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A correlation between the magnetic properties and the crystal structure of the nitronyl nitroxide (NN) radical 2-(6-ethynyl-2-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (6-HCtCPyNN, 1) and its imino nitroxide analogue 2-(6-ethynyl-2-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl (6-HCtCPyIN, 2) has been shown. Magnetic susceptibility measurements on these compounds are indicative of ferromagnetic interactions between molecular units. The imino nitroxide 2 is an organic ferromagnet with critical temperature T C ) 0.19 K, whereas 1 behaves as an antiferromagnet ordering at the Ne ´el temperature, T N ) 0.54 K. Compounds 1 and 2 are isostructural and crystallize in the P2 1 /n space group. Their crystal packings are best described as chains of molecules linked by CtCH‚‚‚O hydrogen bonds running along the [1 h01] direction. A comparative analysis of the structures and magnetic properties of both compounds suggests that the coupling between the molecular units along the chain is ferromagnetic. Thus, the magnetic susceptibilities of 1 and 2 have been interpreted in terms of isotropic ferromagnetic Heisenberg linear chains of S ) 1/2 spins. Weak values of the coupling 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 transmission of magnetic interactions. As in other NN radicals, the spin density is concentrated in the two nitroxide groups and a small negative population is observed in the central carbon atom of the O-N-C-N-O fragment. However, two striking differences with respect to other nitroxides so far studied have been detected. First, the spin density is not equally shared between the two N-O functions. The oxygen atom O1, which participates in the formation of the hydrogen bond, is less populated than the "free" oxygen, O2. Ab initio spin density calculations clearly show that this effect is correlated to the presence of CtCH‚‚‚O hydrogen bonds in the crystal. Second, a positive spin density has been detected in the ethynylic hydrogen atom, H16. These two features indicate a spin density transfer from one nitroxide group to the alkyne function of the neighboring molecule.