Isomers and Conformers of H(NH 2 BH 2 ) n H Oligomers: Understanding the Geometries and Electronic Structure of Boron-Nitrogen-Hydrogen Compounds as Potential Hydrogen Storage Materials Jun Li,* ,² Shawn M. Kathmann, ‡ Gregory K. Schenter, ‡ and Maciej Gutowski* ,‡,§,⊥ W. R. Wiley EnVironmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, Chemical Sciences DiVision, Pacific Northwest National Laboratory, Richland, Washington 99352, Chemistry-School of Engineering and Physical Sciences, Heriot-Watt UniVersity, Edinburgh EH14 4AS, United Kingdom, and Department of Chemistry, UniVersity of Gdan ´ sk, 80-952 Gdan ´ sk, Poland ReceiVed: September 27, 2006; In Final Form: December 23, 2006 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. Introduction On-board hydrogen storage (HS) is one of the major tech- nological bottlenecks in developing a hydrogen economy. 1-4 Boron-nitrogen-hydrogen (BNH x ) materials such as ammonia borane, NH 3 BH 3 , are potentially important candidates for chemical hydrogen storage because of their high gravimetric and volumetric densities of hydrogen. 5-11 One of the key challenges in investigating BNH x is to understand the geometric and electronic factors that govern the stabilities, reactivities, and inter- and intramolecular interactions. Computational modeling is uniquely suited to provide a molecular-level understanding of the electronic structure and stabilities of these materials. There are many theoretical studies of the gas-phase geometry and electronic structure of the NH 3 BH 3 (n ) 1) monomer, which is known to have a staggered structure as the most stable conformation. 12-15 Due to Pauli exchange repulsion, the eclipsed gas-phase conformer has a slightly longer B-N distance (∼0.03 Å) and is energetically higher in energy, by a few kilocalories/ mole. From our PW91 calculation, the rotation barrier from the staggered to the eclipsed conformation is 1.94 kcal/mol, in excellent agreement with the experimental value of 2.07 kcal/ mol. 16 The H(NH 2 BH 2 ) n H oligomers (n g 2), which are products of n consecutive dehydrocoupling reactions involving NH 3 BH 3 molecules, are good starting points for understanding the structures and stabilities of the partially dehydrogenated material. There are theoretical calculations on infinite polyaminoborane (NH 2 BH 2 ) ∞ polymers, 17,18 but little is understood of the structures and the stabilities of the finite BNH x oligomers. The only previous theoretical effort to understand the electronic structure of these oligomers is a recent study on the properties of four types of selected linear chain structures. 19 It is not clear if these structures are the most stable or if other nonchain structures are stable as well. Therefore, an in-depth theoretical investigation is necessary to explore the geometries, stabilities, and electronic structure of these oligomers. Because of significant differences in the electronegativities and the number of valence electrons in B and N, the -BH 2 and -NH 2 groups are electron-deficient and electron-rich, respec- tively. As a result, the H(NH 2 BH 2 ) n H oligomers, with the Lewis structures of H-(H 2 Br:NH 2 ) n -H, have large dipole moments if they adopt linear chain configurations. These configurations would become less stable as the oligomer chains grow longer because the dipole moment increases as n increases. Isomer- ization, conformational changes, and ground-state spin crossing of the oligomers are possible pathways to transform linear chain oligomers into more stable species with smaller dipole moments. Here we report a computational investigation of the oligo- meric molecules H(NH 2 BH 2 ) n H(n ) 1-6) using DFT methods with Slater-type and planewave basis sets. In particular, we investigate the stabilities of oligomers with respect to confor- mational and isomeric degrees of freedom. In addition to coiling and branching, we also consider biradicalization via electron transfer from the negative to the positive charge end of the closed-shell H(NH 2 BH 2 ) n H molecule. We probe the energetics of this pathway via the singlet-triplet energy splitting. We will show that the oligomers energetically prefer coiled structures, * Corresponding authors. E-mail: m.gutowski@hw.ac.uk and jun. li@pnl.gov. ² W. R. Wiley Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory. ‡ Chemical Sciences Division, Pacific Northwest National Laboratory. § Heriot-Watt University. ⊥ University of Gdan ´sk. 3294 J. Phys. Chem. C 2007, 111, 3294-3299 10.1021/jp066360b CCC: $37.00 © 2007 American Chemical Society Published on Web 02/07/2007