Use of an electron equivalent relationship between bond length and bond order to study chemical bonding. Part II. A study of bond orders, bond lengths and aromaticity in polycyclic aromatic hydrocarbons

Abstract

Using the recently introduced concept of electron equivalent bond order, the characteristic bond orders of benzenoid hydrocarbons were studied based on experimental data. Fragmentation schemes can be used successfully to interpret the results and provide a very simple, consistent and useful valence bond approach for benzenoid hydrocarbons. The characteristic bond orders determined from experimental bond lengths can only assume eight discrete values in a relatively narrow range between 1 2 3 and 1 1 4 and primarily depend on the number of adjacent carbon–carbon bonds in the structure. The bond orders can also be used to define a quantitative scale of aromaticity for bonds, rings or entire molecules. A method, based on the optimization of quantified tendency toward maximum aromaticity and minimum octet-rule violation simultaneously, is also presented for the prediction of bond orders (and therefore bond lengths) of benzenoid hydrocarbons without any previous experimental information.