The Agreement between Clar Structures and Nucleus-Independent Chemical Shift Values in Pericondensed Benzenoid Polycyclic Aromatic Hydrocarbons: An Application of the Y-Rule

Abstract

In the present study, we verify theoretically the performance of a previously introduced qualitative simple rule for aromaticity, the Y-rule, which determines the relative aromaticity of each hexagonal ring in pericondensed benzenoid polycyclic aromatic hydrocarbons (peri-PAHs). The Y-rule extends the sextet−double bond description of Clar's model for peri-PAHs by predicting, in an easy way, the most likely location and total number of aromatic sextets in the structure. The final π-electronic distribution thus obtained corresponds to the superposition of the most important Clar structures. The problem of the number and location of aromatic sextets in peri-PAHs involves the very concept of aromaticity. The π-electronic distribution obtained by the Clar sextet principle (or model) as determined by the Y-rule is compared with the π-electronic distribution obtained from nucleus-independent chemical shift (NICS) calculations for various series of benzenoid peri-PAHs. The NICS values were used to assess the relative aromaticities of the individual rings in the peri-PAH compounds. We found that the simple heuristic Y-rule predicts for all the calculated systems the same relative aromaticities given by the NICS calculations, and for the case of peri-systems with very high symmetry (like the compact peri-PAHs) it reinforces the picture derived from the NICS calculations. In these cases, the NICS results can be complex due to symmetry. The advantages of the qualitative Y-rule are that it is of particular importance for the case of large peri-PAHs, it takes only a few minutes to be applied following a very easy methodology, and it provides a quick answer about the aromaticity and location of the resonant sextets in peri-PAH compounds without having to carry out theoretical quantum chemistry calculations that take time and that can be costly depending on the size of the system.