Dihydrogen bonding under high pressure: A Raman study of BH₃NH₃ molecular crystal

The effects of high pressures (up to 40 kbar) on the dihydrogen-bonded BH₃NH₃ molecular crystal were investigated using Raman spectroscopy in diamond and moissanite anvil cells. The stretching mode frequencies of the NH₃ proton donor groups exhibited moderate red shifts with increasing pressures, as found in many conventional hydrogen-bonded systems of weak to medium strength. The stretching modes corresponding to the BH₃ proton acceptor group, on the other hand, showed large blue shifts with increasing pressure, which, however, are not related to the changes in the N-H ⋯ H-B interactions. The BH₃NH₃ crystals undergo a pressure-induced disorder-order phase transition around 8 kbar, which is facilitated by the presence of dihydrogen bonds.