Ammonia Borane, NH₃ BH₃ : A Threshold Photoelectron-Photoion Coincidence Study of a Potential Hydrogen-Storage Material

We have investigated the photoionization of ammonia borane (AB) and determined adiabatic ionization energy to be 9.26±0.03 eV for the X^{+ 2} E←X ¹ A₁ transition. Although the threshold photoelectron spectrum appears at first glance to be similar to the one of the isosteric ethane, the electronic situation differs markedly, due to different orbital energies. In addition, an appearance energy AE_0K (NH₃ BH₃ , NH₃ BH₂ ⁺ )= 10.00±0.03 eV has been determined, corresponding to the loss of a hydrogen atom at the BH₃ -site. From the data, a 0 K bond dissociation energy for the B-H bond in the cation of 71.5±3 kJ mol^-1 was derived, whereas the one in the neutral compound has been estimated to be 419±10 kJ mol^-1 .