Cyclo[18]carbon : Insight into Electronic Structure, Aromaticity and Surface Coupling

Abstract

Cyclo[18]carbon (C18) is studied computationally at density functional theory (DFT) and ab initio levels to obtain insights into its electronic structure, aromaticity, and adsorption properties on the NaCl surface. DFT functionals with a small amount of Hartree-Fock exchange as well as XMC-QDPT2 calculations fail to determine the experimentally observed polyyne molecular structure revealing a cumulene-type geometry. Exchange-correlation functionals with a large amount of Hartree-Fock ex-change as well as ab initio CASSCF calculations yield the polyyne structure is a ground state while the cumulene structure corresponds to the transition state between the two inverted polyyne structures through the Kekule distortion. The polyyne and the cumulene structures are found to be doubly Hückel aromatic. The calculated adsorption energy of cyclo[18]carbon on the NaCl surface is small (37 meV/C) and almost the same for both structures implying that the surface does not stabilize a particular geometry.