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.