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

Daniel A Thomas,Mariana Rossi,Gerard Meijer,Gert Von Helden,Maike Lettow,Eike Mucha

doi:10.1021/jacs.8b13542

Abstract

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.

Highlights

The strong hydrogen bonds formed by halides are crucial in directing fundamental processes ranging from solvation to chemical reactivity
Ions generated by nanoelectrospray ionization (nESI) of a 1 mM aqueous solution of sodium fluoride were exposed to a flow of gas-phase carbon dioxide introduced at the atmospheric pressure inlet of a quadrupole time-of-flight mass spectrometer
The ion at m/z 61 is readily identified as hydrogen carbonate, HCO3−, which may be generated from dissolved carbonic acid in the sample or from the exothermic reaction of the hydroxide ion with carbon dioxide.[76]

Summary

Introduction

The strong hydrogen bonds formed by halides are crucial in directing fundamental processes ranging from solvation to chemical reactivity. The strength of the halide−water hydrogen bond strongly influences the local solvation structure and may affect the solvent dynamics beyond the first solvation shell.[1−4] In choline halide-based deep eutectic solvents, strong halide hydrogen bonding interactions are the key structural motif leading to freezing -point suppression.[5−9] The fluoride anion, as a result of its small ionic radius and high Lewis basicity, forms strong hydrogen bonds, leading to local solvation structures in protic solvents distinct from those of larger halides. Fluoride can act as a potent nucleophile and has been used as a model reactant for studying nucleophilic substitution (SN2) reactions.[14−17] Strong hydrogen bonding is likewise important in this context in which the complexation of fluoride with only a single water molecule significantly decreases the reaction rates.[14,17,18] In addition, the noncovalent capture of fluoride by hydrogen bond donor catalysts has been utilized to tune the enantioselectivity in nucleophilic substitution reactions.[19]

Methods

Results

Conclusion