An experimental and theoretical study of the electronic structure of zinc thiophenolate-capped clusters

The electronic structure of a series of thiophenolate-capped ionic/neutral clusters ([Zn(SPh)(4)](2-)(1); [Zn-4(mu(2)-SPh)(6)(SPh)(4)](2-)(2); Zn-10(mu(3)-S)(4)(mu(2)-SPh)(12) (3); and [Zn-10(mu(3)-S)(4)(mu(2)-SPh)(12)(SPh)(4)](4-)(4), Ph = phenyl), indicated as supertetrahedral fragments and possible molecular models of cubic ZnS, has been investigated by coupling density functional calculations to UV electronic and X-ray photoelectron (XP) spectroscopy. Theoretical outcomes indicate that, on passing from the tetrametallic to the decametallic clusters, there is a modification in the nature of the outermost occupied and lowermost unoccupied molecular orbitals. Actually, both in 1 and in 2 the frontier orbitals are delocalized and mainly composed of the S 3p pairs strongly mixed with the Ph pi levels (the HOMOs) and of the linear combinations of Ph pi* orbitals, the LUMOs. At variance to that, in 3 and 4 both the HOMO and LUMO are highly localized, the former on mu(3)-S atoms occupying C-3 nu coordinatively unsaturated tetrahedral positions and the latter on peripheral Zn atoms. The nature of the electronic levels involved in the UV absorption bands is discussed, and the agreement between theory and experiment is satisfactory. Neither experimental nor theoretical electronic excitation energies are influenced by the cluster size. Moreover, XPS data match quite well variations of the Zn and S gross atomic charges along the series. The different Zn-S bonding scheme characterizing terminal, mu(2)-bridging, and mu(3)-pyramidal S atoms allows a rationalization of the cluster behavior in solution. Along the investigated series, the only species reasonably mimicking both the structural arrangement and the electronic structure of the solid ZnS is Zn10(mu(3)-S)(4)(mu(2)-SPh)(12), which can be considered a molecular model of ZnS nonpolar surfaces.