Chemistry of Cyclo-[2n]-Carbon: A Many-Particle Quantum Mechanics Investigation

Direct observations of cyclo-[2n]-carbon molecules (C10, C14, C16 and C18) experimentally prepared by atom manipulation technique raised immense attraction in the general chemistry society. The cyclic π-conjugated systems, namely in-plane and out-plane, construct dual aromaticity for odd n while dual anti-aromaticity for even n. In this work, we performed electronic structure investigation into cyclo-[2n]-carbon with n = 3 ~ 9 by comparing results obtained by density functional theory (DFT) and density matrix renormalization group (DMRG) method. By using Huckel molecular orbital (HMO) theory and the particle in a ring model, the electronic feature of such innovative carbon allotrope was clearly and chemically presented. Comparison showed that DFT results are considered to be incorrect for C6, which has a unique bonding structure among all cyclo-[2n]-carbon. The bond length alternating (BLA) phenomenon results from difference in electron correlation intensity and spatial distribution observed in pairs of bonding and anti-bonding orbitals. Therefore, results suggested that the dual anti-aromaticity in cyclo-[2n]-carbon with even n should be attributed to electron correlation effect, instead of decreased geometric symmetry, which actually exists in all cyclo-[2n]-carbon molecules and does not point out the essence.