Theoretical investigations on the formation and dehydrogenation reaction pathways of H(NH2BH2)nH (n = 1-4) oligomers: Importance of dihydrogen interactions

Jun Li, Shawn M. Kathmann, Han S. Hu, Gregory K. Schenter, Tom Autrey, MacIej Gutowski

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Abstract

The H(NH2BH2)nH oligomers are possible products from dehydrogenation of ammonia borane (NH3BH3) and ammonium borohydride (NH4BH4), which belong to a class of boron-nitrogen-hydrogen (BNHx) compounds that are promising materials for chemical hydrogen storage. Understanding the kinetics and reaction pathways of formation of these oligomers and their further dehydrogenation is essential for developing BNHx-based hydrogen storage materials. We have performed computational modeling using density functional theory (DFT), ab initio wave function theory, and Car-Parrinello molecular dynamics (CPMD) simulations on the energetics and formation pathways for the H(NH 2BH2)nH (n = 1-4) oligomers, polyaminoborane (PAB), from NH3BH3 monomers and the subsequent dehydrogenation steps to form polyiminoborane (PIB). Through computational transition state searches and evaluation of the intrinsic reaction coordinates, we have investigated the B-N bond cleavage, the reactions of NH 3BH3 molecule with intermediates, dihydrogen release through intra- and intermolecular hydrogen transfer, dehydrocoupling/cyclization of the oligomers, and the dimerization of NH3BH3 molecules. We find that the formation of H(NH2BH2) n+1H oligomers occurs first through reactions of the H(NH 2BH2)nH oligomers with BH3 followed by reactions with NH3 and the release of H2, where the BH3 and NH3 intermediates are formed through dissociation of NH3BH3. We also find that the dimerization of the NH3BH3 molecules to form cyclic c-(NH2BH 2)2 is slightly exothermic, with an unexpected transition state that leads to the simultaneous release of two H2 molecules. The dehydrogenations of the oligomers are also exothermic, typically by less than 10 kcal/(mol of H2), with the largest exothermicity for n = 3. The transition state search shows that the one-step direct dehydrocoupling cyclization of the oligomers is not a favored pathway because of high activation barriers. The dihydrogen bonding, in which protic (HN) hydrogens interact with hydridic (HB) hydrogens, plays a vital role in stabilizing different structures of the reactants, transition states, and products. The dihydrogen interaction (DHI) within the R-BH2(? 2-H2) moiety accounts for both the formation mechanisms of the oligomers and for the dehydrogenation of ammonia borane. © 2010 American Chemical Society.

Original languageEnglish
Pages (from-to)7710-7720
Number of pages11
JournalInorganic Chemistry
Volume49
Issue number17
DOIs
Publication statusPublished - 6 Sep 2010

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