POLYCYCLIC AROMATIC-HYDROCARBONS WITH 5-MEMBERED RINGS - MODELING OF EXPERIMENTAL X-RAY AND NEUTRON-DIFFRACTION STRUCTURES

Several of the readily available theoretical programs are evaluated as tools for modeling the structures of polycyclic aromatic hydrocarbons with five-membered rings (CPAHs). The experimentally determined bond lengths and angles are compared to calculated values. Experimental bon d lengths are also compared to Pauling and Huckel molecular orbital (H MO) bond orders. Previously published experimental X-ray and neutron-d iffraction structures of acenaphthene, acenaphthylene, fluoranthene, c yclopent[o,p,q,r]benz[c]phenanthrene, and corannulene are modeled by t he programs MMX, AM1, MNDO, and PM3, and previously reponed STO-3G and 6-31G data are also evaluated. In general, the error differences bet ween the experimental and calculated results for all of the semiempiri cal programs were small. However, PM3 performed slightly better than A M1 and MMX, while MNDO generated structures which exhibited the larges t deviation from experiment. Although the standard deviations for all programs are shown to be of comparable magnitude, a particular bond le ngth or bond angle in any given theoretical calculation can exhibit si gnificant error from the experimental data. The scatter in the bond or der data computed from Huckel molecular orbital theory and valence bon d theory is contrary to results obtained with alternant systems. It ap pears that these approaches are less successful at modeling accurately the nonalternant hydrocarbon systems described in this paper.