Characterization of a trans–trans Carbonic Acid–Fluoride Complex by Infrared Action Spectroscopy in Helium Nanodroplets

The high Lewis basicity
and small ionic radius of fluoride promote
the formation of strong ionic hydrogen bonds in the complexation of
fluoride with protic molecules. Herein, we report that carbonic acid,
a thermodynamically disfavored species that is challenging to investigate
experimentally, forms a complex with fluoride in the gas phase. Intriguingly,
this complex is highly stable and is observed in abundance upon nanoelectrospray
ionization of an aqueous sodium fluoride solution in the presence
of gas-phase carbon dioxide. We characterize the structure and properties
of the carbonic acid–fluoride complex, F–(H2CO3), and its deuterated isotopologue, F–(D2CO3), by helium nanodroplet
infrared action spectroscopy in the photon energy range of 390–2800
cm–1. The complex adopts a C2v symmetry structure with the carbonic acid
in a planar trans–trans conformation and both OH groups forming
ionic hydrogen bonds with the fluoride. Substantial vibrational anharmonic
effects are observed in the infrared spectra, most notably a strong
blue shift of the symmetric hydrogen stretching fundamental relative
to predictions from the harmonic approximation or vibrational second-order
perturbation theory. Ab initio thermostated ring-polymer molecular
dynamics simulations indicate that this blue shift originates from
strong coupling between the hydrogen stretching and bending vibrations,
resulting in an effective weakening of the OH···F– ionic hydrogen bonds.