triplet state and the first singlet excited state of D(8h) COT (local minima on the T(1) PES and the S(1) PES, respectively) exhibit surprisingly similar magnetic properties. These, in turn, are very close to the magnetic properties of benzene, which is a strong indication of a high degree of aromaticity.

Recent Studies on the Aromaticity and Antiaromaticity of Planar Cyclooctatetraene

“…The value is +30.1 at the GIAO/HF/6-31+G* level using the B3LYP/6-31G* optimized geometry, suggesting that the considerable antiaromatic paratropicity is generated in the planar COT ring even with the bond alternated structure. Similar result (+29.3) was obtained for D 4h COT at the GIAO/HF/6-311+G*//HF/6-31G** level [43]. However, the NICS(0) value was shown to be considerably reduced when the CASSCF-GIAO method was used.…”

“…The NICS calculations also supported the aromaticity of D 8h COT at T 1 state. The NICS(0) value is -12.4 at the GIAO/HF/6-31+G*//B3LYP/6-311+G** [42], which is, however, more negative than that (-8.9) of CASSCF-GIAO method [43]. Another study based on scanning NICS over a distance and separating them into in-plane and out-of plane contributions also confirmed the aromaticity of D 8h COT at T 1 state [49].…”

“…The value (+16.1) of D 4h COT at the CASSCF-GIAO level is almost half of that of HF-GIAO method. It is also much smaller than that (+40.7) of D 8h COT at open-shell S 0 state by factor of 2.3 [43]. For the estimation of NICS value of D 8h COT at S 0 state, the use of CASSCF-GIAO is essential, and hence such a large discrepancy is only observed in the method.…”

“…The degrees of aromaticity of tub-shaped D 2d (singlet (S 0 )) and planar D 4h (S 0 ) and D 8h (open-shell S 0 and triplet (T 1 ) states) COT that have been assessed using NICS criteria are summarized in Table 1, together with the bond lengths of each optimized geometries [38,42,43]. The NICS(0) value of tubshaped D 2d COT is +3.0 at the GIAO/HF/6-31+G* level using the B3LYP/6-311+G** optimized geometry [42].…”

Influence of fluoro and cyano substituents in the aromatic and antiaromatic characteristics of cyclooctatetraene

“…The aromaticity of its planar triplet and transition state as well as the aromaticity/antiaromaticity inversion observed between the ground and the excited states of COT have been previously studied. [18][19][20][21] Additionally, the properties of COT's hetero-derivatives as well as those of its dianion have also been explored in the literature. 11,[22][23][24][25] Besides, the reactivity of metallic derivatives of COT with carbon monoxide has been additionally explored at theoretical level.…”