(H3N-BH3)4: the ammonia borane tetramer

Area of Science:

• Computational chemistry
• Quantum mechanics
• Materials science

Background:

• Ammonia borane (NH3BH3) is a molecule with potential applications in hydrogen storage.
• The behavior of ammonia borane clusters in the gas phase is not well understood.
• Experimental data for larger ammonia borane aggregates is scarce.

Purpose of the Study:

• To predict and characterize the stable structure of the isolated ammonia borane tetramer in the gas phase.
• To investigate the nature and strength of stabilizing interactions within the tetramer.
• To explore the possibility of novel bonding interactions in such systems.

Main Methods:

• Quantum mechanical calculations using the MP2/6-311++G(d,p) level of theory to determine the structure.
• High-level energy computation using the Coupled Cluster Singles Doubles and Triples (CCSD(T)) method with Basis Set Superposition Error (BSSE) correction.
• Quantum Theory of Atoms in Molecules (QTAIM) analysis to study electron density and bonding interactions.
• Application of Koch-Popelier rules to characterize hydrogen bonds.

Main Results:

• A stable structure for the ammonia borane tetramer ((H3N-BH3)4) with S4 point group symmetry was proposed.
• A significant binding energy of 40.1 kcal mol(-1) was calculated for the tetramer.
• The structure is stabilized by a network of eight N-H···H-B dihydrogen bonds, confirmed by QTAIM and Koch-Popelier rules.
• Two potentially controversial B-H···H-B interactions were also identified through topological analysis.

Conclusions:

• The study successfully predicted a stable gas-phase structure for the ammonia borane tetramer.
• Dihydrogen bonds, including N-H···H-B and B-H···H-B types, play a crucial role in stabilizing the tetramer.
• The findings provide valuable insights into the aggregation behavior of ammonia borane and potential new bonding motifs.