Experimental and theoretical characterization of M Si 16 − , M Ge 16 − , M Sn 16 − , and M Pb 16 − ( 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 M Si 16 , M Ge 16 , and M Sn 16 were produced specifically, but the yield of M Pb 16 was low. The anion photoelectron spectra revealed that M Si 16 , M Ge 16 , and M Sn 16 neutrals have large highest occupied molecular orbital-lowest unoccupied molecular orbital gaps of 1.5 - 1.9 eV compared to those of M Pb 16 ( 0.8 - 0.9 eV ) , implying that M Si 16 , M Ge 16 , and M Sn 16 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.