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 (S0), lowest triplet states (T1), and the first and second singlet excited states (S1 and S2) of benzene (C6H6) and square cyclobutadiene (C4H4) 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 S0 states
of C6H6 and C4H4 represent
aromaticity and antiaromaticity “fingerprints” which
are reproduced in other electronic states of the two molecules and
allow classification of these states as aromatic (S0 and
S2 for C6H6, T1 and S1 for C4H4) or antiaromatic (S0 and S2 for C4H4, T1 and
S1 for C6H6). S2 C6H6 is predicted to be even more aromatic than S0 C6H6. 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.