Ab initio studies of borazine and benzene cyclacenes

Abstract

Can the borazine cyclacene system provide a conceptual framework for the investigation of the properties of hexagonal BN materials? We perform both Hartree-Fock (HF) and unrestricted density functional theory calculations for the borazine and benzene cyclacenes system to obtain insights into the structural and electronic properties as a function of the number of rings present in cyclacene. The properties of the borazine cyclacene show little dependence on the number of rings ( R ∼4-12) in the peripheral circuits, in contrast to the benzene cyclacene system. The frontier HOMO-LUMO gap in borazine cyclacene increases steadily with R and stabilizes at ∼6.7 eV for R>9, in contrast to the benzene cyclacene system where the gap decreases and approaches that of the semiconducting carbon nanotube system (∼1.13 eV) for R>8. The aromaticity of the rings is investigated by examining the energy separations in the frontier molecular orbitals. Vibrational modes in the borazine cyclacene systems were also calculated using HF method (HF/3-21G) and compared with experimental Raman measurements on hexagonal BN materials. © 2003 Elsevier Science B.V. All rights reserved.