Hydrogen Bond Dynamics of Water

Water is composed of an ever-changing network of hydrogen bonds, whose structural evolution underlies its unusual physical properties, its important role in guiding aqueous chemistry and charge transport, and its role in hydrophobicity and biological folding and self-assembly. Our group has used 2D IR spectroscopy and other ultrafast nonlinear infrared spectroscopies to monitor the fluctuations of this network and build a mechanistic picture of how water’s structure evolves with time. The OH frequency of dilute HOD dissolved in D₂O is sensitive to the hydrogen bonding configuration at the proton since the formation of a hydrogen bond tugs on the intermolecular potential and lowers the OH frequency. By tracking this frequency over time using 45 fs infrared pulses we gain insight into how water’s fastest intermolecular motions act to break and form hydrogen bonds. In particular, 2D IR has been used to track how molecules at an initial frequency, ω₁, and a final frequency, ω₃, after a waiting period, τ₂. We complement our experiments with spectroscopic modeling that draws on molecular dynamics simulations.

Fig. 1 displays a waiting time series of 2D IR spectra for the OH stretching vibration. At short waiting times, the lineshape appears diagonally elongated because molecules have not yet had sufficient time to sample their environments. As the waiting time increases the lineshape becomes more symmetric (i.e. round) as molecules are able to spectrally diffuse. More interesting are the frequency dependent changes to the lineshape. The antidiagonal linewidth for frequencies corresponding to strained or broken hydrogen bonds broadens within 100 fs, showing the inherent instability of these configurations. This presents a picture of water in which hydrogen bonds are broken only transiently during the exchange of hydrogen bonding partners due to the concerted motion of multiple water molecules.

Our current work is aimed at better understanding this concerted rearrangement mechanism by directly visualizing the correlated motion between adjacent molecules using the coupling between the OD and OT stretches of HOD and HOT dissolved in H₂O.

Shining light on the rapidly evolving structure of water A. Tokmakoff, Science, 317, 5834 (2007).

Multidimensional infrared spectroscopy of water. II. Hydrogen bond switching dynamics J. J Loparo, S. T. Roberts, and A. Tokmakoff, J. Chem. Phys. 125, 194522, (2006).