Isomer-Specific, Cryogenic Ion Vibrational Spectroscopy Investigation of D₂- and N₂-Tagged, Protonated Formic Acid Complexes Using Two-Color, IR-IR Photobleaching

Here we analyze cryogenic ion vibrational spectra of tagged protonated formic acid (PFA) with electronic structure and anharmonic vibrational calculations to establish the isomers generated by electrospray ionization (ESI) followed by buffer gas cooling to ∼25 K. Two isomers are identified (the <i>trans</i> form (E,Z) and the <i>cis</i> form (E,E)) and generated in comparable abundance despite the fact that the calculated E,E structure lies 6.40 kJ mol^-1 above the E,Z form. A large (∼60 kJ mol^-1) barrier separates them such that the E,E form can be kinetically trapped upon cooling in the ion trap. The anticooperativity between the H-bonds of the OH groups is explored by measuring the shift in the D₂-bound OH fundamental when a second D₂ is attached. Both isomers are observed in the N₂-tagged counterparts, displaying the expected red-shifted OH bands. These results indicate that ESI generates both isomers and both must be considered when analyzing cluster spectra based on the PFA core ion.