The implementation of the n-center electron delocalization indices, n-DIs, and n-order electron localization indices, n-LIs, within the framework of the quantum theory of atoms in molecules, QTAIM, is performed. n-DIs are shown to be very useful to study the local aromaticity in monocyclic and polycyclic compounds. Total and π n-DIs from n = 4 to 7 were computed for a series of typical 4, 5, 6, and 7-center aromatic and antiaromatic rings. For n ≥ 5 the π n-DI accounts for the 95% of the total n-DI and can be employed alone to measure the aromaticity. A scaling factor on the n-DIs is required in order to compare the aromaticity of [5c-6e] and [6c-6e] rings, the same correction allows to estimate the relative aromatic stabilization of polycyclic compounds using the sum of its values for individual rings. This is called Effective Scaled Electron Delocalization, ESED. The comparison with other aromaticity indices reflects a good correlation between ESED and both resonance energies, and HOMA indices. The most important differences between scaled π n-DIs and NICS(0) indices are found for compounds that contain rings with different number of centers or π electrons. © 2006 Wiley Periodicals, Inc. J Comput Chem, 2007

A study of the intramolecular hydrogen bond (IHB) in 1,2-ethanediol and 1,2-dihydroxybenzene (catechol) was carried out using the QTAIM theory. Atomic and bond properties defined within this theory were calculated for different donor–acceptor distances, H^d⋯O^a, in both molecules, and different H–O^a–C–C dihedral angles for 1,2-dihydroxybenzene, optimising the remaining geometry. Though no conformer of both compounds present IHB, it appears when the H^d⋯O^a distance is reduced in both molecules or when the H–O^a–C–C angle is rotated in 1,2-dihydroxybenzene. The evolution of integrated and local properties follows the criteria of Koch and Popelier for hydrogen bond formation when the interatomic distance is modified, but display the opposite trends when the IHB is formed by rotating the dihedral angle.