Structural Criteria for Aromaticity

The values are derived from butadiene, which is thereby the implicit standard of the definition. For C-C bonds, a= 251.1 kr and R = 1.388 A. Further definition of bond lengths is required for structures with heteroatoms. 

Another aromaticity index based on bond lengths was devised by Bird. The input into the index are bond order values derived from bond lengths. (See Section 1.4.6 to review the relationship between bond length and bond order.) The formulation of the index is 

The index 7 is assigned as 100 for benzene. The values are scaled for ring size and ring fusions, and increase with ring size so that naphthalene (142) and anthracene (206) have higher indices than benzene. This method is particularly useful for comparing heterocyclic compounds with hydrocarbons (see Section 8.6). 

---