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(NH 2 BH 2 ) n H oligomers result from dehydrogenation of NH 3 BH 3 and NH 4 BH 4 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(NH 2 BH 2 ) 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. Dihydrogen bonding in oligomers, where protic H δ+ (N) hydrogens interact with hydridic H δ- (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.