High Pressure Study Of Dihydrogen Bonds In B-N-H Hydrogen Rich Materials

As hydrogen rich materials,B-N-H compounds with very high hydrogen capacity have attracted enormous research attention from scientists in recent years.In B-N-H compounds,there widely existed a weak intermolecular interaction,dihydrogen bonds,which played a key role in the crystal structure of B-N-H compounds.The charge transfer between H^δ-and H^δ+resulted the release of hydrogen in B-N-H compounds upon heating.Pressure can effectively reduce the distance between molecules and strengthen the intermolecular interactions,is an ideal strategy to investigate the weak intermolecular interactions.The distortion and rearrangement of intermolecular interactions under high pressure even change the crystal structures.The high pressure study on dihydrogen bonds in B-N-H compounds will explore the nature of this weak interaction and is of significance for physics,chemistry and material science.The most importantly,the enhancement of dihydrogen bonds and the generation of new dihydrogen bonds will benefit the release of hydrogen.So the investigation of dihydrogen bonds in B-N-H compounds under high pressure can give insight into the application of B-N-H hydrogen rich materials.We systematically studied the dihydrogen bonds in B-N-H hydrogen rich materials under high pressure.Presure can markedly influence the dihydrogen bonds,lead the structural change of B-N-H compounds and give experimental evidence for the existence of weak C—H···H—B dihydrogen bonds.Meawhile,the enhancement of dihydrogen bonds upon compression will optimize the dehydrogenation of B-N-H hydrogen rich materials.We firstly investigated the pressure-induced polymorphic phase transition of typical B-N-H compound,hydrazine bisborane（HBB）,and studied the variation of dihydrogen bond in HBB upon compression.HBB has a hydrogen capacity of 16.8wt%and exists in two phases（the room-temperature phase and the low-temperature phase）.Dihydrogen bonds and van der Waals forces are the main intermolecular interaction in the crystal structure of HBB.The synchrotron X-ray diffraction and Raman scattering experiments indicated that HBB experienced a reversible phase transition at 0.4 GPa.The Rietveld refinement showed the high-pressure phase and the low-temperature phase exhibited the same crystal structure.Comparing the crystal structure and dihydrogen bonded networks before and after the phase transition,we founded that with the rotation and distortion of the NH₂—NH₂ groups,the initial dihydrogen bonds broke and new dihydrogen bonds generated.Further Hirshfeld surface and fingerprint suggested that dihydrogen bonds were enhanced and changed after the phase transition.By the help of synchrotron X-ray diffraction,Raman scattering and the first-principles calculation,the high pressure behavior of supramolecular material guanidinium borohydride（GBH）was further investigated.GBH is a B-N-H derivative and the main intermolecular interactions in GBH are dihydrogen bonds and van der Waals forces.The guanidinium cation and the borohydride anion are connected one by one in a plane through the close dihydrogen bonds and form a tape.The primary effect between the tapes is van der Waals interaction.The in situ high pressure synchrotron X-ray diffraction and Raman spectra suggested that GBH underwent a phase transition at 0.5 GPa.And the high pressure phase remained stable at 5.0 GPa.After total release of pressure,GBH recovered to the original structure.The intermolecular interactions with the increase of pressure were simulated through first-principles calculation.At ambient pressure,the main interactions between GBH tapes were van der Waals forces.As the pressure increased,the distance between GBH tapes decreased.When the distance reduced to a certain value,the dihydrogen bonds between different tapes generated.The formation of new dihydrogen bonds resulted the phase transition of GBH,and the 3D dihydrogen bonding networks of GBH generated instead of one-dimension GBH tapes.The evolvement of weak C—H···H—B dihydrogen bonds in dimethylamine borane（DMAB）under high pressure and the structural change of DMAB upon compression were subsequently studied.The C—H···H—B dihydrogen bond is so weak that this weak interaction is only investigated by theoretical researches.The experimental evidence of this interaction has been still limited thus far.Previous study considered that only N—H···H—B dihydrogen bonds and van der Waals forces existed in the crystal structure of DMAB.DMAB molecules linked in a head-to-tail arrangement via dihydrogen bonding and formed a tape.The main interactions between different tapes are van der Waals forces.The in situ high pressure Raman scattering suggested that partial CH stretching modes exhibited red shift before 0.54GPa.This red shift gives strong evidence for the existence of C—H···H—B dihydrogen bonds.Further Hirshfeld surface analysis also suggested the existence of this dihydrogen bonds.In the pressure range from 0.63 to 1.49 GPa,in situ high pressure Raman scattering,in situ high pressure infrared absorption and in situ synchrotron X-ray diffraction spectra varied a lot,which indicated DMAB experienced a phase transition.The high pressure phase is stable even at 5.08 GPa and the crystal structure can be recovered upon decompression.The first-principles calculation was performed on DMAB.The C—H···H—B dihydrogen bonds underwent rotation and the H···H distance of this weak interaction decreased with the increase of pressure.The variation of C—H···H—B dihydrogen bonds resulted in the phase transition of DMAB.While the N—H···H—B dihydrogen bonds changed very little upon compression.The enhancement of dihydrogen bonds in hydrazine borane（HB）under high pressure and the influence of the strengthened dihrdrogen bonds on the thermal dehydrogenation of HB were investigated at last.Both in situ Raman scattering and synchrotron XRD measurements suggested that HB didn’t experience a phase transformation in the pressure range of 1 atm to 3.0 GPa.High-pressure and high-temperature Raman experiments indicated that pressure changed the thermal decomposition processes,increased the reaction temperature and promoted the breakage of NH bonds to release more hydrogen.EDS was utilized on the decomposition product of HB after heating to 250 ^oC at various pressures.From the ambient pressure to 2.0 GPa,the ratio of B:N decreased gradually,which indicated that pressure would partly restrain the release of N₂H₄.When pressure increased to2.5 GPa,this ratio declined sharply,only 5%of HB would generate N₂H₄.The first-principles calculation and Hirsheld surface analysis suggested that dihydrogen bonds and BN bonds were both strengthened in some extent.We considered that the enhancement of dihydrogen bonds promoted the breakage of NH bonds.And the enhancement of dihydrogen and BN bonds restrained the release of N₂H₄.Our investigation proved that pressure is a green and effective method to promote the dehydrogenation of HB,and give insight to increase the hydrogen release efficiency of B-N-H hydrogen rich materials.