The aromaticity and antiaromaticity of the ground state(S_0),lowest triplet state(T1),and first singlet excited state(S1)of benzene,and the ground states(S_0),lowest triplet states(T1),and the first and second singlet excited states(S1 and S2)of square and rectangular cyclobutadiene are assessed using various magnetic criteria including nucleus-independent chemical shifts(NICS),proton shieldings,and magnetic susceptibilities calculated using complete-active-space self-consistent field(CASSCF)wave functions constructed from gauge-including atomic orbitals(GIAOs).These magnetic criteria strongly suggest that,in contrast to the well-known aromaticity of the S0 state of benzene,the T1 and S1 states of this molecule are antiaromatic.In square cyclobutadiene,which is shown to be considerably more antiaromatic than rectangular cyclobutadiene,the magnetic properties of the T1 and S1 states allow these to be classified as aromatic.According to the computed magnetic criteria,the T1 state of rectangular cyclobutadiene is still aromatic,but the S1 state is antiaromatic,just as the S2 state of square cyclobutadiene;the S2 state of rectangular cyclobutadiene is nonaromatic.The results demonstrate that the well-known"triplet aromaticity"of cyclic conjugated hydrocarbons represents a particular case of a broader concept of excited-state aromaticity and antiaromaticity.It is shown that while electronic excitation may lead to increased nuclear shieldings in certain low-lying electronic states,in general its main effect can be expected to be nuclear deshielding,which can be substantial for heavier nuclei.
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