Density, degeneracy, delocalization-based index of aromaticity (D3BIA)

Aromaticity has been exhaustedly discussed for several years and it remains as a misterious issue. In this work it is proposed a new index of aromaticity named density, degeneracy and delocalization-based index of aromaticity or simply D(3)BIA in an attempt to cast new insight and perspective over this theme. This index is based on AIM (atoms in molecules) theory and it is somewhat supported by SC (spin-coupled) theory. Aromaticity decreases as the number of heteroatoms in the aromatic molecule increases since degeneracy decreases and it decreases as the ring size of an aromatic compound increases because it disfavors overlap of single-electron states. The relation between planar structures, electron density and aromaticity is also observed. The attractive interaction of 6 pi-electron resonance in cyclobutadiene dianion compensate its carbon-to-carbon repulsive interaction while in its dicationic parent the 2 pi-electron resonance is insufficient to counterbalance its carbon-to-carbon repulsive interaction and it adopts a puckered structure.