Spin Distribution of Organic High-Spin Molecules as Studied by Endor/Triple

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Abstract
Single-crystal 1H-ENDOR and TRIPLE resonance spectroscopies have been applied to the first organic high-spin molecule, m-phenylenebis(phenylmethylene), 1 in its quintet ground state. Both the magnitude and absolute sign of all the proton hyperfine coupling constants have been determined from the ENDOR/TRIPLE data, yielding crucial information on the spin density distribution of its pi-electron network. The spin distribution thus determined demonstrates the important role of topological symmetry in organic high-spin molecules. Spin densities on carbon atoms of other typical high-spin molecules such as 3,4′-diphenylmethylenebis(phenylmethy-lene), 2 have also been determined for their ground states or low-lying excited states. Spin-prediction theories have been tested in terms of the spin density distribution, showing a UHF generalized Hubbard model Hamiltonian to be appropriate for the evaluation of spin densities in macromolecules with large spins and those in organic high-spin molecules which are unit moieties of organo-magnetic materials.