The rotational dynamics of water in super- and subcritical conditions is investigated by measuring the spin-lattice relaxation time 1 of heavy water (D2O). The experimentally determined 1 is shown to be governed by the quadrupolar relaxation mechanism even in the supercritical conditions and to provide the second-order reorientational correlation time 2R of the O-D axis of a single water molecule. It is then found that while 2R decreases rapidly with the temperature on the liquid branch of the saturation curve, it 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 oC. The comparison of 2R with the angular momentum correlation time shows that the rotational dynamics is not diffusive in supercritical water. The dependence of 2R on the hydrogen bonding state is also examined in combination with molecular dynamics simulations, and the effect of the hydrogen bonding on the rotational dynamics in supercritical water is found to be weaker than but to be on the same order of magnitude as that in ambient water on the relative scale
