Isomer-Specific,\nCryogenic Ion Vibrational Spectroscopy\nInvestigation of D<sub>2</sub>- and N<sub>2</sub>‑Tagged, Protonated\nFormic Acid Complexes Using Two-Color, IR-IR Photobleaching

Here we analyze cryogenic ion vibrational spectra of\ntagged protonated\nformic acid (PFA) with electronic structure and anharmonic vibrational\ncalculations to establish the isomers generated by electrospray ionization\n(ESI) followed by buffer gas cooling to ∼25 K. Two isomers\nare identified (the <i>trans</i> form (E,Z) and the <i>cis</i> form (E,E)) and generated in comparable abundance despite\nthe fact that the calculated E,E structure lies 6.40 kJ mol<sup>–1</sup> above the E,Z form. A large (∼60 kJ mol<sup>–1</sup>) barrier separates them such that the E,E form can be kinetically\ntrapped upon cooling in the ion trap. The anticooperativity between\nthe H-bonds of the OH groups is explored by measuring the shift in\nthe D<sub>2</sub>-bound OH fundamental when a second D<sub>2</sub> is attached. Both isomers are observed in the N<sub>2</sub>-tagged\ncounterparts, displaying the expected red-shifted OH bands. These\nresults indicate that ESI generates both isomers and both must be\nconsidered when analyzing cluster spectra based on the PFA core ion.