An analysis of the changes in aromaticity and planarity along the reaction path of the simplest Diels-Alder reaction. Exploring the validity of different indicators of aromaticity

In this work, we analyze the changes in aromaticity and planarity along the reaction path of the Diels-Alder reaction between ethene and 1,3-butadiene. To this end, a new index that quantifies the planarity of a given ring is defined. As expected, the planarity of the ring being formed in the Diels-Alder cycloaddition increases along the reaction path from reactants to product. On the other hand, the aromaticity of the ring formed is measured using several well-established indices of aromaticity such as the nucleus independent chemical shift (NICS), the harmonic oscillator model of aromaticity (HOMA), and the para-delocalization index (PDI), as well as a recently defined descriptor of aromaticity: the aromatic fluctuation index (FLU). The results given by the NICS and PDI indices, at variance with those obtained by means of the HOMA and FLU indicators of aromaticity, confirm the existence of an aromatic transition state for this reaction. The reasons for the failure of some of the descriptors of aromaticity employed are discussed. The results support the multidimensional character of aromaticity.