Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington, D.C. 20015, USA.
J Phys Chem A. 2011 Oct 13;115(40):11033-8. doi: 10.1021/jp204642b. Epub 2011 Sep 20.
Raman spectroscopy and synchrotron X-ray diffraction are used to examine the high-pressure behavior of tetramethylammonium borohydride (TMAB) to 40 GPa at room temperature. The measurements reveal weak pressure-induced structural transitions around 5 and 20 GPa. Rietveld analysis and Le Bail fits of the powder diffraction data based on known structures of tetramethylammonium salts indicate that the transitions are mediated by orientational ordering of the BH(4)(-) tetrahedra followed by tilting of the (CH(3))(4)N(+) groups. X-ray diffraction patterns obtained during pressure release suggest reversibility with a degree of hysteresis. Changes in the Raman spectrum confirm that these transitions are not accompanied by bonding changes between the two ionic species. At ambient conditions, TMAB does not possess dihydrogen bonding, and Raman data confirms that this feature is not activated upon compression. The pressure-volume equation of state obtained from the diffraction data gives a bulk modulus [K(0) = 5.9(6) GPa, K(0)' = 9.6(4)] slightly lower than that observed for ammonia borane. Raman spectra obtained over the entire pressure range (spanning over 40% densification) indicate that the intramolecular vibrational modes are largely coupled.
拉曼光谱和同步加速器 X 射线衍射被用来研究室温下四甲基铵硼氢化（TMAB）在 40 GPa 下的高压行为。测量结果表明，在 5 和 20 GPa 左右存在较弱的压力诱导结构转变。基于已知的四甲基铵盐结构的粉末衍射数据的 Rietveld 分析和 Le Bail 拟合表明，这些转变是由 BH(4)(-)四面体的取向有序化以及随后的（CH(3))(4)N(+)基团倾斜介导的。在减压过程中获得的 X 射线衍射图谱表明具有一定滞后的可逆性。拉曼光谱的变化证实这些转变不伴随着两种离子物种之间的键合变化。在环境条件下，TMAB 不具有双氢键，拉曼数据证实该特征在压缩时不会被激活。从衍射数据得到的状态方程给出了一个体弹模量[K(0) = 5.9(6) GPa，K(0)’= 9.6(4)]，略低于氨硼烷的观察值。在整个压力范围内（超过 40%的致密化）获得的拉曼光谱表明，分子内振动模式在很大程度上是耦合的。
