Dehydrogenation of Ammonia Borane Impacts Valence and Core Electrons: A Photoemission Spectroscopic Study

Abstract

Ammonia borane (H ₃BNH ₃) is a promising material for hydrogen storage and release. Dehydrogenation of ammonia borane produces small boron–nitrogen hydrides such as aminoborane (H ₂BNH ₂) and iminoborane (HBNH). The present study investigates ammonia borane and its two dehydrogenated products for the first time using calculated photoemission spectra of the valence and core electrons. It is found that a significant decrease in the dipole moment was observed associated with the dehydration from 5.397 D in H ₃BNH ₃, to 1.942 D in H ₂BNH ₂, and to 0.083 D in HBNH. Such reduction in the dipole moment impacts properties such as hydrogen bonding, dihydrogen bonding, and their spectra. Dehydrogenation of H ₃BNH ₃ impacts both the valence and core electronic structure of the boron–nitrogen hydrides. The calculated valence vertical ionization energy (VIE) spectra of the boron–nitrogen hydrides show that valence orbitals dominated by 2p-electrons of B and N atoms exhibit large changes, whereas orbitals dominated by s-electrons, such as (3a ₁4a ₁5a ₁/3σ4σ5σ) remain less affected. The first ionization energy slightly increases from 10.57 eV for H ₃BNH ₃ to 11.29 eV for both unsaturated H ₂BNH ₂ and HBNH. In core space, the oxidative dehydrogenation of H ₃BNH ₃ affects the core electron binding energy (CEBE) of borane and nitrogen oppositely. The B1s binding energies increase from 194.01 eV in H ₃BNH ₃ to 196.93 eV in HBNH, up by 2.92 eV, whereas the N1s binding energies decrease from 408.20 eV in H ₃BNH ₃ to 404.88 eV in HBNH, dropped by 3.32 eV.