Nakano Satoshi, Sano-Furukawa Asami, Hattori Takanori, Machida Shinichi, Komatsu Kazuki, Fujihisa Hiroshi, Yamawaki Hiroshi, Gotoh Yoshito, Kikegawa Takumi
National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan.
J-PARC Center, Japan Atomic Energy Agency (JAEA), 2-4 Shirakata, Tokai, Naka, Ibaraki 319-1195, Japan.
Inorg Chem. 2021 Mar 1;60(5):3065-3073. doi: 10.1021/acs.inorgchem.0c03345. Epub 2021 Feb 15.
High-pressure X-ray and neutron diffraction analyses of an ambient-pressure phase (AP) and two high-pressure phases (HP1 and HP2) of ammonia borane (i.e., NHBH and NDBD) were conducted to investigate the relationship between their crystal structures and dihydrogen bonds. It was confirmed that the hydrogen atoms in AP formed dihydrogen bonds between adjacent molecules, and the H-H distance between the hydrogen atoms forming this interaction was shorter than 2.4 Å, which was nearly 2 times larger than the van der Waals radius of hydrogen. In the case of half of the hydrogen bonds, a phase transition from AP to the first high-pressure phase (HP1) at ∼1.2 GPa resulted in an increase in the H-H distances, which suggested that the dihydrogen bonds were broken. However, when HP1 was further pressurized to ∼4 GPa, all of the H-H distances became shorter than 2.4 Å again, which implied the occurrence of pressure-induced re-formation of the dihydrogen bonds. It was speculated that the re-formation was consistent with a second-order phase transition suggested in previous studies by Raman spectroscopy and X-ray diffraction measurement. Furthermore, at ∼11 GPa, HP1 transformed to the second high-pressure phase (HP2), and its structure was determined to be 2 ( = 2). In this phase transition, the inclination of the molecule axis became larger, and the number of types of dihydrogen bonds increased from 6 to 11. At 18.9 GPa, which was close to the upper pressure limit of HP2, the shortest dihydrogen bond decreased to ∼1.65 Å. Additionally, the X-ray diffraction results suggested another phase transition to the third high-pressure phase (HP3) at ∼20 GPa. The outcomes of this study confirmed experimentally for the first time that the structural change under pressure causes the breakage and re-formation of the dihydrogen bonds of NHBH.
对氨硼烷（即NHBH和NDBD）的常压相（AP）以及两个高压相（HP1和HP2）进行了高压X射线和中子衍射分析，以研究它们的晶体结构与双氢键之间的关系。结果证实，常压相中的氢原子在相邻分子之间形成了双氢键，形成这种相互作用的氢原子之间的H-H距离短于2.4 Å，这几乎是氢的范德华半径的2倍。在一半的氢键中，在约1.2 GPa下从常压相到第一个高压相（HP1）的相变导致H-H距离增加，这表明双氢键被破坏。然而，当HP1进一步加压至约4 GPa时，所有的H-H距离再次变得短于2.4 Å，这意味着发生了压力诱导的双氢键重新形成。据推测，这种重新形成与先前拉曼光谱和X射线衍射测量研究中提出的二级相变一致。此外，在约11 GPa时，HP1转变为第二个高压相（HP2），其结构确定为2（ = 2）。在这个相变过程中，分子轴的倾斜度变大，双氢键的类型数量从6种增加到11种。在接近HP2压力上限的18.9 GPa时，最短的双氢键减小至约1.65 Å。此外，X射线衍射结果表明在约20 GPa时向第三个高压相（HP3）发生了另一个相变。本研究的结果首次通过实验证实，压力下的结构变化导致了NHBH双氢键的断裂和重新形成。
