Magnetic Shielding Study of Bonding and Aromaticity in Corannulene and Coronene

Peter B Karadakov

doi:10.3390/chemistry3030063

Abstract

Bonding and aromaticity in the bowl-shaped C5v and planar D5h geometries of corannulene and the planar D6h geometry of coronene are investigated using 3D isosurfaces and 2D contour plots of the isotropic magnetic shielding σiso(r) and, for planar geometries, of the out-of-plane component of the shielding tensor σzz(r). Corannulene and coronene both feature conjoined shielded “doughnuts” around a peripheral six-membered carbon ring, suggesting strong bonding interactions and aromatic stability; a deshielded region inside the hub ring of corannulene indicates that this ring is antiaromatic, more so in planar corannulene. The switch from the planar to the bowl-shaped geometry of corannulene is shown to enhance both bonding and the local aromaticities of the five- and six-membered rings; these factors, in addition to ring strain reduction, favour the bowl-shaped geometry. The most and least shielded bonds in both corannulene and coronene turn out to be the spoke and hub bonds, respectively. The higher π electron activity over spoke bonds in planar corannulene and coronene is supported by σzz(r) contour plots in planes 1 Å above the respective molecular planes; these findings about spoke bonds are somewhat unexpected, given that ring current studies indicate next to no currents over spoke bonds.

Highlights

The polycyclic aromatic hydrocarbon (PAH) dibenzo[ghi,mno]fluoranthene (C20H10), in which a five-membered central ring is surrounded by five six-membered rings, was synthesised by Barth and Lawton in 1966 [1], who gave it the trivial name under which it is currently known, corannulene
The aim of this paper is to investigate aromaticity and chemical bonding in corannulene at the two stationary points on its ground-state potential energy surface, the bowlshaped local minimum of C5v symmetry and the planar bowl-inversion transition state of D5h symmetry, by analysing, for each geometry, the changes in the off-nucleus isotropic shielding, σiso(r), within the space surrounding the molecule by means of 3D isosurfaces and 2D contour plots
The C5v geometry of corannulene and the D6h geometry of coronene used throughout this paper are shown in Figure 2, which includes details of the D5h geometry of corannulene and B3LYP Mulliken charges calculated with the def2-TZVP and 6-311++G(d,p) basis sets

Summary

Introduction

The polycyclic aromatic hydrocarbon (PAH) dibenzo[ghi,mno]fluoranthene (C20H10), in which a five-membered central ring is surrounded by five six-membered rings, was synthesised by Barth and Lawton in 1966 [1], who gave it the trivial name under which it is currently known, corannulene (see [2]). Interest in the aromaticity of corannulene was one of the incentives for its initial synthesis: Barth and Lawton thought that ionic resonance structures such as that in Figure 1d could give rise to two charged concentric conjugated systems, an inner cyclopentadienyl anion and an outer cyclopentadecaheptenyl cation, each of which satisfies Hückel’s 4n + 2 rule. This “annulene-within-an-annulene” (AWA) model could be expected to apply to planar coronene (C24H12), which has inner and outer rings with 6 π and 14 π electrons, respectively. As explained by Zanasi and coworkers [6], the AWA model would work if the inner and outer annulene rings were uncoupled, which is an unrealistic assumption as the carbon–carbon spoke bonds connecting the two rings have been found to be shorter than the hub, flank and rim bonds (see Figure 1a for definitions)

Objectives

Methods

Results

Conclusion