The Onset for Vibrationally Induced, Intramolecular Proton Transfer at Three Water Molecules in Microhydrated 4-Aminobenzoic Acid.

Protonation of 4-aminobenzoic acid occurs at either the amino group (the N-protomer, N) or at the carboxylic acid C═O group (the O-protomer or O). N lies higher in energy than O in the isolated ion but becomes the more stable of the 4ABAH+·(H2O)n clusters in the size range n ∼ 5. We report the size-dependent onset of intramolecular proton transfer upon vibrational excitation of cold (20 K), size-selected 4ABAH+·(H2O)n clusters in a cryogenically cooled ion trap. The onset occurs with three water molecules, which are retained after protomer interconversion. While we do not definitively identify a mechanism for the transfer, path integral molecular dynamics simulations indicate that it may proceed by proton shuttling across a bridge of at least three water molecules. By capturing protomer interconversion in a closed, finite system with well-defined internal energy, we provide a useful benchmark for furthering our understanding of water-mediated proton transport.