Kinetics of water-mediated intra-cluster proton transfer in a temperature-controlled, mass-selective ion trap

Solvent-mediated chemical reactions are challenging systems for theory because the exchange of thermal energy between solvent molecules and reactants leads to a large available phase space in which only a small fraction leads to reaction. Cryogenic radiofrequency ion traps provide a powerful platform upon which to study the mechanics of solvent-mediated chemical reactions in isolated systems with well-defined composition and internal energy. This is carried out using a custom-built experimental configuration that incorporates a suite of nanosecond IR and UV pump-probe schemes to measure the kinetics of chemical and physical processes occurring on timescales ranging from nanoseconds to milliseconds. The approach is demonstrated by measuring the kinetics of long-range proton transfer between two protomers of 4-aminobenzoic acid that is mediated by water molecules. In the case of reaction at constant temperature, the kinetics of the forward and reverse reactions that support dynamic equilibrium are revealed using an adaptation of the classic T-jump approach pioneered by Eigen in the 1950s. This yields rates that are averaged over a Gaussian distribution of internal energies. The microcanonical rates, k(E), can also be accessed, however, using cryogenic trapping methods. That is accomplished by first isolating reactant and product species at low temperature and then photoinitiating reaction through protomer-selective vibrational excitation. Forward and reverse rates are observed on the microsecond time scale for the case of six water molecules. Because these relatively slow reactions occur in a finite system at a well-defined energy, they provide an important benchmark for theoretical methods that treat the entire ensemble: reactants, products and solvent molecules, on an equal footing.

Speaker: Mark Johnson, Yale University