title: Magnetic Shielding, Aromaticity, Antiaromaticity, and Bonding in the Low-Lying Electronic States of Benzene and Cyclobutadiene.
abstract: 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 (S1and 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 S0states of C6H6and C4H4represent aromaticity and antiaromaticity "fingerprints" which are reproduced in other electronic states of the two molecules and allow classification of these states as aromatic (S0and S2for C6H6, T1and S1for C4H4) or antiaromatic (S0and S2for C4H4, T1and S1for C6H6). S2C6H6is predicted to be even more aromatic than S0C6H6. 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.
authors: Department of Chemistry, University of York , Heslington, York YO10 5DD, U.K.
authors: Peter B
authors: PB
authors: Karadakov
authors: Department of Chemistry, University of York , Heslington, York YO10 5DD, U.K.
authors: Peter
authors: P
authors: Hearnshaw
authors: Department of Chemistry, University of York , Heslington, York YO10 5DD, U.K.
authors: Kate E
authors: KE
authors: Horner