Dihydrogen Bonding: Donor-Acceptor Bonding (AH···HX) versus the H₂Molecule (A-H₂-X)

Dihydrogen bonds (DHBs) play a role in, among others, crystal packing, organometallic reaction mechanisms, and potential hydrogen-storage materials. In this work we have analyzed the central H-H bond in linear H₄, LiH···HX, BH₄ ^-···HX, and A1H₄ ^-···HX complexes with various X by using the quantitative molecular orbital model contained in Kohn-Sham density functional theory at the BP86/ TZ2P level of theory. First, we address the questions of if and how one can distinguish, in principle, between a H⋯H donor-acceptor DHB and the formation of an H₂ molecule by using the simple H ₄ model system. The results of these analyses have been used to gain an understanding of the bonding in more realistic model systems (some of which have been studied experimentally), and how this differs from the bonding in H₄.