Do crown ethers fully dissociate ion pairs in aqueous solution?
Abstract
In alkali fluoride solutions, crown ethers are widely assumed to generate "naked" anions by capturing cations, yet direct evidence for complete ion-pair dissociation is limited. We combine 19F Nuclear Magnetic Resonance (NMR) and multiscale modeling on aqueous NaF and KF with 18crown-6 (18C6). Fluoride diffusion slows on crown addition, scaling linearly with the crownbound cation fraction across both salts and extrapolating to zero without complexation. The 19F chemical shifts independently establish this retained ion pairing. For KF, the signal moves away from the free-ion baseline, the wrong direction for liberation. While NaF responds more weakly, the theoretical chemical shift profile shows that the crown-bound cation carries a shielding contact (CIP) region and a deshielding solvent-shared (SIP) region whose cancellation flattens the response. Molecular dynamics confirm reduced but persistent contact pairing. Crown ethers thus suppress contact pairing while retaining both solvent-shared and residual contact associations with the sequestered cation.
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BibTeX
@misc{musial2026crown,
title = {{Do crown ethers fully dissociate ion pairs in aqueous solution?}},
author = {Malgorzata Musial and Christopher L. Suiter and Jason Widegren and Demian Riccardi},
howpublished = {ChemRxiv},
year = {2026},
month = sep,
doi = {10.26434/chemrxiv.15008835/v1},
url = {https://chemrxiv.org/doi/full/10.26434/chemrxiv.15008835/v1},
note = {CC BY},
}
RIS
TY - UNPB
TI - Do crown ethers fully dissociate ion pairs in aqueous solution?
AU - Malgorzata Musial
AU - Christopher L. Suiter
AU - Jason Widegren
AU - Demian Riccardi
PY - 2026
DA - 2026/09/15
PB - ChemRxiv
DO - 10.26434/chemrxiv.15008835/v1
UR - https://chemrxiv.org/doi/full/10.26434/chemrxiv.15008835/v1
AB - In alkali fluoride solutions, crown ethers are widely assumed to generate "naked" anions by capturing cations, yet direct evidence for complete ion-pair dissociation is limited. We combine 19F Nuclear Magnetic Resonance (NMR) and multiscale modeling on aqueous NaF and KF with 18crown-6 (18C6). Fluoride diffusion slows on crown addition, scaling linearly with the crownbound cation fraction across both salts and extrapolating to zero without complexation. The 19F chemical shifts independently establish this retained ion pairing. For KF, the signal moves away from the free-ion baseline, the wrong direction for liberation. While NaF responds more weakly, the theoretical chemical shift profile shows that the crown-bound cation carries a shielding contact (CIP) region and a deshielding solvent-shared (SIP) region whose cancellation flattens the response. Molecular dynamics confirm reduced but persistent contact pairing. Crown ethers thus suppress contact pairing while retaining both solvent-shared and residual contact associations with the sequestered cation.
ER -