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. The cold song: The proton‐bound dicarboxylate motif, RCOO−⋅H+⋅−OOCR, is encountered frequently, for example in protic ionic liquids and protein active sites. However, the location of the bridging proton can be challenging to ascertain. Cold‐ion IR spectroscopy reveals that the isolated formate proton‐bound dimer, HCOO−⋅H+⋅−OOCH, exhibits a fully shared proton. Bridging proton motion couples strongly to deformation of the flanking −COO− framework.