hrvatski

Interaction of amidoborane molecular chains with alkali metals – a theoretical study

Ammonia borane (NH3 -BH3 - AB) stands out as a perpepective candidate material for hydrogen storage, with its high gravimetric density and low desorption temperature. Severe problems persist however, namely multiple-step ireversible desorption to stable polymeric species and contamination of desorbed hydrogen with unwanted substances in the process. At least one of many obstacles, like borasine poisoning, can be overcame with the substitution of amido group hydrogens with light alkaline metals, which gives amidoboranes, M(NH2BH3)x (x stands for charge of metal ion) or MAB. During the dehydrogenation process several desorption pathways are observed, one including the synthesis of alkali metal - A2B3 (BH3-NH2-BH2-NH2-BH3 ) compound. Another, non-substitutional, bonding mechanism is proposed [ref: Fijalkowski et al.], with alkaline metal ionically bind within tetrahedral environement of acidic hydrogens surrounding borane. In this paper, we have performed systematic electronic structure calculations and the analysis of charge density topology of AB, AB2, A2B2 and A2B3 chains and their Na – H substitutional counterparts with alkaline metal at symmetry allowed H positions on amide sites. Additionally, electronic structure of A2B3- anion and A2B3 – based molecule with Na bind to borane hydrogens have been performed. The goal is to investigate trends in cohesion, bonding nature and strength of these mostly hypothetical molecules and to explain experimentally evident existence of at least some of them. The results show strong influence of alkali metal on deviation from alkane-like C2v symmetry chains through strong interaction with both original substitution site and surrounding acidic hydrogens. In case of A2B3 Na-acidic H bonding prevails which can explain the observed different kind of bonding in Na-A2B3. On the other hand, the energy promotion due to dihydrogen bonding in molecular crystal is of at least same order of magnitude compared to the value of C2v-C1 relaxation of A2B3 molecule and ion.

Amidoborane ; ammonia borane ; hydrogen storage ; DFT

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