Does Al(4)H(14)(—) cluster anion exist? High-level ab initio study

A comprehensive ab initio investigation using coupled cluster theory with the aug-cc-pVnZ, n = D,T basis sets is carried out to identify distinct structures of the Al(4)H(14)(—) cluster anion and to evaluate its fragmentation stability. Both thermodynamic and mechanistic aspects of the fragmentation reactions are studied. The observation of this so far the most hydrogenated aluminum tetramer was reported in the recent mass spectrometry study of Li et al. (2010) J Chem Phys 132:241103–241104. The four Al(4)H(14)(—) anion structures found are chain-like with the multiple-coordinate Al center and can be viewed approximately as comprising Al(2)H(7)(—) and Al(2)H(7) moieties. Locating computationally some of the Al(4)H(14)(—) minima on the correlated ab initio potential energy surfaces required the triple-zeta quality basis set to describe adequately the Al multi-coordinate bonding. For the two most stable Al(4)H(14)(—) isomers, the mechanism of their low-barrier interconversion is described. The dissociation of Al(4)H(14)(—) into the Al(2)H(7)(—) and Al(2)H(7) units is predicted to require 20-22 (10-13) kcal mol(-1) in terms of ΔH (ΔG) estimated at T = 298.15 K and p = 1 atm. However, Al(4)H(14)(—) is found to be a metastable species in the gas phase: the H(2) loss from the radical moiety of its most favorable isomer is exothermic by 18 kcal mol(-1) in terms of ΔH (298.15 K) and by 25 kcal mol(-1) in terms of ΔG(298.15 K), with the enthalpic/free energy barrier involved being less than 1 kcal mol(-1). By contrast with alane Al(4)H(14)(—), only a weakly bound complex between Ga(4)H(12)(—) and H(2) has been identified for the gallium analogue using the relativistic effective core potential.