Justin Jankunas Doctoral Dissertation Award Finalist: Watching the Water Dance: A New Way to Monitor Slow Reaction Kinetics on the Molecular Level with Temperature-controlled Water Clusters

Cryogenic ion spectroscopy has the unique capability of recording the vibrational spectra of specific isomers and isotopomers of mass selected ions at very low temperature. Such technique provides a new way to identify reaction intermediates and capture clear snapshots of transient structures at the air-water interface. In an exciting recent development, we moved beyond these static pictures by introducing a temperature- and time-dependent ion spectroscopy technique that can follow the kinetics of chemical processes in dynamic equilibrium in a finite system. Think of it as monitoring the kinetics of a reaction in a 20 molecule “nanobeaker” in which the heat required for a chemical process is just another excursion through the phase space volume confined by the free energy landscape. To demonstrate how this method works, we measured the molecular-level trajectories that drive spectral diffusion at the air-water interface. The vibrational spectrum of water in the OH stretching region is very broad because of the wide variations in the H-bond environments. In liquid water, this diffuse spectrum is dynamic in the sense that the frequency of a particular OH oscillator changes rapidly in time and explores the entire envelope in less than 1 ps.  Leveraging on our previous understanding of the spectral behavior of H₂O molecules in a finite hydrogen bonded network, we tracked the time dependent frequency of a single, isolated OH oscillator in a cage of 20 water molecules. The frequency changes are fascinating because, at the onset of spectral dynamics, the oscillator is observed to “blink” between two widely separated frequencies before undergoing more diffusive excursions with increasing temperature as the cluster melts. With this demonstration of the technique, we open a new and exciting chapter on how we can unravel solvent-mediated chemistry in a regime where every atom counts.