Polyradical Character of Triangular Non-Kekulé Structures, Zethrenes, p-Quinodimethane-Linked Bisphenalenyl, and the Clar Goblet in Comparison: An Extended Multireference Study.

Anita Das,Hans Lischka,Thomas Müller,Felix Plasser

doi:10.1021/acs.jpca.5b12393

Abstract

In this work, two different classes of polyaromatic hydrocarbon (PAH) systems have been investigated in order to characterize the amount of polyradical character and to localize the specific regions of chemical reactivity: (a) the non-Kekulé triangular structures phenalenyl, triangulene and a π-extended triangulene system with high-spin ground state and (b) PAHs based on zethrenes, p-quinodimethane-linked bisphenalenyl, and the Clar goblet containing varying polyradical character in their singlet ground state. The first class of structures already have open-shell character because of their high-spin ground state, which follows from the bonding pattern, whereas for the second class the open-shell character is generated either because of the competition between the closed-shell quinoid Kekulé and the open-shell singlet biradical resonance structures or the topology of the π-electron arrangement of the non-Kekulé form. High-level ab initio calculations based on multireference theory have been carried out to compute singlet–triplet splitting for the above-listed compounds and to provide insight into their chemical reactivity based on the polyradical character by means of unpaired densities. Unrestricted density functional theory and Hartree–Fock calculations have been performed for comparison also in order to obtain better insight into their applicability to these types of complicated radical systems.

Highlights

Over the past decade, graphene[1−3] has attracted considerable attention because of its wide range of applications,[4−9] e.g., as chemical sensors, organic semiconductors, energy storage devices, in spintronics and nonlinear optics
Multiconfiguration self-consistent field (MCSCF), mostly in the form of the complete active space self-consistent field (CASSCF) method, MR configuration interaction with singles and doubles (MR-CISD), and multireference averaged quadratic coupled cluster (MR-AQCC) calculations have been performed for the structures shown in Charts 1−3
The symmetry is given both in C2v and D3h notations

Summary

INTRODUCTION

Graphene[1−3] has attracted considerable attention because of its wide range of applications,[4−9] e.g., as chemical sensors, organic semiconductors, energy storage devices, in spintronics and nonlinear optics. A characteristic feature of the zethrenes and structure 7 is the competition between a closed-shell quinoid Kekulé form and an open-shell biradical resonance form (Charts 2 and 3).[32] Because of the presence of these two resonance structures, two interesting questions arise: which resonance form is dominating aThe benzene ring in red represents Clar’s aromatic sextet ring. If in different valence bond (VB) structures for a given PAH the biradical form contains more aromatic sextet rings than the closed-shell quinoid structure, as discussed in ref 32 and shown in Charts 2 and 3, its enhanced stability should be the source of a greater singlet biradical character. One major effort is dedicated to the clarification of which electronic configuration, either the closed-shell quinoid Kekulé or the open-shell biradical form, better describe the ground state of structures 4−7. By means of comparison with our multireference ab initio results, DFT is considered in the present context in order to obtain better insight into its applicability to these difficult questions concerning the correct description of biradical systems

COMPUTATIONAL DETAILS

RESULTS AND DISCUSSION

CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES