ИСТИНА

Low-lying electronic energy levels of finite-length zigzag carbon nanotubes [C8]qM with special modification of edges were calculated within CASSCF approximation. Systems [Cn]qM are finite carbon nanotubes (n,0) where one edge is monohydrogenated (Fujita edge) and second edge is dihydrogenated (Klein edge). The point symmetry group of investigated systems is C8v. According to analytical estimations within Huckel model, boundary orbitals of these systems are localized at edges, forming n-multiple degenerated cluster. These MOs tend to single occupation by electrons and form the low-lying many-electron states for the whole system. Each of systems [C8]qM (q=4-6) possess 3 MOs (e3, b1) localized at the Fujita edge and 5 MOs (a1, e1, e2) localized at the Klein edge; these MOs are included in the active space of CASSCF calculations. Low-lying states of investigated systems form tier structure, that can be interpreted as the result of weak interaction of hypothetic magnetic states assigned to each edge of the hydrocarbon fragment. We assign the hypothetical electron energy levels for each of the edges, that originated from configurations {e3, b1}3 for Fujita edge and {a1, e1, e2}5 for Klein edge. We assume that the lowest energy states of edges must have the highest multiplicity, it gives the following configuration: e32b11 for Fujita edge; and a11e12e22 for Klein edge, that corresponds to the terms of 4B2 and 6A1, respectively. Consequently, the lowest states of the whole fragment are obtained by combining the terms of Fujita and Klein edges, videlicet 6A1X4B2. This production gives states 3,5,7,9B2. Ground state for all q is maximum multiplicity state 9B2, that realizes ferromagnetic type of edge spins coupling. Lowest excited states are obtained by other types of spin coupling, that approximately can be described by Heisenberg Hamiltonian. Energy splittings within this tier are decreased with increasing q. Results of CASSCF calculation are agreed qualitatively with broken symmetry unrestricted DFT calculation (B3LYP), that also indicates ferromagnetic type of ground state coupling. Zigzag carbon NT with asymmetric edges can be considered as promising nanoscale materials with magnetic properties. Synthesis and subsequent experimental studies of such hydrocarbons are very interesting, as well as various chemical edge modifications for their stabilization. This work was supported by RFBR (grant No 10-03-00665).