A NEW AROMATICITY DESCRIPTOR BASED ON ELECTRON DELOCALIZATION EMPLOYING THE DISTRIBUTED MULTIPOLE ANALYSIS (DMA)

Aromatic compounds are energetically stable and have low reactivity, but an aromaticity concept is difficult to establish because it cannot be measured. For this reason, different descriptors have been developed to rationalize and quantify this phenomenon. Given that cyclic electron delocalization is an essential property of aromatic compounds, in this work, we propose six new aromatic descriptors based on Stone's distributed multipole analysis (DMA) to partition the molecular electron density on electric multipoles localized on different sites of a molecule. The new aromatic descriptors are based on different components of DMA quadrupole electric moment tensor Q_2, the first term of the DMA multipole expansion having contributions from the out-of-plane electron density. The proposed descriptors are straightforward to obtain because the DMA method is implemented on different popular electronic structure packages. For users of Gaussian, the formatted checkpoint with the calculated molecular electron density is used as input for the GDMA2 program of Stone to compute the necessary Q_2 components. The computer protocol for computing the Q_2-based descriptors in either way is presented. To assess the performance of the aromaticity descriptors, we used 12 tests of the Girona benchmark developed by the Solà group involving different distortions of benzene, substitutions, complexation, ring size dependence, atom size dependence, heteroatomic species, Clar systems, and fulvenes. The correct aromaticity trends of the six new indices were predicted entirely in 78% of the cases; for 16%, most trends were predicted and only failed utterly in 6%. The failed cases were related to the appreciable contribution of in-plane sigma electrons. Our proposal joins others to contribute to understanding the important and complex chemical concept of aromaticity.