High-pressure study on symmetric hydrogen bond in gas hydrates

在甲烷水合物的“填充ICG”相，理论上估计压力诱导的氢键对称化发生在40 Gpa，比ICE-VII低20 Gpa。然而，到目前为止，还没有实验证据证明甲烷水合物的对称氢键，因为还没有人观察到甲烷水合物“充满冰”相的O-H伸缩振动峰。因此，本研究的目的是利用高压拉曼光谱技术生长甲烷水合物的单晶“充冰”，探索甲烷水合物“充冰”相中氢键的对称性。通过对上述单晶的高压拉曼光谱研究，我们最终观察到了在10 Gpa以上的“填充冰”相中的O-H伸缩振动。虽然我们还没有直接观察到对称的氢键，但我们从O-H伸缩模拉曼频移的压力依赖关系发现，甲烷水合物中氢键的对称化可能发生在50 Gpa左右。

In the "filled icg" phase of methane hydrate, it is theoretically estimated that the pressure-induced symmetrization of hydrogen bond occurs at 40 GPa which is 20 GPa lower than that of ice-VII. However, until now there is no experimental evidence for symmetric hydrogen bond of methane hydrate, because no one observed the O-H stretching vibrational peak in "filled ice" phase of methane hydrate. Therefore, the purposes of this research are to grow a single crystalline "filled ice" of methane hydrate and to explore the symmetrization of hydrogen bond in "filled ice" phase of methane hydrate using high-pressure Raman spectroscopy.We succeeded in growing small single crystals of "filled ice" of methane hydrate using a temperature controlled high-pressure diamond anvil cell. By applying the high-pressure Raman spectroscopy to the above single crystals, we finally observed the O-H stretching vibration in "filled ice" phase above 10 GPa. Although we did not still observed the symmetric hydrogen bond directly, we found the possibility that the symmetrization of hydrogen bond in methane hydrate should occur at about 50 GPa from the pressure dependence of Raman frequency shifts for O-H stretching mode at pressures from 10 to 15 GPa.