The first ENDOR spectrum of naked metal clusters: 7Li3, 6Li 7Li2 and 7Li3(H2O)x in an adamantane matrix

The electron double resonance (ENDOR) spectra of naked and hydrated neutral homonuclear metal clusters are reported. 7Li3, 6Li 7Li2, and 7Li3(H2O)x clusters have been prepared in an adamantane matrix using the rotating cryostat variant of the matrix isolation technique. The centre of the ENDOR doublet for the major cluster, 7Li3, occurs at 46.15 MHz giving an isotropic hyperfine interaction (hfi) of 92.30 MHz which is close to that of 92.70 MHz determined previously from the EPR spectrum of Li3. Weaker ENDOR transitions from 6Li 7Li2 give hfis of 37.1 MHz for 6Li and 51.84 MHz for 7Li. These are consistent with the values for 7Li3 if account is taken of the statistical factor of two favouring the location of the single 6Li nucleus in 6Li 7Li2 at the terminal positions of the obtuse angled structures in the minima of the ‘Mexican hat’ hypersurface of the cluster. A weaker doublet, centred at 36.59 MHz is assigned to a hydrated cluster complex 7Li3(H2O)x. The reduction in the hfi value compared with that in the neutral cluster 7Li3 indicates a 20% transfer of unpaired electron spin density to the coordinated water molecules. A complex series of doublets centred at the proton frequency (14.31 MHz) indicate the presence of a number of hydrated Li3 clusters. These first observations of the ENDOR transitions of neutral trimeric metal clusters suggest that it may be possible to observe the spectra of other, larger and more complex, clusters.