The rotational dynamics of water in super- and subcritical conditions is in
vestigated by measuring the spin-lattice relaxation time T-1 of heavy water
(D2O). The experimentally determined T-1 is shown to be governed by the qu
adrupolar relaxation mechanism even in the supercritical conditions and to
provide the second-order reorientational correlation time tau (2R) of the O
-D axis of a single water molecule. It is then found that while tau (2R) de
creases rapidly with the temperature on the liquid branch of the saturation
curve, it remains on the order of several tens of femtoseconds when the de
nsity is varied up to twice the critical at a fixed supercritical temperatu
re of 400 degreesC. The comparison of tau (2R) with the angular momentum co
rrelation time shows that the rotational dynamics is not diffusive in super
critical water. The dependence of tau (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 a
s that in ambient water on the relative scale. Actually, although tau (2R)
is divergent in the limit of zero density, it is observed to increase with
the density when the density is above similar to1/3 of the critical. (C) 20
01 American Institute of Physics.