Slow hydrogen-bond switching dynamics at the water surface revealed by theoretical two-dimensional sum-frequency spectroscopy

Abstract

Using our newly developed explicit three-body (E3B) water model, we simulate the surface of liquid water. We find that the timescale for hydrogen-bond switching dynamics at the surface is about three times slower than that in the bulk. In contrast, with this model rotational dynamics are slightly faster at the surface than in the bulk. We consider vibrational two-dimensional (2D) sum-frequency generation (2DSFG) spectroscopy as a technique for observing hydrogen-bond rearrangement dynamics at the water surface. We calculate the nonlinear susceptibility for this spectroscopy for two different polarization conditions, and in each case we see the appearance of cross-peaks on the timescale of a few picoseconds, signaling hydrogen-bond rearrangement on this timescale. We thus conclude that this 2D spectroscopy will be an excellent experimental technique for observing slow hydrogen-bond switching dynamics at the water surface. Interfaces play important roles in many disciplines of science. The water liquid/vapor interface, for example, is of great interest in chemistry, biology, and earth science and is an important model system for water in a heterogeneous environment. Of particular interest is understanding the extent to which the structure and dynamics, and ultimately reactivity, of water at the interface differ from those in the bulk. For example, how does the distribution of hydrogen bonds differ between interfacial and bulk water? How anisotropic is the orientation of the water molecules at the interface? In terms of dynamics, how do the diffusion constant, rotational relaxation time, and hydrogen-bond rearrangement time vary as the interface is approached? One can also consider vibrational dynamics processes such as energy relaxation and transfer.One important technique for addressing these questions is computer simulation. Models used in these calculations for the water surface range from rigid, fixed-point-charge two-body models (1-3), to fluctuating charge or polarizable models (4, 5), to ab initio molecular dynamics calculations (6-10). Regarding static properties, for example, some effort has been expended toward understanding what fraction of H atoms in the surface layer are hydrogen bonded, and what fraction of molecules do not donate any hydrogen bonds (nondonors or "acceptor-only" molecules) (6, 9). In terms of dynamics, it is generally found that diffusion is faster at the interface than in the bulk (1, 4, 10), and rotational relaxation is also faster (3,6,7,10). On the other hand, two studies with fixed-charge two-body models show that hydrogenbond rearrangement is slower at the interface (2, 3), whereas one study with a fluctuating-charge model shows that hydrogen-bond rearrangement is faster (5). In this latter study the authors conclude that this is generally true for polarizable models.Because of its surface sensitivity, vibrational sum-frequency generation (SFG) spectroscopy (11, 12) has become one of the most powerful experimental techniques for the study of interfaces, including the one separa...

Mechanism of vibrational energy dissipation of free OH groups at the air–water interface

“…Summing the forward and backward time constants for each process extracted from the rate model allows direct comparison with the exponential analysis offered above. We find that the time constant of vibrational relaxation of the free OH due to IET calculated using the rate model is k Prior work suggests that, although inhomogeneous broadening is important in determining the H-bonded OH spectral response at the air-water interface, it is relatively unimportant in understanding the free OH (21,27,46). This then implies that we are justified in describing the VSF spectral response of the free OH at both the air/H 2 O and air/H 2 O:HDO:D 2 O interface as a sum of Lorentzians:…”

“…Because of its environmental ubiquity, experimental simplicity, and its relevance to understanding hydrophobic solvation more generally, much work has focused on the spectral response of interfacial water at the air-water interface (21)(22)(23). A VSF intensity spectrum (Fig.…”

Communication: Vibrational sum-frequency spectrum of the air-water interface, revisited

“…We eagerly await these experimental results, as well as 2DSFG results on the same system. 29 We thank the University of Wisconsin Foundation for support of this work. We are grateful to Professor Tahara and Professor Shen for sending preprints of their experimental work.…”

Skin Supersolidity of Water and Ice

“…The surface does not have a long-range impact on water. The timescale for H-bond switching dynamics at the surface is about three times slower than that in the bulk [19] because of the strong polarization induced viscosity. Vibrational sum frequency spectroscopy (VSFS) and MD simulations [20] suggested that the upper most two layers of water molecules are ordered 'ice like' with specific frequency of 3,217 cm -1 at the air/water interface.…”