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

Abstract

Ammonia borane (H 3 BNH 3 ) is a promising material for hydrogen storage and release. Dehydrogenation of ammonia borane produces small boron−nitrogen hydrides such as aminoborane (H 2 BNH 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 3 BNH 3 , to 1.942 D in H 2 BNH 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 3 BNH 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 1 4a 1 5a 1 /3σ4σ5σ) remain less affected. The first ionization energy slightly increases from 10.57 eV for H 3 BNH 3 to 11.29 eV for both unsaturated H 2 BNH 2 and HBNH. In core space, the oxidative dehydrogenation of H 3 BNH 3 affects the core electron binding energy (CEBE) of borane and nitrogen oppositely. The B1s binding energies increase from 194.01 eV in H 3 BNH 3 to 196.93 eV in HBNH, up by 2.92 eV, whereas the N1s binding energies decrease from 408.20 eV in H 3 BNH 3 to 404.88 eV in HBNH, dropped by 3.32 eV.

Photoemission and X-ray Absorption Investigation of Ammonia-Borane in the Gas Phase

“…The binding energy is in good agreement with the theoretical value of Chong and Wang, 408.2 eV. 3 There is a hint of unresolved vibrational structure, with a spacing of 120 meV, but the statistics are not sufficiently good to be certain of this.…”

“…The N 1s spectrum of AB is shown in Figure a, and the peak has a binding energy of 407.96 eV and a width of 0.7 eV. The binding energy is in good agreement with the theoretical value of Chong and Wang, 408.2 eV . There is a hint of unresolved vibrational structure, with a spacing of 120 meV, but the statistics are not sufficiently good to be certain of this.…”

“…We fitted the third band with two Gaussian functions to represent the 4a 1 and 1e 1 bands and obtained the energies listed in Table . As seen in Figure , the agreement with the calculated vertical ionization energies using the ΔPBE0 method is very good.…”

“…They denoted this degenerate orbital HOMO-π x and HOMO-π y , and the second highest molecular orbital HOMO− 1; the HOMO is also denoted as 2e 1 and the HOMO−1 as 5a 1 . 3 From the charge density maps (Figure 3 of Schleier et al 2 ), it is evident that both orbitals are localized more strongly on the BH 3 moiety than on the NH 3 moiety. This is consistent with the calculation of the electronic structure of AB in the solid state 4 in which the bands appear at energy separations similar to those of the gas-phase bands but disperse as a function of momentum in the condensed state.…”

“…Schleier et al recently reported the threshold valence electron spectrum of AB and noted that the spectrum of the highest occupied molecular orbital (HOMO) displayed vibronic structure, indicating that the ionic ground state is bound. They denoted this degenerate orbital HOMO-π x and HOMO-π y , and the second highest molecular orbital HOMO–1; the HOMO is also denoted as 2e 1 and the HOMO–1 as 5a 1 . From the charge density maps (Figure 3 of Schleier et al), it is evident that both orbitals are localized more strongly on the BH 3 moiety than on the NH 3 moiety.…”