Understanding Magnetic Interaction Pathways: An Experimental Determination of Electron Density in an Alkyne-Substituted Nitronyl Nitroxide Radical

In order to understand magnetic interaction pathways, an experimental electron density distribution was determined in a purely organic free radical, m -(ethynyl)pyridyl nitronyl nitroxide, using accurate X-ray diffraction measurements. The intermolecular contacts were characterized by a topological analysis of the experimental electron density. Hydrogen bonds involving the O−N−C−N−O fragment reveal a 3D character. Atomic charges obtained by integration over the topological atomic basins indicate a charge transfer from the two oxygen atoms to the C⋮C triple bond by electron conjugation through the molecular skeleton, but also possibly through a short hydrogen bond. The X-ray results are compared to theoretical DFT calculations and to experimental spin densities obtained by polarized neutron diffraction.