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-(HH)-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.