Mechanisms of Hydrogen Generation from Tetrameric Clusters of Lithium Amidoborane.

The first-principles study of dehydrogenation mechanism of tetrameric clusters of lithium amidoborane LiNH2BH3, (LiAB)4, is presented. The choice of tetramer is based on the suspicion that dimeric cluster models used in previous theoretical studies are too small to capture the essence of the reaction. Dehydrogenation pathways starting from three isomers of (LiAB)4 tetramers were explored by applying the artificial force induced reaction (AFIR) method at the M06 level of theory. All obtained reaction pathways feature initial dimerization of two LiAB molecules in the tetramer. Formation of intermediates containing the Li3H moiety is a very characteristic feature of all pathways. In the succeeding rate-limiting step of the release of H2 molecule, a hydridic H atom of the Li3H moiety activates a protic H atom of the NH2 group with formation of the Li2H2 moiety in transition state. The most kinetically favorable pathway has the activation enthalpy of 26.6 kcal mol(-1), substantially lower than that found for dimeric cluster. The obtained results suggest that only three LiAB molecules directly participate in the elementary reactions.