Watching the Water Dance: A New Way to Monitor Slow Reaction Kinetics on the Molecular Level with Temperature-controlled Nanodroplets*

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. 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. 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.