ReStricted geometry optimization for estimating stabilization energies of polycyclic aromatic hydrocarbons

Abstract

At the B3LYP/6-31G* level, our program of geometry optimization under the restrictions of π-orbital interactions was applied to five homologous series such as acene-, phenanthrene, triphenylene, chrysene, benzopyrene series, and seven additional molecules such as tri- and tetra-benzanthracene, benzo- and naphtho-pentaphene, dibenzopenta-cene and tribenzotetracene, providing each of the 35 polycyclic aromatic hydrocarbons (PAHs) with two types of localized reference geometries: (i) the 'GL' geometry where all double bonds are localized and (ii) m different 'GE-m' geometries, where only a specific pair of double bonds are permitted to conjugate and all other bonds are localized.. Each GE-m resulted from the local π interaction between a pair of double bonds in the GL geometry. The total sum of molecular energy differences, ([E(Ground) - E(GL)] - σE(GE-m) - E(GL)]), is defined as the extra stabilization energy (ESE) for a PAH. The corrected ESE (CESE) then resulted from considering πinteractions between pairs of nonbonded double bonds. In this work, the position, energy, and GL sextet rules were developed to ensure that the GL geometry chosen from its candidates as well as the GE-m geometries is no longer arbitrary. Therefore, it is the GL and GE-m geometries of the PAH itself, rather than nonaromatic compounds, that serve as reference structures for calculating CESE. As a result, for each of the four homologous series (acene, phenanthrene, triphenylene, and benzopyrene), CESE/p can be fitted as a linear function of the p band frequency vtilde;. For each of the four isomeric series with molecular formulas of C18H12,C22H14,C 26H16, and C30H18, CESE/π can also be fitted as a second-order polynomial function of the p band frequency ṽ as well as of the number of GL sextets. Copyright © 2009 John Wiley & Sons, Ltd.