Magnetic characterization of the geometry and chemistry of small metal clusters

Abstract

AbstractThe application of electron paramagnetic resonance (EPR) spectroscopy in determining the geometric and electronic structures of small naked clusters, Mn, of the alkali and coinage metals (n < 9) is assessed. The EPR spectra and the paramagnetic parameters derived therefrom provide stringent tests of quantitative molecular orbital calculations. Perturbations of the cluster shape by interaction with substrate molecules are readily detected and have a bearing on the dynamic nature of the active sites of heterogeneous catalysts. Previous studies of the metal trimers in hydrocarbon matrices are extended to the preparation of an unusual acute Na3 cluster associated with water and the heteronuclear clusters Na2Li and Au2Ag. Both adopt static obtuse 2B2 geometries and show no fluxionality, in contrast to the homonuclear pseudo‐rotating Jahn—Teller clusters Li3 and Na3. There is a reversal in paramagnetic parameters in the series Cu3, Ag3, Au3 which is due in part to the emergence of relativistic effects because of the high charge on the gold nucleus. The first ENDOR spectrum is reported of a metal cluster Li3, the parameters fitting exactly those determined from the more complex EPR spectrum of Li3. A new heteronuclear septameric species, Na2K5, has been examined which has the pentagonal bipyramidal structure seen for homonuclear septamer clusters. There is an unpaired spin density of 0.41 on the apical sodium nuclei with only 0.004 at the basal potassium nuclei.