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。然后发现氢键在超临界温度下仍然存在，并且在超临界密度下水分子的平均氢键数至少为一个。超临界温度下化学位移的密度依赖性表明，低密度下氢键在空间上更加不均匀。在水分子间势模型的类spc和类tip4p框架内，进一步估计了水分子在超临界状态下的偶极矩。通过测量重水（D_2O）的自旋晶格弛豫时间T_1，研究了水在超临界和亚临界条件下的旋转动力学。实验结果表明，即使在超临界条件下，T_1也受四极性弛豫机制的支配。在400℃的固定超临界温度下，当密度变化到临界的两倍时，重定向相关时间保持在几十飞秒左右。重定向相关时间与角动量相关时间的比较表明，旋转动力学在超临界水中不具有弥漫性。系统地研究了水热条件下的非催化反应。通过聚焦1,4-丁二醇脱水成四氢呋喃，确定了热水中解离质子的作用。人们普遍认为，在环境条件下酸催化的反应在水热条件下由于解离质子浓度的增加而加速。在我们的反应体系中，我们保持密度和温度不变，通过加入盐酸来改变质子的浓度。然后我们确定了对反应的非催化作用，并发现水在水热条件下以其中性形式促进反应。

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.