Isomer‐Specific Vibrational Spectroscopy of Microhydrated Lithium Dichloride Anions: Spectral Fingerprint of Solvent‐Shared Ion Pairs

Abstract

The vibrational spectroscopy of lithium dichloride anions microhydrated with one to three water molecules, [LiCl2(H2O)1–3]−, is studied in the OH stretching region (3800–2800 cm−1) using isomer‐specific IR/IR double‐resonance population labelling experiments. The spectroscopic fingerprints of individual isomers can only be unambiguously assigned after anharmonic effects are considered, but then yield molecular level insight into the onset of salt dissolution in these gas phase model systems. Based on the extent of the observed frequency shifts ΔνOH of the hydrogen‐bonded OH stretching oscillators solvent‐shared ion pair motifs (<3200 cm−1) can be distinguished from intact‐core structures (>3200 cm−1). The characteristic fingerprint of a water molecule trapped directly in‐between two ions of opposite charge provides an alternative route to evaluate the extent of ion pairing in aqueous electrolyte solutions.

Impact of anion polarizability on ion pairing in microhydrated salt clusters

“…Summarizing, the interactions at play in salt-water complexes, in the order of decreasing bond strengths, are: 37,46 hard ion-hard ion > hard ion-water > water-water > soft ion-water > soft ion-soft ion. Please do not adjust margins Please do not adjust margins Figure 12 summarizes the structures that were assigned in the present and our previous study 35 on lithium dihalide anions microhydrated with up to three water molecules. For n = 1, we mainly observe the competition between two driving forces that determine the relative isomer stabilities, in particular the stability of isomers 1.0.0a/b vs. 1.0.0c/d.…”

“…Finally, even though the IRPD spectra of all three n = 1 systems appear similarly complex, IR 2 MS 2 measurements provide no evidence for multiple isomers, in contrast to the two isomers reported for [LiCl2(H2O)]ˉ. 35 The n = 2 spectra, on the other hand, show greater similarity, suggesting similar structures, i.e. the solvent-shared structure containing two bridging water molecules previously identified for [LiCl2(H2O)2]ˉ.…”

“…Hence, similar to the [LiCl2(H2O)1-3]ˉ anions, anharmonic effects, possibly combined with the contribution from multiple isomers, need to be considered. 35 All three spectra (see Figure 1a to 1c) exhibit absorption in region I, indicating the presence of free O-H stretching oscillators. However, band a1 (n = 1, 3652 cm -1 ) is close to the typical cut-off value for a free O-H stretching frequency, indicating the onset of weak H-bonding.…”

“…34 In our recent study on [LiCl 2 (H 2 O) n ] − (n = 1-3), we established that SIP vs. CIP formation can be distinguished based on the extent of the observed frequency shis of the hydrogen-bonded O-H stretching oscillators. 35 In the present work, we extend our study to include iodide anions and focus on the microhydration of archetypical saltwater anions of the type [LiXX ′ (H 2 O) n ] − with XX ′ = I 2 , ClI and Cl 2 and n = 1-3. We investigate these systems in the gas-phase at cryogenic temperatures, which enables us to dissect the interaction between salt ions and a few water molecules as a function of anion polarizability.…”

“…28,37 Methodologically, we utilize vibrational action spectroscopy of D 2 -tagged [LiXX ′ (H 2 O) 1-3 ] − anions complemented by ab initio simulations. The inclusion of anharmonic effects is crucial for a reliable interpretation of the experimental vibrational spectra of these systems, 35 and we perform this here both in a static and in a dynamic fashion.…”