Topological Estimation of Aromatic Stabilities of Polyacenes and Helicenes: Modeling of Resonance Energy and Benzene Character

The distance-based topological indices viz Wiener (W)-, Szeged (Sz)-, Padmakar-Ivan (PI)- and Sadhana (Sd)-indices have been used for estimating aromatic stabilities as well as % benzene character of polyacenes and helicenes. Excellent models are obtained in combined set and there was no need to split the data set into polyacenes and helicenes. The newly introduced Sd index was found the best index among all the four indices used. In bi-parametric modeling with the combination of the Sd index with the PI index yielded excellent results. The results are discussed critically on the basis of variety of statistical parameters.     

Local aromaticity in polyacenes manifested by individual proton and carbon shieldings: DFT mapping of aromaticity

Exponential dependencies between locally calculated geometric and magnetic indexes of aromaticity, harmonic oscillator model of aromaticity (HOMA) and nucleus independent chemical shifts (NICS)(0), NICS(1) and NICS(1)zz, and the number of conjugated benzene rings in linear acenes, from benzene to decacene were observed at B3LYP/6-311+G** level of theory. Correlations between HOMA and NICS indexes showed exponential dependencies and were fitted with simple three-parameter function. Similar correlations between both indexes of aromaticity and proton and carbon nuclear isotropic shieldings of individual acene rings were observed. Contrary to proton data, the predicted ¹³C nuclear isotropic shieldings of carbon atoms belonging to inner rings in polyacenes were less shielded, indicating lower aromaticity and therefore, higher reactivity.     

Aromaticity and cyclic conjugation of three pairs of strongly subspectral series of conjugated molecules as models for graphene armchair and zigzag edges

It is shown that armchair edges possess greater aromaticity and cyclic conjugation than do zigzag edges in model nanoribbons. These results are consistent with the zigzag graphene edges being more prone to rapid oxidation when oxygen is present than armchair graphene edges. These results were obtained using Aihara's BRE and Bosanac and Gutman's CCE methods for measuring aromaticity of individual rings. New numerical results are given for three pairs of strongly subspectral series.