A theoretical study on the aromaticity of benzene and related derivatives incorporating a C-CC-C fragment

Continuing with our interest in aromaticity, we have studied the influence that replacement of formal C-C single bonds by C-CC-C fragments, in a series of mono- (cyclobutadiene, benzene, and cyclooctatetraene) and fused-carbocycles (naphthalene and azulene), has in their properties, focusing mostly on NMR and aromaticity. We have analyzed the effect of such substitution not only in the aromaticity of the different structures, but also in the influence of low and high spin states by means of NICS values over the rings and 3D NICS isosurfaces. We have found that, in most of the cases, the substitution induces a loss of aromaticity in singlet states (both restricted and unrestricted) that can be recovered when triplet states are taken into account.