High Resolution NMR on Organic Radical Salts

Abstract

NMR (Nuclear Magnetic Resonance) is one of the most versatile spectroscopic techniques for the investigation of solids because it can provide information about local magnetic and electric fields at the atomic level as well as fluctuations of these fields caused e.g. by conduction electrons in organic conductors. In ordinary organic materials proton and carbon NMR spectroscopy in solution has become a routine method to study the diamagnetic interaction of these nuclei with electrons in closed shells (chemical shift). The NMR investigation of organic radical salts, however, is aimed at gathering information about the interaction of delocalized unpaired electron spins (hyperfine interaction) thereby probing basically the electronic structure of the conduction band which can be viewed as a linear combination of the singly occupied molecular orbitals (SOMO). In isolated organic radicals the hyperfine coupling is usually very large, whereas in organic conductors it is rather small due to electron delocalization (organic conductors) or due to a strong exchange coupling among the electron spins (Mott Hubbard insulators). In organic conductors high resolution NMR spectroscopy is therefore mandatory in order to observe the hyperfine interaction between conduction electrons and the nuclei resulting in the Knight shift.