Abstract
To explore how the ion-pair in a single salt molecule evolves with the addition of water, infrared (IR) spectra of complexes composed of NaCl and multiple water molecules have been recorded for the first time. The NaCl(H2O)n complexes were formed and probed in liquid helium nanodroplets, and IR spectra were recorded for n = 1 → 4. The spectra for n = 1, 2, and 3 are consistent with formation of the lowest energy contact-ion pair structures in which each water molecule forms a single ionic hydrogen bond to an intact Na(+)Cl(-) ion-pair. Alternative structures with hydrogen bonding between water molecules become energetically competitive for n = 4, and the IR spectrum indicates likely the coexistence of at least two isomers.
Highlights
To explore how the ion-pair in a single salt molecule evolves with the addition of water, infrared (IR) spectra of complexes composed of NaCl and multiple water molecules have been recorded for the first time
Alternative structures with hydrogen bonding between water molecules become energetically competitive for n = 4, and the IR spectrum indicates likely the coexistence of at least two isomers
Analysis of the spectra was consistent with a contact ionpairs (CIPs)-like complex with the steady elongation of the Na–Cl bond as water molecules are added
Summary
To explore how the ion-pair in a single salt molecule evolves with the addition of water, infrared (IR) spectra of complexes composed of NaCl and multiple water molecules have been recorded for the first time. The spectra for n = 1, 2, and 3 are consistent with formation of the lowest energy contact-ion pair structures in which each water molecule forms a single ionic hydrogen bond to an intact Na+Cl− ion-pair.
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