Binding between ground-state aluminum ions and small molecules: A1+·(H2/CH4/C2H 2/C2H4/C2H6). n,. Can Al+ insert into H2?

Paul R. Kemper, John Bushenell, Michael T. Bowers, Gregory I. Gellene

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Binding energies and entropies have been measured for the attachment of up to four H2 ligands and six small hydrocarbons to ground-state Al+ ions (1S, 3s2). Bond energies are typically very weak compared with analogous transition metal ion or the isovalent boron ion systems. Bond energies for the first ligand addition to Al+ are 1.4 (H2), 6.1 (CH4), 9.3 (C2H6), 14.0 (C2H2), and 15.1 kcal/mol for C2H4. The origin of the weak bonding lies primarily in the large, repulsive 3s orbital, which prevents close approach by the ligands. In addition, the lack of low-energy acceptor orbitals on the Al+ ion minimizes electron donation to the metal ion and also reduces the Al+/ligand attraction. Finally, the lack of low lying, occupied π-type orbitals prevents donation from the Al+ to the σ* orbitals on the ligands. A very detailed theoretical examination of the Al+(H2)n cluster energetics was also made. The purpose was to investigate the possibility of insertion by the Al+ into the H-H bond via σ bond activation, as is found with the isovalent B+ ion. The calculations showed that the inserted HAlH+ ion is stable but that its formation is endothermic by 10.9 kcal/mol with respect to the separated reactants. The inserted HAlH+(H2)2 ion, however, appears to be almost isoenergetic with the uninserted Al+(H2)3 isomeric cluster.

Original languageEnglish
Pages (from-to)8590-8597
Number of pages8
JournalJournal of Physical Chemistry A
Issue number44
StatePublished - Oct 29 1998

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