Molecular structure and thermal stability of B2H4 and B2H+4 species

B2H4 has been produced by the reaction of F atoms with B2H6, in successive abstraction reactions. The B2H4 species was detected and analyzed by photoionization mass spectrometry. The adiabatic ionization potential of this species (9.70±0.02 eV) and the vertical value (∼10.4 eV) are obtained from the photoion yield curve. These values, and the shape of this curve, are consistent with a doubly bridged, C2v structure for both the neutral and ionic species. The fragment ion B2H+2 is observed, with an appearance potential of 11.535±0.03 eV. This value, combined with previous results, yields D0(B2H4–H)≊40.1 kcal/mol, whereas D0(B2H5–H)≲102.7 kcal/mol. The B2H+2 fragment may have as its neutral precursor an isomeric B2H4 (D2d), with approximately the same stability as the C2v species. An earlier value for the appearance potential of B2H+4 from B2H6 is shown to be too high, due to a very small formation probability at the thermochemical threshold.