Abstract
In a more synthetical approach to the study of ion-specific phenomena, four dipodal bis(guanidinium) siloxanes have been synthesized starting from glycine, β-alanine, γ-aminobutanoic acid, L-proline and 1,3-bis(3-aminopropyl)tetramethyldisiloxane. Together with their non-amide progenitor they were comparatively studied in regards to their interactions with nine different anions: sulphate, chromate, molybdate, benzoate, chloride, azide, nitrite, nitrate and thiocyanate. Their aqueous solubilities, form, 1H NMR and FT-IR spectra were examined while searching for anion-specific interactions falling in- or outside of the Hofmeister series. We show that although the “chao-” and “kosmotropic” ions affect the properties of solutions in a predictable way, more selective cation-anion pairing is responsible for phase separation and crystallinity. As a prominent example, crystal structure of one of the benzoate salts was successfully obtained and reveals a synergy of hydrophobic packing, ionic and hydrogen bonding. Immobilized but still flexible siloxane bridges give rise to crystals described by P 42/n space group and neatly segregated into hydro- and lipophilic sections.
Highlights
In a more synthetical approach to the study of ion-specific phenomena, four dipodal bis(guanidinium) siloxanes have been synthesized starting from glycine, β-alanine, γ-aminobutanoic acid, L-proline and 1,3-bis(3-aminopropyl)tetramethyldisiloxane
When we recorded the spectrum for GUA, despite the disappearance of coupling, all three signals remained separate. We believe this proves that another truly ion-specific interaction is at play here[26,27], we did not find a reliable explanation for why the presence of an amide bond would assist in equalization of guanidinium protons in the presence of azide
Much larger differences in band positions observed for salts of that cation further strengthen the notion that the amide group has a “buffering” effect on the molecule - or rather the whole organic phase.While in solution nitrate behaved very much according to its place in the Hofmeister series, in thin films it becomes evident that this particular anion has some degree of affinity for guanidinium
Summary
In a more synthetical approach to the study of ion-specific phenomena, four dipodal bis(guanidinium) siloxanes have been synthesized starting from glycine, β-alanine, γ-aminobutanoic acid, L-proline and 1,3-bis(3-aminopropyl)tetramethyldisiloxane. While it was proved that the chemical shifts of solvent’s atoms can change in the presence of ions in a manner not directly translatable to Hofmeister series rationale[24], the solvent-mediated impact on other solutes remains in agreement with the notion that poorly solvated anions render the polar solvent less aggressive[25] (by allowing it to retain its typical, hydrogen-bonded structure or, in the case of DMSO, one based on other strong dipole-dipole interactions).
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