Aqueous Solution Chemistry at Supercritical Conditions

水的质子化学位移是在很宽的温度和密度范围内测量的。结果表明，在超临界温度下，氢键仍然存在，在超临界密度下，水分子的平均氢键数至少为1。超临界温度下化学位移的密度依赖关系表明，在低密度下氢键在空间上更加不均匀。在类SPC和类TIP4P的水分子间势模型框架内，进一步估算了超临界状态下水分子的偶极矩.通过测量重水(D2O)的自旋晶格驰豫时间T1，研究了超临界和亚临界条件下水分子的转动动力学.实验确定的T_1即使在超临界条件下也受四极弛豫机制的支配。结果表明，在400℃的固定超临界温度下，当密度变化到临界值的两倍时，重定向关联时间保持在几十飞秒量级。重定向关联时间与角动量关联时间的比较表明，超临界水中的转动动力学不是扩散的。系统地研究了水热条件下的非催化反应。以1，4-丁二醇脱水为四氢呋喃为例，考察了热水中的离解质子对反应的影响。人们普遍认为，由于离解质子浓度的增加，在常温条件下被酸催化的反应在水热条件下会加速。在我们的反应体系中，我们保持密度和温度不变，并通过加入盐酸来改变质子浓度。然后我们测定了反应的非催化效应，发现水热条件下的水以中性形式促进了反应。

The proton chemical shift of water is measured over a wide range of temperatures and densities. It is then found that the hydrogen bonding persists at supercritical temperatures and that the average number of hydro gen bonds is at least one for a water molecule in the supercritical densities. The density dependence of the chemical shift at a supercritical temperature shows that the hydrogen bonding is spatially more inhomogeneous at lower densities. The dipole moment of a water molecule at supercritical states is further estimated within the SPC-like and TIP4P-like frameworks of the water intermolecular potential model.The rotational dynamics of water in super- and subcritical conditions is investigated by measuring the spin-lattice relaxation time T_1 of heavy water (D_2O). The experimentally determined T_1 is shown to be governed by the quadrupolar relaxation mechanism even in the supercritical conditions. It is then found that the reorientational correlation time remains on the order of several tens of femtoseconds when the density is varied up to twice the critical at a fixed supercritical temperature of 400℃. The comparison of the reorientational correlation time with the angular momentum correlation time shows that the rotational dynamics is not diffusive in supercritical water.Noncatalytic reactions at hydrothermal conditions are investigated systematically. By focusing the dehydration of 1,4-butanediol into tetrahydrofuran, we determined the effect of the dissociated proton in hot water. It is widely believed that a reaction which is acid-catalyzed in ambient conditions is accelerated in hydrothermal conditions due to the increased concentration of dissociated proton. In our reaction system, we kept the density and temperature constant and varied the proton concentration by adding hydrochloric acid. We then determined the noncatalytic effect on the reaction and found that water at hydrothermal conditions promotes the reaction in its neutral form.