3: "Ferromagnetism" in Solid Free Radicals (CIT)
In odd alternate aromatic radicals the spin density alternates in sign as one moves from one atom to the next. It occurred to me that if a 3D array of these radicals were stacked on top of one another with a geometry so that each negative spin was paired with a neighboring molecular positive spin, the spins would arrange in such a way that the state of lowest energy would have the largest spin — a potential state of molecular “ferromagnetism.” (89) Years later this concept was elegantly verified [3, 4] by Japanese workers who synthesized molecules with S = 1 ground states from suitably stacked odd alternate radicals each with S = ½. In fact this little paper stimulated a great deal of experimental work but unfortunately no significant practical applications. For a confirmatory theoretical analysis of the problem and many references, see M. Deumal et al., and Yoshizawa et. al.
Odd alternate radicals have many interesting properties. For example I predicted that the triplet ground state of the biradical trimethylene methyl would be one in which the spin on the central carbon atom would be negative. Assuming D3h symmetry for the radical, I predicted that the fine structure splitting for the radical would be small due to a cancellation of positive contributions of spin-spin repulsions on the CH2 groups, and negative contributions due to the negative spin density on the central carbon atom (76).
A small fine structure splitting was indeed found to be the case in the work of Claesson et al. Unfortunately, in my paper on the subject I failed to mention the importance of the proton hyperfine splittings on the CH2 groups. Claesson et al., report isotropic proton hyperfine splittings of -26 MHz for these protons, leading to an estimated spin density of -26/Q = -26/-63 = 0.41 for the spin density on each these three atoms. (Valence bond theory predicts 0.416 for these spin densities.) Thus there is 3(0.41) spin density on the methylene protons, which implies 2-(3)(0.41)=0.77 missing spin density, corresponding to a negative spin density of -0.77 on the central carbon atom. (My manuscript erroneously gives this negative spin density as -0.25, which should instead be -0.75.). Strangely, I have not found any discussion of these spin densities in this interesting radical since my 1961 paper (76).