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

Abstract

AbstractThe 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.