Effect of environment on hydrogen bond dynamics in liquid water.

It is generally accepted that the distinctive properties of water can be ascribed to hydrogen bonding [1]. The fundamental dynamical process of the liquid is the making and breaking of hydrogen bonds. This dynamics, however, is far from being completely understood. Experiments such as infrared absorption and Raman scattering [2], depolarized light scattering [3], and inelastic neutron scattering [4] probe hydrogen bond dynamics indirectly and can be interpreted in only a qualitative way [5]. While limited to classical models, the method of molecular dynamics can be used to explore hydrogen bond dynamics at the microscopic level [6]. Factors controlling the dynamics can be determined from trajectory calculations of correlation functions. Different types of hydrogen bond correlation functions, as proposed by Stillinger [7], have been computed for liquid water. Different conclusions for the long time relaxation have been drawn. Some workers conclude this relaxation is purely exponential; others conclude it is nonexponential [8]. In this Letter, we demonstrate that the long time dynamics of a single hydrogen bond in ambient liquid water is indeed characterized by significant nonexponential relaxation, and this complex relaxation is essentially uncorrelated to the specific bonding patterns near the tagged hydrogen bond. Our results are based upon an analysis of a large number of molecular dynamics simulations employing the single point charge (SPC) [9] intermolecular potential model for liquid water. A configurational criterion for whether a particular pair of water molecules is bonded allows the construction of a hydrogen bond population operator, h. It is unity when the particular tagged pair of molecules is hydrogen bonded, according to the adopted definition [10], and is zero otherwise. Two water molecules are chosen as being hydrogen bonded only if their interoxygen distance is less than 3.5 Å, and simultaneously the O—H . . . O angle is less than 30± [11]. The average number of hydrogen bonds in an equilibrium of N water molecules is 12 NsN 2 1d khl, where khl denotes the time average of h. In the dynamical equilibrium of liquid water, the hydrogen bond population operator fluctuates in time. These fluctuations are characterized by the correlation function cstd ­ khs0dhstdlykhl . (1)