Infrared photodissociation spectroscopy of D₂-tagged CH₃CO₂⁻(H₂O)₀₋₂ anions

ABSTRACT

Infrared photodissociation spectroscopy of D₂-tagged anions is used to obtain the vibrational spectra of microsolvated acetate, $\text{C}{\text{H}}_3{\text{CO}}_2^{-}({\text{H}}_2\text{O}{)}_n$ (n = 0–2), in the CH/OH stretching (∼4000–2500 cm⁻¹) and fingerprint (∼1800–800 cm⁻¹) spectral regions. These results are analysed by comparison to anharmonic IR spectra from MP2 calculations as well as Born-Oppenheimer molecular dynamics (BOMD) simulations. In agreement with prior work, we find that the first water molecule adds to the acetate anion by donating two hydrogen bonds, yielding a symmetrical structure involving a six-membered hydrogen-bonded ring. Two nearly degenerate binding motifs that differ in energy by less than 1 kJ/mol are identified for n = 2 anion, where the lowest-energy geometry has two ion-water hydrogen bonds as well as a water-water hydrogen bond. The molecular dynamics simulations confirm that this lower-energy structure is preferred over a slightly higher-lying configuration possessing three ion-water hydrogen bonds and no water-water interactions. Analysis of the molecular motion contributing to peaks in the BOMD spectra via a generalised normal mode approach provides assignment of all observed transitions to the lower-energy structure, and enables distinction of the vibrational signatures associated with ion-water and water-water intermolecular motions.

GRAPHICAL ABSTRACT

KEYWORDS:

• Ion spectroscopy
• vibrational spectroscopy
• infrared photodissociation
• microsolvation

Acknowledgements

Y.L. thanks the Alexander-von-Humboldt Foundation for a post-doctoral research fellowship, and L.M.M. acknowledges the Zuckerman STEM Leadership Fellowship.. J.A.D. acknowledges NIH grant number S10OD023532 for funding the computational facilities used in this work, and thanks John Kelly for helpful conversations regarding molecular dynamics simulations.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was funded by the US Air Force Office of Scientific Research under grant number FA9550-16-1-0097 and the German Research Foundation DFG (Deutsche Forschungsgemeinschaft) as part of the individual research grant number AS133/3-1 ‘‘Spectroscopic Characterization of Salt Dissolution in Microhydrated Cluster Ions and at the Water/Vapor Interface.’.