Ion Pair Supramolecular Structure Identified by ATR‐FTIR Spectroscopy and Simulations in Explicit Solvent**

Abstract

The present work uses ATR-FTIR spectroscopy assisted by simulations in explicit solvent and frequency calculations to investigate the supramolecular structure of carboxylate alkalimetal ion pairs in aqueous solutions. ATR-FTIR spectra in the 0.25-4.0 M concentration range displayed cation-specific behaviors, which enabled the measurement of the appearance concentration thresholds of contact ion pairs between 1.9 and 2.6 M depending on the cation. Conformational explorations performed using a non-local optimization method associated to a polarizable force-field (AMOEBA), followed by high quantum chemistry level (RI-B97-D3/dhf-TZVPP) optimizations, modedependent scaled harmonic frequency calculations and electron density analyses, were used to identify the main supramolecular structures contributing to the experimental spectra. A thorough analysis enables us to reveal the mechanisms responsible for the spectroscopic sensitivity of the carboxylate group and the respective role played by the cation and the water molecules, highlighting the necessity of combining advanced experimental and theoretical techniques to provide a fair and accurate description of ion pairing.

Lamellarity-Driven Differences in Surface Structural Features of DPPS Lipids: Spectroscopic, Calorimetric and Computational Study

“…The displacement of the ν as COO − band of DPPS during the L c → L α transition in MLV (maximum at 1638 cm −1 at 35 °C appears at 1610 cm −1 at 65 °C) suggests a considerable increase in ionization degree [ 21 , 93 , 95 ] upon melting of MLV. As this finding does not apply for LUV (maximum at 1599 cm −1 at 35 °C appears at 1577 cm −1 at 65 °C), the melting of which starts from L β phase, it might be that different kinds of ion pairs are formed among COO − moiety of MLV/LUV and counterions present in solution [ 96 , 97 , 98 ] or that its hydration in MLV/LUV is qualitatively different which reflects on the bond length (see Section 5 ).…”

Stepwise dissociation of ion pairs by water molecules: cation-dependent separation mechanisms between carboxylate and alkali-earth metal ions

“…Finally, the frequency of the ν a (CO 2 − ) transition of free acetate ions, 1551.5 cm −1 , 10 is almost reached for (Ca 2+ , AcO − )(H 2 O) 8 clusters (1547 cm −1 ), despite the fact that ions are far from being fully dissociated. This result echoes that of Denton et al for (Ca 2+ , d -PrO − )(H 2 O) 12 clusters, illustrating the difficulty to follow the complete dissociation process by IR spectroscopy, as already reported for alkali cations.…”

“…[6][7][8] A few rare studies managed to push further the interpretation of experimental data at the cost of an advanced theoretical work with explicit solvation. 9,10 In addition to these studies in solution, the investigation of ion pairs in gas-phase experiments provide a unique opportunity to decompose, and thus better understand, the processes at play in ion pairing. [11][12][13][14][15][16] In particular, microsolvation experiments, 13,14,[16][17][18][19][20] where a controlled number of water molecules are attached to an ion pair, provide valuable information on the role played by the first surrounding water molecules on the supramolecular structure, and have been applied to investigate the pairing properties of the carboxylate group.…”

“…6–8 A few rare studies managed to push further the interpretation of experimental data at the cost of an advanced theoretical work with explicit solvation. 9,10…”

“…This result echoes that of Denton et al for (Ca 2+ , d -PrO − )(H 2 O) 12 clusters, illustrating the difficulty to follow the complete dissociation process by IR spectroscopy, as already reported for alkali cations. 9,10 Consequently, the investigation of larger clusters by this approach should be carried out at an advanced theoretical level capable to reproduce experimental frequencies with an accuracy close to the wavenumber scale.…”