Abstract
The hydrogen bonding structure and dynamics of supercritical water is studied by means of high-temperature and high-pressure NMR. The proton chemical shift is measured over a wide range of thermodynamic conditions and is related to the number of hydrogen bonds in supercritical water with the help of computer simulations. It is found that the hydrogen bonding persists at supercritical temperatures and that the average number of hydrogen bonds is at least one in the supercritical densities. The spin-lattice relaxation time is further measured to determine the reorientational correlation time. It is shown that while the reorientational relaxation proceeds on the order of picosecond in ambient water, it does on the order of several tens of femtoseconds in supercritical water. The role of water in noncatalytic reactions in hydrothermal conditions is also examined by focusing on the dehydration of 1, 4-butanediol into tetrahydrofuran, which proceeds under the presence of a strong acid at ambient conditions. The water-induced and acid-catalyzed rate constants are separately determined, and water is proven to promote the reaction in its undissociated form.
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