Understanding the borane analogy in Al(n)H(n+4) (n = 5-19): unprecedented stability of a non-Wade-Mingos cluster Al(8)H(12) fused by two T(d)-like Al(4)H(6).

Contrasting the boranes B(n)H(n+4) with rich chemistry, the alanes Al(n)H(n+4) remain largely unknown in laboratory, except for the simplest Al(2)H(6). Though recent experimental and theoretical studies have proved Al(n)H(n+2) to be the borane analogues, whether or not the borane analogy can exist for the more complicated Al(n)H(n+4) is still unclear. In this paper, we find that at the B3PW91/TZVP level, Al(n)H(n+4) each has a nido-single cluster ground structure as B(n)H(n+4) for n < 12. For n >or= 12, the fusion cluster becomes energetically more competitive than the single cluster also as B(n)H(n+4). Thus, concerning the ground structures, the alanes Al(n)H(n+4) (n = 5-19) could be considered as the borane analogues. Remarkably, Al(8)H(12) has a novel closo(4)-closo(4) cluster fused by two T(d)-like subunits Al(4)H(6), lying only 0.49 kcal/mol above the single cluster. The Born-Oppenheimer molecular dynamic simulation shows that the closo(4)-closo(4) fusion cluster intrinsically has high kinetic stability, which can be ascribed to the rigidity of the T(d)-Al(4)H(6) subunit. Since T(d)-Al(4)H(6) has been experimentally characterized in a gas phase very recently, we strongly recommend that the unprecedented non-Wade-Mingos alane Al(8)H(12) can be effectively formed via the direct dimerization between two T(d)-Al(4)H(6), with the reaction energy (-39.65 kcal/mol) very similar to that of the known dialane (2AlH(3) --> Al(2)H(6), -35.27 kcal/mol).