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

摘要

The levels of aromaticity of the most important geometries on the ground-state(S_0),lowest triplet-state(T1),and first singlet excited-state(S1)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 T1 PES and the S1 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.