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 (BNH_x) materials are polar analogues of hydrocarbons with potential applications as media for hydrogen storage. As H(NH₂BH₂)_nH 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 BNH_x-based hydrogen storage materials. In this work we have performed computational modeling on the H(NH₂BH₂)_nH (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. Dihydrogen bonding in oligomers, where protic H^d+(N) hydrogens interact with hydridic H ^d-(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. © 2007 American Chemical Society.