The potential energy surface for the BH 4 radical has been 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 UMP 4 level using the 6-311G( df, p) basis set. It was found that BH 4 has only two stable structures, one of C 2v ( 2 B 1) symmetry and the other of C 3v ( 2 A 1) symmetry. The C 2v structure is the global minimum, lying about 55 kJ mol −1 below the C 3vstructure. The C 3v structure is predicted to have only fleeting existence, dissociating to BH 3 and H with negligible activation energy. The C 2v structure is predicted to be kinetically stable to dissociation at 77 K, in agreement with the ESR data for BH 4. However, it should rapidly dissociate to BH 3 and H above 250 K. Intramolecular hydrogen scrambling pathways in C 2v BH 4 were also investigated. Two such processes were identified: homochiral scrambling (hydrogen exchange with retention of configuration at boron), and heterochiral scrambling (hydrogen exchange proceeding with inversion of configuration at boron). The activation energy for heterochiral scrambling is greater than that required for competing dissociation of C 2vBH 4 into BH 3 and H; consequently, this scrambling mechanism is not predicted to be observed. In principle, the homochiral mechanism could be observable, but would be too slow to be detected by ESR spectroscopy at 77 K. The ESR spectrum Of C 2v BH 4 is predicted, therefore, to correspond to a static C 2vstructure at 77 K, which is in agreement with the experimental data. The reaction between the .BH 2 radical and H 2, to give BH 3 and H, is predicted to be exothermic and to take place, not via a concerted pathway, but rather via a nonconcerted route, involving the intermediacy of the C 2v and C 3v structures of BH 4.