Separation of the energetic and geometric contributions to aromaticity. Part VI. Changes of the aromatic character of the rings in naphthalene, anthracene, phenanthrene and pyrene derivatives induced by the charged substituent CH 2+

Abstract

Ab initio 6-31G ∗ optimised geometries of naphthalene, anthracene, phenanthrene, pyrene and all their CH 2 + mono-substituted derivatives have been examined in respect to estimate the changes of their aromatic character due to these kinds of substitution. CH 2 + substituent attached to aromatic hydrocarbons causes dramatic changes in their molecular geometry. This implies great changes in both local (i.e. referring to a single ring) and global aromaticity. The substituted rings always lose some part of their original aromatic character, but if the position of the substitution permits formation of the quinoidal structure via a short CC bond which is spread over the larger part of the molecule, the decrease of aromatic character is greater than in the other cases. Formation of quinoidal structures in a molecule leads to a long-range intramolecular charge transfer.