Structure and stability of the Al₁₄ halides Al ₁₄i_n^- (n=1-11): Can we regard the Al ₁₄ core as an alkaline earthlike superatom?

We have studied the structures and stabilities of Al₁₄I _n^- (n=1-11) clusters at the density functional level of theory. The experimentally observed Al₁₄I_n^- (n=3, 5,7, 9, and 11) [Bergeron et al., Science 307, 231 (2005)] are found to be stable both kinetically and thermodynamically. Al₁₄I ₃^-, not Al₁₄I^-, is the first member of the Al₁₄I_n^- series in the mass spectrometric experiment, which is ascribable to the low kinetic stability of the Al ₁₄I^- cluster. The Al₁₄ core in Al ₁₄I₃^- is close to neutral Al₁₄, both electronically and structurally. Population analysis shows that charge transfer occurs from the Al cluster to the I atoms, where the populations for Al ₁₄ vary from -0.70(Al₁₄I^-) to +0.96(Al ₁₄I₁₁^-). The Al₁₄I₅ ^- and Al₁₄I₇^- clusters have the structure of Al₁₄I₃^- as a core framework, but, for n = 9 and 11, we found many more stable isomers than the isomers having the Al₁₄I₃^- core. In particular, the shape of Al₁₄ in the Al₁₄I₁₁^- cluster is a hexagonal wheel-shaped form, which was observed in the x-ray experiment for the metalloid complex [Al₁₄{N(SiMe₃)₂} ₆I₆Li(OEt₂)₂] ^-[Li(OEt₂)₄]⁺ toluene [Köhnlein et al., Angew. Chem., Int. Ed. 39, 799 (2000)]. We have demonstrated that a simple jellium model cannot describe the structure and stability of the iodine-doped aluminum clusters, although it is successful for describing those of aluminum clusters. The electronic and geometric changes of the Al ₁₄^- cluster due to the presence of iodines are very similar to the case of a magic cluster Al₁₃^-. It can be concluded from our electronic and structural analysis that one cannot regard the Al₁₄ core as an alkaline earthlike superatom in the Al₁₄ iodide clusters.