Unraveling the Anomalous Solvatochromic Response of the Formate Ion Vibrational Spectrum: An Infrared, Ar-Tagging Study of the HCO2-, DCO2-, and HCO2-center dot H2O Ions

Reductive activation offers an attractive synthetic route for conversion of CO2 to transportable fuels, a process that often involves creation of the formate ion as an intermediate. We carry out an Ar-tagging infrared spectroscopic study of isolated HCO2¯ and its first hydrate, HCO2¯·H2O, and analyze the resulting band patterns with electronic structure and vibrationally anharmonic calculations. Strong vibronic interactions and intramolecular mode couplings are identified that are responsible for the deceptively complex solvation behavior of this familiar ion. In particular, the CH stretch fundamental is found to be anomalously low in energy in the isolated ion and to dramatically blue shift (by hundreds of cm–1) upon solvation. These two effects are traced to the large dependence of the electronic wave function on the CH bond length, reminiscent of the classic curve-crossings that dominate the dissociation behavior of neutral salt molecules