Understanding the chemical bonding representation and the effect of heteroatoMS substitution in coronene

Abstract

The bonding patterns in coronene are complicated and controversial. Among the different proposed descriptions, the two most representative are those generated by Clar's aromatic π-sextet and Adaptative Natural Density Partitioning (AdNDP) models. The chapter 1 reports the detailed quantum-chemical calculations at the density functional theory level to evaluate the model that gives a better representation of coronene. In addition, the analysis of the molecular structure of coronene, quantification of the aromaticity using various local aromaticity descriptors and assessment of the Diels-Alder reactivity with cyclopentadiene have been carried out. It has been concluded that Clar's π-sextet model provides the representation of coronene that better describes the physicochemical behaviour of this molecule. The recent studies show that properties of coronene could be tuned by isoelectronic substitution procedures. Thus, the effect of single, double and triple BN pair substitution on the aromaticity and stability of coronene is investigated in Chapter 2. The nucleus-independent chemical shift (NICS)-based method, has been used for its aromaticity assessment. The study reveals that the positional isomers with adjacently placed BN units are the most stable structure. Charge separation and different bond strength together play an important role in rationalizing the relative stability trends. The aromatic character of all BN substituted rings decreases w.r.t unsubstituted coronene except in case of triple BN pair substitution in inner ring, where the NICSzz(1) values of outer rings increase. It is found that depending upon the position of BN pairs, the HOMO-LUMO gap of coronene could be modulated. Thus, it is proposed to study the opto-electronic properties of these molecules.