On the origin of planarity in Al-5(-) and Al-5 clusters: The importance ofa four-center peripheral bond

Ab initio calculations were combined with anion photoelectron spectroscopy to unravel the structural origin of Al-5 and Al-5(-). Well-resolved photoel ectron spectra of Al-5(-) were obtained and compared to theoretical calcula tions performed at various levels of theory. It was shown that the best agr eement between the experimental and theoretical data is for a planar C-2v s tructure. Analyses of the electronic structure and molecular orbitals revea led that the planarity in Al-5 and Al-5(-) are due to the presence of a fou r-center peripheral bond that is common in a whole family of planar pentaat omic species recently uncovered. (C) 2000 American Institute of Physics. [S 0021-9606(00)01837-7].