Experimental and theoretical characterization of MSi16-, MGe16-, MSn16-, and MPb16- (M = Ti, Zr, and Hf):: The role of cage aromaticity

Silicon (Si), germanium (Ge), tin (Sn), and lead (Pb) clusters mixed with a group-4 transition metal atom [M= titanium (Ti), zirconium (Zr), and hafnium (Hf)] were generated by a dual- laser vaporization method, and their properties were analyzed by means of time- of- flight mass spectroscopy and anion photoelectron spectroscopy together with theoretical calculations. In the mass spectra, mixed neutral clusters of MSi16, MGe16, and MSn16 were produced specifically, but the yield of MPb16 was low. The anion photoelectron spectra revealed that MSi16, MGe16, and MSn16 neutrals have large highest occupied molecular orbital - lowest unoccupied molecular orbital gaps of 1.5 - 1.9 eV compared to those of MPb16 (0.8 - 0.9 eV), implying that MSi16, MGe16, and MSn16 are evidently electronically stable clusters. Cage aromaticity appears to be an important determinant of the electronic stability of these clusters: Calculations of nucleus- independent chemical shifts (NICSs ) show that Si-16(4-), Ge-16(4-), and Sn-16(4-) have aromatic characters with negative NICS values, while Pb-16(4-) has an antiaromatic character with a positive NICS value. (c) 2008 American Institute of Physics.