The hydrogen-depleted graphs of polycyclic aromatic hydrocarbons contain two types of vertices (with regard to their degree) and correspondingly three types of edges. The respective sums of these edges reflect the molecular topology of the hydrocarbons and were used for constructing a new topological index for kata-annellated aromatic hydrocarbons that correlates well with their topological resonance energies per electron. Dependent on the degrees of their first neighbours, the vertices of degree 2 of polycyclic hydrocarbons can be distinguished as to high, medium and low chemical reactivity of the corresponding carbon atoms, in agreement with the results from MO theory and experiments.
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