Investigating Kinetics of Messenger Tag Rearrangement in Cryogenically Cooled Pyridinium-(CH₂)-COOH Cations

Cryogenic ion traps have been used to study kinetics of molecular interactions, such as proton migration on hydrated 4-aminobenzoic acid and Eigen-Zundel interconversion in H⁺(H₂O)₆. Here we utilize carboxyl-functionalized ionic liquids [HOOC-(CH₂)_n-Py⁺] (n = 1-7) to study the kinetics of messenger tag rearrangement in a cryogenically cooled 3D Paul trap. In investigating the chain-length dependent intramolecular polarization contributions to the pKa’s of carboxylic acids, two features arose in the OH stretch region. Isomer-specific double resonance confirmed that the two features are due to two isomers associated with different tag locations. The redshifted OH feature arises from the isomer in which the tag molecule is attached to the OH group. The higher energy OH stretch is due to the isomer in which the tag is bound closer to the pyridinium ring, leaving the OH free. While these two isomers can be isolated using isomer specific double resonance in a Wiley-McLaren time-of-flight, experiments in trap reveal that depletion of one isomer resulted in depletion of the other isomer as well. We report the kinetics of this tag rearrangement on Py⁺(CH₂)COOH.