(H3N-BH3)4: the ammonia borane tetramer

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 S-4 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 delta+ . . . H delta--B bonding interactions characterized as hydrogen bonds by application of the Koch-Popelier rules; in addition, two highly controversial B-H delta- . . . H delta--B interactions are also predicted by a topological analysis of the electron density.