Separation of the energetic and geometric contributions to aromaticity. Part VIII. Changes in aromaticity and the evidence for the multi-dimensionality of the aromatic character of benzene rings in para- and meta-cyclophanes

Seven quantitative measures of aromaticity: I-6, HOMA, EN, GEO, HOSE, BE and BAC, for benzene itself and 39 benzene rings in 26 meta- and para-cyclophane derivatives are used for the study of the aromaticity of the benzene ring in these systems. The indices calculated from molecular geometry and supplemented by RHF/6-31G* energy calculations for each ring geometry are the subject of correlation and factor analyses. As a result evidence of a formal two-dimensional character of aromaticity is clearly shown. The first factor describing 49.8% of the total variance is mostly contributed by energetic indices (EN, BE, HOSE and RHF calculated energy, and to a lesser extent HOMA), the next factor describing 44.8% of the total variance depends mostly on geometric indices (I-6, GEO, HAC and partly HOMA). The HOMA index being composed of two contributions, energetic and geometric, correlates very well with RHF energies, which also take into account all conributions due to the deformations of the ring geometry. Deformations of benzene rings in cyclophanes due to substitution decrease aromaticity only slightly. (C) 1998 Elsevier Science Ltd. All rights reserved.