Theoretical Design of High‐Spin Polycyclic Hydrocarbons

Abstract

High-spin organic structures can be obtained from fused polycyclic hydrocarbons, by converting selected peripheral HC(sp(2)) sites into H(2)C(sp(3)) ones, guided by Ovchinnikov's rule. Theoretical investigation is performed on a few examples of such systems, involving three to twelve fused rings, and maintaining threefold symmetry. Unrestricted DFT (UDFT) calculations, including geometry optimizations, confirm the high-spin multiplicity of the ground state. Spin-density distributions and low-energy spectra are further studied through geometry-dependent Heisenberg-Hamiltonian diagonalizations and explicit correlated ab initio treatments, which all agree on the high-spin character of the suggested structures, and locate the low-lying states at significantly higher energies. In particular, the lowest-lying state of lower multiplicity is always found to be higher than kT at room temperature (at least ten times higher). Simplification of the ferromagnetic organization based on sets of semilocalized nonbonding orbitals is proposed. Molecular architectures are thus conceived in which the ferromagnetically-coupled unpaired electrons tally up to one third of the involved conjugated carbons. Connecting such building blocks should provide bidimensional materials endowed with robust magnetic properties.

Kekulé versus Lewis: When Aromaticity Prevents Electron Pairing and Imposes Polyradical Character

“…[6] The possibility of cutting graphene sheets to create magnetic nanoflakes to substitute, saturate, or subtract carbon atoms in the graphene itself is now a hot topic. [7] The authors have proposed some logical recipes to create high-spin fused polycyclic aromatic systems, [8] and to assemble them into ferromagnetic or antiferromagnetic architectures, which may be discrete or periodic. [9] A guiding tool to predict the spin multiplicity of any alternant hydrocarbon (i.e., free from odd-membered rings) is the so-called Ovchinnikov's rule, proposed in 1978, [10] later established more rigorously by Lieb, and known in solid-state physics as Lieb's theorem.…”

Communication: A dramatic transition from nonferromagnet to ferromagnet in finite fused-azulene chain

“…23,24 Our very recent studies on high-spin polyaromatic hydrocarbon molecules showed satisfactory agreements between EVB calculations and high level ab initio methods, such as differencededicated configuration interaction and complete active space with second order perturbation theory. 25 Recently, we employed it to study the open-shell singlet ground state of polyacene, 26 by solving the model Hamiltonian with the density matrix renormalization group method (DMRG), [27][28][29] and the results agreed with DMRG on the complete-active-spaceself-consistent-field (CASSCF) or the active-space variational two-electron reduced-density-matrix-CASSCF. 11,13,26,30,31 …”

High-Spin Chains and Crowns from Double-Exchange Mechanism

“…Values of and have also been calculated in non-doped nickelates = 0.6 and = 30 . Much smaller values of = 0.3 are observed in organic magnetic systems [22] as the magnetic orbitals are delocalized over several carbon centers.…”