Determination of the Size-Dependent Temperature Onsets for Long Range, Water-Mediated Proton Transfer in Microhydrated 4-Aminobenzoic Acid*

Water-mediated translocation of “excess” protons is the basis of the well-known “Grotthuss” or proton-relay mechanism that accounts for the enhanced proton mobility that is observed in water. Here we report the temperature onset for spontaneous transfer of a proton between the two isomers (protomers) formed upon protonation of 4-aminobenzoic acid (H⁺4ABA). This is accomplished by isolating size-selected water clusters of H⁺4ABA in a temperature-controlled, radiofrequency ion trap and photodepleting one of them (the isomer protonated at the acid group, denoted the O-protomer) by excitation of the electronic band unique to this species at ~310 nm. We then follow the spontaneous depletion of the isomer protonated at the amino group (the N-protomer) and repopulation of the O-protomer as functions of time and temperature. The relative isomer populations are monitored by recording the vibrational spectra of the ion ensemble after ejection from the ion trap into a double-focusing, tandem time-of-flight photofragmentation mass spectrometer. The temperature onset of large amplitude proton motion is observed to be strongly dependent on cluster size, becoming much lower as cluster size increases from three to six water molecules.