Aqueous Solution Chemistry at Supercritical Conditions

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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.