Excited-State Aromaticity Reversals in Naphthalene and Anthracene.

Aromaticity reversals between the electronic ground (S 0 ) and low-lying singlet (S 1 , S 2 ) and triplet (T 1 , T 2 , T 3 ) states of naphthalene and anthracene are investigated by calculating the respective off-nucleus isotropic magnetic shielding distributions using complete-active-space self-consistent field (CASSCF) wavefunctions involving gauge-including atomic orbitals (GIAOs). The shielding distributions around the aromatic S 0 , antiaromatic S 1 ( 1 L b ), and aromatic S 2 ( 1 L a ) states in naphthalene are found to resemble the outcomes of fusing together the respective S 0 , S 1 , and S 2 shielding distributions of two benzene rings. In anthracene, 1 L a is lower in energy than 1 L b , and as a result, the S 1 state becomes aromatic, and the S 2 state becomes antiaromatic; the corresponding shielding distributions are found to resemble extensions by one ring of those around the S 2 and S 1 states in naphthalene. The lowest antiaromatic singlet state of either molecule is found to be significantly more antiaromatic than the respective T 1 state, which shows that it would be incorrect to assume that the similarity between the (anti)aromaticities of the S 1 and T 1 states in benzene, cyclobutadiene, and cyclooctatetraene would be maintained in polycyclic aromatic hydrocarbons.