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 bon d lengths of 18 benzene rings taken from 10 benzenoid hydrocarbons (be nzene, naphthalene. anthracene, tetracene, phenanthrene, chrysene, tri phenylene, pyrene, perylene and coronene) computed by ab initio at RHF 6-31G* level of theory. The following indices were computed: three e nergetic indices (BE, EN and Hartree Fock energy for a constrained bon d 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 len gths taken from the rings of benzenoid hydrocarbons) and one which is a combination of geometric and energetic indices - HOMA. The applicati on of correlation and factor analyses to the above data matrix (8 indi ces for 18 rings) led to the conclusion that two orthogonal factors mu st be used to explain 96.4% of the total variance. The first factor wh ich describes 66.8% is composed mostly of energetic indices (HF, EN an d BE), HOMA (which contains both energetic and geometric contributions ) and NICS. The second factor describes 29.8% and is built up of geome tric indices (GEO, I-6 and BAG). The best correlation coefficient is f ound for a regression between HF and HOMA values (r-= 0.98). This supp orts the high utility of the HOMA model in describing aromaticity. NIC S correlates well with energetic indices including HF and HOMA and it supports Schleyer's concept of the great importance of magnetic indice s in describing aromaticity. (C) 1998 Elsevier Science Ltd. All rights reserved.