Polarized neutron diffraction and its application to spin density studies

Spin density distributions in molecular compounds containing unpaired electrons have been studied by polarized neutron diffraction (PND). The spin density distributions provide a unique perspective of the magnetic properties of the compounds. The background and fundamentals of polarized neutron diffraction are summarized in this review, followed by examples of applications in inorganic and organic chemistry. Spin densities in several compounds that are obtained by polarized neutron diffraction are highlighted. Spin densities in single molecular magnet Fe₈O₂(OH)₁₂(tacn)₆]⁸⁺ and cyano-bridged K₂Mn(H₂O)₂]₃Mo(CN)₇]₂·6H₂O demonstrate how to obtain magnetic interaction in the complexes by PND. PND studies of Ru(acac)₃, containing one single unpaired electron, show small spin densities in this complex. Finally the use of PND in studying nitronyl nitroxide radicals is given. Our goal in this review is to illustrate how PND functions and how it serves as a sensitive tool in directly probing spin density in molecules.