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

TY - JOUR

T1 - Magnetic Shielding, Aromaticity, Antiaromaticity and Bonding in the Low-Lying Electronic States of Benzene and Cyclobutadiene

AU - Karadakov, Peter Borislavov

AU - Hearnshaw, Peter

AU - Horner, Kate Elizabeth

N1 - © 2016, American Chemical Society. This is an author-produced version of the published paper. Uploaded in accordance with the publisher’s self-archiving policy. Further copying may not be permitted; contact the publisher for details.

PY - 2016/11/18

Y1 - 2016/11/18

N2 - 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.

AB - 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.

UR - http://www.scopus.com/inward/record.url?scp=84996563955&partnerID=8YFLogxK

U2 - 10.1021/acs.joc.6b02460

DO - 10.1021/acs.joc.6b02460

M3 - Article

SN - 0022-3263

VL - 81

SP - 11346

EP - 11352

JO - Journal of Organic Chemistry

JF - Journal of Organic Chemistry

IS - 22

ER -