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

The proton‐bound dicarboxylate motif, RCOOˉ·H+·ˉOOCR, is a prevalent chemical configuration found in many condensed phase systems. We study the archetypal proton‐bound formate dimer, HCOOˉ·H+·ˉOOCH, utilizing cold‐ion infrared action spectroscopy in the photon energy range of 400‐1800 cm^-1. The spectrum obtained at ~0.4 K utilizing action spectroscopy of ions captured in helium nanodroplets is compared to that measured at ~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, however, reveal that the bridging proton motion is strongly coupled to the flanking ‐COOˉ framework, an effect that is qualitatively in line with the expected change in ‐C=O bond rehybridization upon protonation