A Quantum-Chemical Study on Understanding the Dehydrogenation Mechanisms of Metal (Na, K, or Mg) Cation Substitution in Lithium Amide Nanoclusters

The hydrogen-releasing activity of (LiNH2)(6)-LiH nanoclusters and metal (Na, K, or Mg)-cation substituted nanoclusters (denoted as (NaNH2)(LiNH2)(5), (KNH2)(LiNH2)(5), and (MgNH)(LiNH2)(5)) are studied using ab initio molecular orbital theory. Kinetics results show that the rate-determining step for the dehydrogenation of the (LiNH2)(6)-LiH nanocluster is the ammonia liberation from the amide with a high activation energy of 167.0 kJ mol(-1) (at B3LYP/6-31 + G(d,p) level). However, metal (Na, K, Mg)-cation substitution in amide hydride nanosystems reduces the activation energies for the rate-determining step to 156.8, 149.6, and 144.1 kJ mol(-1) (at B3LYP/6-31 G(d,p) level) for (NaNH2)(LiNH2)(5), (KNH2)(LiNH2)(5), and (MgNH)(LiNH2)(5), respectively. Furthermore, only the -NH2 group bound to the Na/K cation is destabilized after Na/K cation substitution, indicating that the improving effect from Na/K-cation substitution is due to a short-range interaction. On the other hand, Mg-cation substitution affects all NH2 groups in the nanocluster, resulting in weakened N-H covalent bonding together with stronger ionic interactions between Li and the -NH2 group. The present results shed light on the dehydrogenation mechanisms of metal-cation substitution in lithium amide-hydride nanoclusters and the application of (MgNH)(LiNH2)(5) nanoclusters as promising hydrogen-storage media.