Remarkable Structures of Dialane(4), A1₂H₄

The potential energy surface of dialane(4), A1₂H₄, has been studied by ab initio molecular orbital theory at the MP4 (SDTQ)/ 6-31G//HF/6-31G* + ZPE level. Five minima (1, 2, 5,7, and 8) and three transition structures (3, 6, and 9) were determined at the HF/6-31G* level. The most stable isomers are salt-like structures: i.e., 1 (C_3v), being the face complex of Al⁺ with tetrahedral A1H₄^-, and the 1.0 kcal/mol less stable 2 (C_2v), where Al⁺complexes to an A1H₄^-edge. Structure 5 (D_2d) is 10.7 kcal/mol less stable than 1, with a barrier for Al-Al bond rotation of only 1.8 kcal/mol (6), and has an energy difference with 7 (H₃AIAIH) of 11.3 kcal/mol; the μ-hydrido structure 8 is of intermediate energy. The doubly hydrogen-bridged isomer 9 is the energetically least favored (transition) structure, in strong contrast to B₂H₄, where it represents a minimum with nearly the same energy as the D_2d isomer.