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

Daniel A Thomas; Mateusz Marianski; Eike Mucha; Gerard Meijer; Mark A Johnson; Gert von Helden

doi:10.1002/anie.201805436

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

Angew Chem Int Ed Engl. 2018 Aug 13;57(33):10615-10619. doi: 10.1002/anie.201805436. Epub 2018 Jul 12.

Authors

Daniel A Thomas¹, Mateusz Marianski¹, Eike Mucha¹, Gerard Meijer¹, Mark A Johnson², Gert von Helden¹

Affiliations

¹ Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195, Berlin, Germany.
² Sterling Chemistry Laboratory, Yale University, 225 Prospect Street, New Haven, CT, 06520, USA.

PMID: 29923287
DOI: 10.1002/anie.201805436

Abstract

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.

Keywords: carboxylates; cold-ion spectroscopy; coupling; helium nanodroplets; shared protons.