Natural bond-bond polarizability: a Hückel-like electronic delocalization index

We show how the bond-bond polarizability index, as originally introduced by Coulson and Longuet-Higgins in the Hückel-theoretic context, can be generalized in the natural bond orbital (NBO) framework to ab initio molecular orbital and density functional theory levels. We demonstrate that such a "natural bond-bond polarizability" (NBBP) index provides a flexible and quantitative descriptor for a broad spectrum of delocalization effects ranging from strong π aromaticity to weak intra- and intermolecular hyperconjugative phenomena. Illustrative applications are presented for representative delocalization effects in saturated and unsaturated species, chemical reactions, and hydrogen-bonding interactions.