Rotational motion in liquid water is anisotropic: A nuclear magnetic resonance and molecular dynamics simulation study

Experimental NMR measurements of the deuterium and O-17 T-1 relaxation time s in deuterium-enriched liquid water have been performed. from 275 to 350 K . These relaxation times can yield rotational correlation times of appropri ate molecule-fixed unit vectors if the quadrupole coupling constants and as ymmetry parameters are known. We determine the latter from ab initio studie s of water clusters and experimental chemical shift measurements. We find t hat the rotational correlation time for the OD bond vector in (D2O)-O-16 va ries from 5.8 ps at 275 K to 0.86 ps at 350 K, and that the rotational corr elation time for the out-of-plane vector of dilute (D2O)-O-17 in (D2O)-O-16 varies from 4.4 ps at 275 K to 0.64 ps at 350 K. These results indicate th at the rotational motion of water is anisotropic. Molecular dynamics simula tions of liquid water are in good agreement with these experiments at the h igher temperatures, but the simulation results are considerably faster than experiment at the lower temperatures.