ON GLOBAL AND LOCAL PROPERTIES OF CLAR Pi-ELECTRON SEXTETS

The individual rings in benzenoid systems are studied via a new structural index, called Clar's ring character. The new index, which is based on selected Kekule valence structures, differs significantly in a number of cases from similar ring characterizations based on all Kekule valence structures. It is related to previously introduced quantities based on MO functions, but it can be obtained simply (i.e. without the need of a computer). We have illustrated the new ring index for several families of compounds, and report a number of regularities for a homologous series of molecules.