Molecular orbital studies on the spin states of nitroxide species: Bis- and trisnitroxymetaphenylene, 1,1-bisnitroxyphenylethylene, and 4,6-dimethoxy-1,3-dialkylnitroxy-benzenes

Abstract

rnWe describe by approximate MO calculation a number of species for which high spin states are either the ground state or lie very low in energy. These are models for the spin coupling in projected ferromagnetic organic materials. The theory guiding their construction is based on planar conjugated systems, while experimentally realized systems are often far from planarity. We can by appeal to steric decoupling explain the failure of the prediction that methoxy subsitution on metaphenylene-coupled nitroxides should stabilize the triplet. However, we find in general that the qualitative rules derived from discussion of planar systems are robust; drastic departures from planarity are required before they lose their value. 0 1996 John Wiley & Sons, Inc. netic (i.e., spin-aligning) coupling between sites. A number of simple theoretical methods and principles have helped to guide and rationalize experimental design. These include the Huckel analysis of Longuet-Higgins [2] and the valence-bond treatment of Ovchinnikov [3], which permit the prediction of possible high-spin states in planar conjugated hydrocarbon species. The topology of rr systems is the foundation of all these analyses, and lntroduction he possibility of producing ferrmagnetic or-T ganic materials has attracted great interest [l]. The preparation of these materials requires sites with high local spin densities and ferromag-

Magnetic properties of Cr2On− clusters: A theoretical study

“…1 further shows the concentration of spins on the chromium atoms, and are in agreement with ''spin density alternation" in the most stable state as advocated by Trindle et al [26]. The spin density alternation in high-spin cases occurs through an accumulation of major spin densities on Cr atoms and a minor opposite spin density on oxygen atom.…”

Magnetic coupling in bis-nitronylnitroxide radicals: The role of aromatic bridges

“…From the results of Sec. IV, in agreement with those reported in literature, 13,31,43,44 it is evident that the meta and para substitutions trigger different electronic mechanisms in the aromatic bridge that lead to different interactions between the two radical sites. We want to revise here some semiquantitative theories for the prediction of the spin of the ground state and link them to more accurate analysis, such as those which can be achieved with the correlated wave functions computed in the present paper.…”

“…[45][46][47] Trindle and Datta, 31 interpreted this feature on the basis of previous heuristic theories 48 and with more robust arguments based on the spin polarization of the triplet unrestricted Hartree-Fock ͑UHF͒ wave function. According to this theory, if the number of atoms in the chain connecting the two magnetic sites is odd, the triplet is favored since an excess spin alternation along the chain is possible, which strongly stabilizes the triplet state.…”

Theoretical Investigation on the Decaying Behavior of Exchange Interaction in Quinoid and Aromatic Molecular Wires

“…cm −1 .The calculations of the NN-T, NN-TSO, NN-TSO2,allylNN-T, allylNN-TSO, allylNN-TSO2 series led to negative J values. For the NN-2,4T, NN-2,4TSO, NN-2,4TSO2 series, J was found to be positive for odd numbers of n and negative for even numbers of n. This trend satisfies the spin alternation rule that was first proposed byOvchinnikov 31 and later expanded to SCF methodologies by Trindle et al32,33 …”