Is Ferromagnetic Spin Coupling Constant within Homologous Di- and Triradicals?

Abstract

By means of the powerful Lanczos algorithm, the nonempirical valence bond (VB) model has been applied to investigate the low-lying electronic states for three typical types of homologous π-conjugated di-and triradicals. For small radicals including trimethylenemethane, cyclopentadienyltrimethylenemethane, mbenzoquinodimethane, and trimethylenebenzene, our VB results are in remarkable agreement with those obtained from previous sophisticated ab initio molecular orbital calculations. By using the energy difference between the high-spin ground and first excited states for these molecules, the strength of ferromagnetic coupling in these diradicals and their homologous triradicals has been discussed in detail. Our results reveal that ferromagnetic coupling in diradicals TMM and BQDM will decrease to different extent in their homologous triradicals. However, ferromagnetic coupling strength between two "unpaired" electrons in DCPP is found to be 9 times larger than that in its component diradical CPTMM.

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

“…Nonempirical valence bond (EVB) model, 21,22 which is a geometry-dependant Heisenberg Hamiltonian, has proved to be a predictive tool for the study of the low-lying states of conjugated hydrocarbons and in particular for studying spin coupling in radicalar conjugated hydrocarbons. 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.…”

HEISENBERG MODEL AND ITS APPLICATIONS TO Π-Conjugated SYSTEMS

“…Comparing the energies of these states, the first excited and other low-lying excited states can be picked out. 20 Let's describe this recursion process. By operating withĤ on ψ 0 , we define a state ψ 1 as follows…”

“…Even though, there are still several threads of development concerning the Heisenberg model in understanding many physical and chemical properties of conjugated molecules. 6,[12][13][14][15][16][17][18][19][20][21][22] On the one hand, since for conjugated systems the π-electrons are mainly responsible for their special properties, 23,24 thus the Heisenberg model is reasonably appropriate. On the other hand, with the advent of powerful computers, use of the graphical unitary-group technique has led to the exact Heisenberg ground-state energies for benzenoid hydrocarbons (BHs) up to N = 24 sites.…”

Theory of photoinduced high-spin states in organic molecules

“…[15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31] The topological rule based on the dynamical spin polarization effect has been proved to be effective to explain spin alignment in the ground state. [32][33][34][35] The topological rule here implies that spin correlation between arbitrary two sites is positive or negative depending on whether the number of bonds along any one of the links between them is even or odd.…”

“…However, the topological rule cannot be applied to doped or excited molecules, and only a few special cases, such as the strongcorrelation limit, 36 can be analytically solved. Most discussions on spin alignment in the excited or doped state have been given on the basis of a valence-bond study, [16][17][18][19][20] meanfield approaches, 23-28 ab initio calculations, 30,31 or molecular orbitals treatment. [37][38][39][40][41][42][43] Clarifying spin alignment in a molecular magnet from a general point of view has become an important theoretical subject.…”