(H3N–BH3)4: the ammonia borane tetramer

Abstract

For this special edition of PCCP dealing with prediction of new molecules using quantum mechanical methods, we propose a structure for (H₃N–BH₃)₄, the isolated ammonia borane tetramer in gas phase, for which there are no experimental reports. The structure, belonging to the S₄ point group, was found at the MP2/6-311++G(d,p) level of theory; the total energy was computed at the CCSD(T) level including BSSE correction, affording a binding energy of 40.1 kcal mol⁻¹. The tetramer is stabilized by a network of dihydrogen bonds. We study the stabilizing interactions via QTAIM, obtaining eight N–H^δ+⋯^δ−H–B bonding interactions characterized as hydrogen bonds by application of the Koch–Popelier rules; in addition, two highly controversial B–H^δ−⋯^δ−H–B interactions are also predicted by a topological analysis of the electron density.