Dynamics of confined water molecules (original) (raw)
2005, Proceedings of the National Academy of Sciences
We present femtosecond midinfrared pump-probe measurements of the molecular motion and energy-transfer dynamics of a water molecule that is enclosed by acetone molecules. These confined water molecules show hydrogen-bond and orientational dynamics that are much slower than in bulk liquid water. This behavior is surprising because the hydrogen bonds to the CAO groups of the acetone molecules are weaker than the hydrogen bonds in bulk water. The energy transfer between the OOH groups of the confined water molecules has a time constant of 1.3 ؎ 0.2 ps, which is >20 times slower than in bulk water. We find that this energy transfer is governed completely by the rate at which hydrogen bonds are broken and reformed, and we identify the short-lived molecular complex that forms the transition state of this process. hydrogen bonding ͉ infrared pump-probe spectroscopy ͉ energy transfer W ater plays an essential role in many chemical and biological processes. Over the last decades, this notion has motivated a lot of work on the dynamical properties of bulk liquid water (1-7). However, the role of water in (bio)chemical processes is often played by a limited number of water molecules in a strongly restricted molecular environment. For example, the stability, structure, and biological function of proteins are largely determined by only a few surrounding layers of water molecules (8). When the water molecules participate directly in a reaction, the number of involved water molecules is even smaller. For example, the proton-pumping function of bacteriorhodopsin involves changes of the hydrogen network that is formed by particular amino acids of the protein and only a few confined water molecules (9-12). Recently, the dynamics of water in restricted environments was studied by comparing the spectral dynamics of an optically excited probe molecule embedded in a hydrated (bio)molecule with the spectral dynamics of the same probe molecule in bulk water (13). The spectral dynamics reflect the collective rearrangement of the solvating water, and were found to be much slower within the hydrated (bio)molecule than in bulk water. In this article, we present a study of the hydrogen-bond and energy-transfer dynamics of individual H 2 O and 1 H 2 HO molecules in a confined environment. In this study, we probed the dynamics of the water molecules directly with femtosecond midinfrared laser pulses that are resonant with the OOH stretch vibrations.
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