Magnetic Shielding, Aromaticity, Antiaromaticity and Bonding in the Low-Lying Electronic States of Benzene and Cyclobutadiene

Aromaticity, antiaromaticity, and their effects on chemical bonding in the ground states (S 0), lowest triplet states (T 1), and the first and second singlet excited states (S 1 and S 2) of benzene (C 6H 6) and square cyclobutadiene (C 4H 4) are investigated by analyzing the variations in isotropic magnetic shielding around these molecules in each electronic state. All shieldings are calculated using state-optimized π-space complete-active-space self-consistent field (CASSCF) wave functions constructed from gauge-including atomic orbitals (GIAOs), in the 6-311++G(2d,2p) basis. It is shown that the profoundly different shielding distributions in the S 0 states of C 6H 6 and C 4H 4 represent aromaticity and antiaromaticity "fingerprints" which are reproduced in other electronic states of the two molecules and allow classification of these states as aromatic (S 0 and S 2 for C 6H 6, T 1 and S 1 for C 4H 4) or antiaromatic (S 0 and S 2 for C 4H 4, T 1 and S 1 for C 6H 6). S 2 C 6H 6 is predicted to be even more aromatic than S 0 C 6H 6. As isotropic shielding isosurfaces and contour plots show very clearly the effects of aromaticity and antiaromaticity on chemical bonding, these can be viewed, arguably, as the most succinct visual definitions of the two phenomena currently available.