Microcanonical kinetics of water-mediated proton transfer in microhydrated 4-aminobenzoic acid.

Isolated cluster systems can help to elucidate the molecular level description of water-mediated proton transfer. Protonation of neutral 4-aminobenzoic acid (4ABA) occurs at the acid (O-protomer) and amine (N-protomer) functionalities, yielding two distinct species with relative energies dependent on the degree of hydration. Here, we measured the rates of intramolecular proton transfer in 4ABAH+·(H2O)6 ions upon protomer-selective vibrational excitation of initially cold (6 K) cluster ions isolated in a cryogenic ion trap. Interconversion rates were observed on the microsecond time scale. These results quantify the kinetics of proton transfers in the context of a closed, finite system at well-defined internal energies and therefore provide experimental benchmarks for theoretical efforts that are being developed to treat relatively slow, highly cooperative solvent-mediated chemical processes. Editor's summary: Proton transfer is perhaps the most fundamental chemical reaction, underlying numerous more complex transformations in both biochemical and synthetic systems. The process often involves a network of numerous water molecules. To home in on the details in a well-controlled context, Rana et al. studied interconversion of two protonated isomers, or protomers, of 4-aminobenzoic acid in small water clusters. By measuring the kinetics of proton migration between nitrogen and oxygen sites with well-resolved spectroscopic signatures, the authors were able to establish precise benchmarks for future modeling. —Jake S. Yeston [ABSTRACT FROM AUTHOR]