A comparison of approaches to estimate the resonance energy

Abstract

We discuss Ab Initio approaches to calculate the energy lowering (stabilisation) due to aromaticity. We compare the valence bond method and the block-localised wave function approaches to calculate the resonance energy. We conclude that the valence bond approach employs a Pauling-Wheland resonance energy and that the block-localised approach employs a delocalisation criterion. The latter is shown to be more basis set dependent in a series of illustrative calculations.

π-electron content of rings in polycyclic conjugated compounds – A valence bond based measure of local aromaticity

“…Only in the case of 19 the VB-EC and SCI do not agree, whereas the VB-EC and NICS(1) give opposite predictions in 21. In half of the studied systems (17,20,21,22) the results of the HOMA and VB-EC go along, but not for 15, 16, 18 and 19. It should be pointed out that in the previous study [65], it has been shown that the HOMA does not agree with the other aromaticity indices in aza naphthalenes.…”

“…Ring Table 8 Correlation coefficients for the relations between different aromaticity indices and VB-EC of the hexagonal rings in aza derivatives of naphthalene (15)(16)(17)(18)(19)(20)(21)(22). R ef 0.3373 SCI 0.7947 SCI NAO 0.9611 HOMA 0.6068 NICS (1) À0.8123 Table 6 Coulson-Chirgwin weights of Kekulé structures of naphthalene and its aza derivatives obtained with the 6-311G(d,p) basis set ( Fig.…”

“…The choice of the appropriate reference system is crucial in calculations of the energetic aromaticity indices. Even for benzene, as a prototype aromatic molecule, there is a lot of controversy about its resonance energy [20][21][22]. Although the concept of resonance energy originates from Valence Bond theory, nowadays most of the employed aromaticity indices are obtained using MO and DFT calculations.…”

The Nature of the Chemical Bond from a Quantum Mechanical Interference Perspective

“…Benzene is among the most studied systems in chemistry. This emblematic molecule is an annulene composed of 6 carbon atoms displaced on a D 6h symmetric planar ring, and its geometric, electronic and magnetic properties are still reasons for scientific inquiry . From the energetic point of view, the molecule is more stable than their cyclic polyenes analogues.…”

Aromatic character of heptafulvene and its complexes with halogen atoms

“…In fact, the aromatic stabilization energy, ASE, is most often studied. This is estimated in such a way that energy is (usually) only gained through π-electron delocalization [12,29,30].…”