Borstad Gustav M, Yoo Choong-Shik
Institute for Shock Physics, Department of Physics and Department of Chemistry, Washington State University, Pullman, Washington 99164, USA.
J Chem Phys. 2014 Jan 28;140(4):044510. doi: 10.1063/1.4862823.
We have investigated high-pressure behaviors of simple binary mixtures of NH3 and D2 to 50 GPa and CH4 and D2 to 30 GPa using confocal micro-Raman spectroscopy. The spectral data indicate strong proton exchange reactions occur in dense D2-NH3 mixture, producing different isotopes of ammonia such as NH3, NH2D, NHD2, and ND3. In contrast, the proton exchange process in dense D2-CH4 mixture is highly limited, and no vibration feature is apparent for deuterated methane. The vibrational modes of H2 isotopes in D2-NH3 are blue shifted from those of pure H2 isotopes, whereas the modes of D2-CH4 show overall agreement with those in pure D2 and CH4. In turn, this result advocates the presence of strong repulsion and thereby internal pressure in D2-NH3 mixture, which are absent in D2-CH4. In fact, the bond length of hydrogen molecules in D2-NH3, calculated from the present spectral data, is shorter than that observed in pure hydrogen - supporting the enhanced intermolecular interaction in the mixture. Comparing the present spectral results with those previously observed in D2-H2O mixtures further suggests that the strength of repulsive interaction or the magnitude of internal pressure in the mixtures is proportional to the strength of hydrogen bonding in H2O, NH3, and CH4 in decreasing order. Hence, we suggest that the proton exchange is assisted by hydrogen bonding in these molecules.
我们使用共聚焦显微拉曼光谱研究了氨（NH₃）与氘（D₂）的简单二元混合物至50吉帕以及甲烷（CH₄）与氘（D₂）的简单二元混合物至30吉帕的高压行为。光谱数据表明，在高密度的D₂-NH₃混合物中发生了强烈的质子交换反应，产生了不同同位素的氨，如NH₃、NH₂D、NHD₂和ND₃。相比之下，高密度D₂-CH₄混合物中的质子交换过程受到很大限制，氘代甲烷没有明显的振动特征。D₂-NH₃中H₂同位素的振动模式相对于纯H₂同位素发生了蓝移，而D₂-CH₄的模式与纯D₂和CH₄中的模式总体一致。相应地，这一结果表明D₂-NH₃混合物中存在强烈的排斥力，进而存在内压，而D₂-CH₄中不存在这种情况。事实上，根据目前的光谱数据计算得出，D₂-NH₃中氢分子的键长比纯氢中观察到的键长短，这支持了混合物中分子间相互作用增强的观点。将目前的光谱结果与之前在D₂-H₂O混合物中观察到的结果进行比较，进一步表明混合物中排斥相互作用的强度或内压的大小与H₂O、NH₃和CH₄中氢键的强度按降序成正比。因此，我们认为这些分子中的质子交换是由氢键辅助的。
