On the structure and stability of the AlH 4 radical and its potential energy surface for rearrangement and dissociation: an ab initio MO study

Abstract

The potential energy surface for the AlH 4 radical was investigated using ab initio MO theory. Stationary points were located at the UHF and UMP2 levels with the 6–31G( d) and 6–31G( d, p) basis sets, and characterized by harmonic frequency calculations. Single-point calculations on the optimized structures were carried out at the UMP4 (SDTQ) level using the 6–311 + G (MC) (2 df, p) basis set. It was found that AlH 4 has three stable structures, each possessing a different point group symmetry: D 2d( 2 B 2), C 3v( 2 A 1) and C s( 2 A 1). The C s structure is the global minimum, lying about 83 and 145 kJ mol −1 below the C 3v and D 2d structures respectively. However, the C s structure is best regarded as a very weak molecular (van der Waals) complex between .AlH 2 and H 2, the distance between the two groups being about 3.1 Å. In this complex the H 2 molecule freely rotates about the Al p orbital that is perpendicular to the .AlH 2 unit. The C s structure lies only about 4 kJ mol −1 in energy below free .AlH 2 and H 2, and dissociation should occur with no activation energy. It therefore seems unlikely that AlH 4 is capable of existing, except perhaps, fleetingly, in matrices at very low temperatures. In addition to the aforementioned stable structures, three transition structures were also located; for the interconversion between two equivalent C s structures, for the hydrogen abstraction reaction, A1H 3 + H, and for the rearrangement between the C s and C 3v structures. The exothermic reaction of AlH 3 and .H, to give .AlH 2 with H 2, is predicted to take place, not via a concerted pathway but rather via a non-concerted route, involving the intermediacy of the C 3v and C s minima. Finally, a comparison of the BH 4, CH 4 +, AlH 4 and SiH 4 + potential energy surfaces is made, and their similarities and differences are rationalized in terms of second-order Jahn-Teller effects.