Evidence for transmission of ferromagnetic interactions through hydrogen bonds in alkyne-substituted nitroxide radicals: Magnetostructural correlations and polarized neutron diffraction studies

correlation between the magnetic properties and the crystal structure of th e nitronyl nitroxide (NN) radical 2-(6-ethynyl-2-pyridyl)-4,4,5,5-tetrameth ylimidazoxide-1-oxyl-3-oxide (6-HC drop CPyIN, 1) and its imino nitroxide a nalogue 2-(6-ethynyl-2-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl (6-HC drop CPyIN, 2) has been shown. Magnetic susceptibility measurements on the se compounds are indicative of ferromagnetic interactions between molecular units. The imino nitroxide 2 is an organic ferromagnet with critical tempe rature T-C = 0.19 K, whereas 1 behaves as an antiferromagnet ordering at th e Neel temperature, T-N = 0.54 K. Compounds 1 and 2 are isostructural and c rystallize in the P2(1)/n space group. Their crystal packings are best desc ribed as chains of molecules linked by C drop CH ... O hydrogen bonds runni ng along the [101] direction. A comparative analysis of the structures and magnetic properties of both compounds suggests that the coupling between th e molecular units along the chain is ferromagnetic. Thus, the magnetic susc eptibilities of 1 and 2 have been interpreted in terms of isotropic ferroma gnetic Heisenberg linear chains of S = 1/2 spins. Weak values of the coupli ng constants (J = +0.90 K for 1, J = +0.67 K for 2) have been obtained. The spin density distribution of radical 1 has been determined by a polarized neutron diffraction experiment to gain more insight into the mechanism of t ransmission of magnetic interactions. As in other NN radicals, the spin den sity is concentrated in the two nitroxide groups and a small negative popul ation is observed in the central carbon atom of the O-N-C-N-O fragment. How ever, two striking differences with respect to other nitroxides so far stud ied have been detected. First, the spin density is not equally shared betwe en the two N-O functions. The oxygen atom O1, which participates in the for mation of the hydrogen bond, is less populated than the "free" oxygen, O2. Ab initio spin density calculations clearly show that this effect is correl ated to the presence of C drop CH ... O hydrogen bonds in the crystal. Seco nd, a positive spin density has been detected in the ethynylic hydrogen ato m, H16. These two features indicate a spin density transfer from one nitrox ide group to the alkyne function of the neighboring molecule.