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A 2019 study by Christopher Hunter and colleagues derived comparative hydrogen-bond donor parameters for several cations from measured binding equilibria. In that comparison, lithium and guanidinium formed some of the most stable complexes, but the results are parameters for tested solution systems—not a universal ranking that holds regardless of solvent, acceptor, or other conditions.
How the researchers compared cations
The team measured equilibrium constants for cations binding to a set of hydrogen-bond acceptors, then used those measurements to derive a hydrogen-bond donor parameter for each cation. The parameter offers a way to compare how strongly each cation interacts with acceptors in solution.
The cations included guanidinium; primary, tertiary, and quaternary ammonium; imidazolium; methylpyridinium; and the alkali-metal ions lithium, sodium, potassium, rubidium, and caesium. Researchers repeated measurements with different acceptors and solvents to check whether the comparison remained consistent. The study is identified as S. J. Pike et al., Chemical Science (2019), DOI 10.1039/c9sc00721k.
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The Chemistry World account of the study identifies lithium and guanidinium among the cations forming the most stable complexes in the comparison. It also highlights a less intuitive result: charged cations’ hydrogen-bonding abilities in solution fell within the range of neutral hydrogen-bond donors, and some neutral donors could outcompete fully charged species.
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This is a comparison of interaction strengths under the study’s experimental framework, not a claim that every cation follows one fixed order with every acceptor in every solvent. To interpret a cation–acceptor interaction, the relevant context includes the cation’s identity, the acceptor, the solvent, and whether water or a particular counterion is present.
Did water or counterions change the interactions?
The report says the researchers examined adding water and changing anionic counterions. Both effects were negligible in the systems tested. That finding is limited to those tested systems and conditions; it does not establish that water or counterions are irrelevant in other molecular environments.
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Why the parameters may be useful
The study presents the parameters as a tool for estimating free energies of cation–acceptor interactions across different solvents and for validating solvation models. Such estimates could help supramolecular chemists assess whether introducing an interaction is likely to meaningfully change binding affinity. The report also points to possible relevance in aqueous chemistry and catalysis, where ionic interactions and partially charged transition states can matter. These are potential uses, not a guarantee that the parameters predict every complex or catalytic system without further testing.
What the available report does not quantify
The Chemistry World report, published June 13, 2019, refers to a chart but does not provide readable numerical parameter values in its text. Exact values and detailed experimental conditions therefore cannot be stated from that account. The primary paper and supporting information are the appropriate sources for those details; the journal publisher’s landing page could not be accessed for this account, so the quoted numeric values are not independently verified here.
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The report quotes physical-chemistry expert Scott Cockcroft as saying the finding was surprising because some neutral hydrogen-bond donors can outcompete fully charged species. Hunter described the practical aim as quantifying whether adding an interaction changes affinity substantially or by an undetectably small amount. These remarks capture the study’s significance, while the numerical scale and scope of any prediction depend on the underlying experimental data.
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