Ground-State Structure of the Proton-Bound Formate Dimer by Cold-Ion Infrared Action Spectroscopy

The proton-bound dicarboxylate motif, RCOO^- ⋅H⁺ ⋅^- OOCR, is a prevalent chemical configuration found in many condensed-phase systems. The proton-bound formate dimer HCOO^- ⋅H⁺ ⋅^- OOCH was studied utilizing cold-ion IR action spectroscopy in the range 400-1800 cm^-1 . The spectrum obtained at ca. 0.4 K of ions captured in He nanodroplets was compared to that measured at ca. 10 K by photodissociation of Ar-ion complexes. Similar band patterns are obtained by the two techniques that are consistent with calculations for a C₂ symmetry structure with a proton shared equally between the two formate moieties. Isotopic substitution experiments point to the nominal parallel stretch of the bridging proton appearing as a sharp, dominant feature near 600 cm^-1 . Multidimensional anharmonic calculations reveal that the bridging proton motion is strongly coupled to the flanking -COO^- framework, an effect that is in line with the expected change in -C=O bond rehybridization upon protonation.