The reorientation of a water molecule is important for a host of phenomena, ranging over--in an only partial listing--the key dynamic hydrogen-bond network restructuring of water itself, aqueous solution chemical reaction mechanisms and rates, ion transport in aqueous solution and membranes, protein folding, and enzymatic activity. This review focuses on water reorientation and related dynamics in pure water, and for aqueous solutes with hydrophobic, hydrophilic, and amphiphilic character, ranging from tetra-methylurea to halide ions and amino acids. Attention is given to the application of theory, simulation, and experiment in the probing of these dynamics, in usefully describing them, and in assessing the description. Special emphasis is placed on a novel sudden, large-amplitude jump mechanism for water reorientation, which contrasts with the commonly assumed Debye rotational diffusion mechanism, characterized by small-amplitude angular motion. Some open questions and directions for further research are also discussed.

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Molecular Dynamics simulations are used to examine several key aspects of recent ultrafast infrared experiments on liquid water dynamics in an amplified and extended version of a recent communication [J. Phys. Chem. A 2002, 106, 11993]. It is found that the relation between the OH stretch frequency and the length of the hydrogen bond in which the OH is involved is characterized by considerable dispersion. This dispersion, which is in part related to the varying OHO angle of the hydrogen bond, precludes a one-to-one correspondence between the OH frequency and the hydrogen bond length. Further, it is found that the time scale currently interpreted in terms of a stochastic modulation by the surrounding solvent of a highly frictionally damped hydrogen bond system is largely governed by hydrogen bond-breaking and -making dynamics, while the motion of an intact hydrogen-bonded complex is underdamped in character. A detailed analysis of these issues is provided for calculated spectral dynamics after creation of a hole in the ground vibrational state, in terms of a three time analysis (in addition to the H-bond period). Further, the validity of describing the OH frequency fluctuations as a Gaussian random process is examined, as is the character of the associated autocorrelation function of the OH frequency shift.