Separation of the energetic and geometric contribution to aromaticity. Part X. The case of benzene rings in fused polycyclic benzenoid hydrocarbons.

Eight aromaticity indices were estimated by the application of the bond lengths of 18 benzene rings taken from 10 benzenoid hydrocarbons (benzene, naphthalene. anthracene, tetracene, phenanthrene, chrysene, triphenylene, pyrene, perylene and coronene) computed by ab initio at RHF 6-31G** level of theory. The following indices were computed: three energetic indices (BE, EN and Hartree Fock energy for a constrained bond lengths in the ring), two geometric indices (Bird's I-6 and GEO) two magnetic indices (Schleyer's NICS for benzene rings in the benzenoid hydrocarbons and another one computed for isolated rings with bond lengths taken from the rings of benzenoid hydrocarbons) and one which is a combination of geometric and energetic indices - HOMA. The application of correlation and factor analyses to the above data matrix (8 indices for 18 rings) led to the conclusion that two orthogonal factors must be used to explain 96.4% of the total variance. The first factor which describes 66.8% is composed mostly of energetic indices (HF, EN and BE), HOMA (which contains both energetic and geometric contributions) and NICS. The second factor describes 29.8% and is built up of geometric indices (GEO, I-6 and BAG). The best correlation coefficient is found for a regression between HF and HOMA values (r-= 0.98). This supports the high utility of the HOMA model in describing aromaticity. NICS correlates well with energetic indices including HF and HOMA and it supports Schleyer's concept of the great importance of magnetic indices in describing aromaticity. (C) 1998 Elsevier Science Ltd. All rights reserved.