Isomer- specific vibrational spectroscopy investigation of protonated formic acid complexes with D₂, N₂, and H₂O using two color, IR-IR photobleaching

Protonated formic acid (PFA) is purported to be an active species involved in accelerated condensation reactions at the interface of microdroplets. Here we analyze cryogenic ion vibrational spectra of tagged 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 trans form (E,Z) and the cis form (E,E)) and generated in comparable abundance at low temperature 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 and N₂ tag-bound OH fundamentals when a second tag is attached. Analysis of the isomer-specific patterns displayed by the H/D isotopically labeled PFA-H₂O-D₂ isomers indicates that the water molecule preferably binds to the E OH. Two distinct, non-interconverting rotamers arise from the orientation of the D₂-bound OH taking on the E or Z form. This assignment scheme corrects a previous theoretical analysis that invoked a scenario in which structures with E- and Z-bound water molecules interconvert at low (20 K) temperature.