Isomers and conformers of H(NH2BH2)nH oligomers:: Understanding the geometries and electronic structure of boron-nitrogen-hydrogen compounds as potential hydrogen storage materials

Boron-nitrogen-hydrogen (BNHx) materials are polar analogues of hydrocarbons with potential applications as media for hydrogen storage. As H(NH2BH2)),H oligomers result from dehydrogenation of NH3BH3 and NH4BH4 materials, understanding the geometries, stabilities, and electronic structure of these oligomers is essential for developing chemical methods of hydrogen release and regeneration of the BNRx-based hydrogen storage materials. In this work we have performed computational modeling on the H(NH2BH2)(n)H (n = 1-6) oligomers using density functional theory (DFT). We have investigated linear chain structures and the stabilizing effects of coiling, biradicalization, and branching through Car-Parrinello molecular dynamics simulations and subsequent geometry optimizations. We find that the zigzag linear oligomers are unstable with respect to the coiled, square-wave chain, and branched structures, with the coiled structures being the most stable. Dilrydrogen bonding in oligomers, where protic H delta+(N) hydrogens interact with hydridic H delta-(B) hydrogens, plays a crucial role in stabilizing different isomers and conformers. The results are consistent with structures of products that are seen in experimental NMR studies of dehydrogenated ammonia borane.