Identification of ion pairs in solution by IR spectroscopy: crucial contributions of gas phase data and simulations

• Published in: Physical chemistry chemical physics : PCCP 2019, Vol.21 (24), p.12798-1285
• Main Authors: Habka, Sana, Very, Thibaut, Donon, Jeremy, Vaquero-Vara, Vanesa, Tardivel, Benjamin, Charnay-Pouget, Florence, Mons, Michel, Aitken, David J, Brenner, Valérie, Gloaguen, Eric
• Format: Article
• Language: English
• Subjects: Amoeba; Cesium; Chemical Physics; Chemical Sciences; Harmonic functions; Infrared spectroscopy; Ion pairs; Ions; Mathematical analysis; Mechanics; Mechanics of materials; or physical chemistry; Organic chemistry; Phenylacetic acid; Physics; Quantum chemistry; Sodium; Solvents; Theoretical and; Vapor phases; Vibrational spectra
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c9cp00700h

In a context where structure elucidation of ion pairs in solution remains a contemporary challenge, this work explores an original approach where accurate gas phase spectroscopic data are used to refine high level quantum chemistry calculations of ion pairs in solution, resulting in an unprecedented level of accuracy in vibrational frequency prediction. First, gas phase studies focus on a series of isolated contact ion pairs (M + , Ph-CH 2 -COO − , with M = Li, Na, K, Rb, Cs) for which conformer-selective IR spectra in the CO 2 − stretch region are recorded. These experiments reveal the interactions at play in isolated contact ion pairs, and provide vibrational frequencies enabling us to assess the accuracy of the theoretical approach used, i.e. , mode-dependent scaled harmonic frequency calculations at the RI-B97-D3/dhf-TZVPP level. This level of calculation is then employed on large water clusters embedding either a free acetate ion or its contact or solvent-shared pairs with a sodium cation in order to simulate the individual vibrational spectra of these species in solution. This study shows that the stretching modes of carboxylate are sensitive to both solvent-shared and contact ion pair formation. FTIR spectra of solutions of increasing concentrations indeed reveal several spectral changes consistent with the presence of specific types of solvent-shared and contact ion pairs. By providing relevant guidelines for the interpretation of solution phase IR spectra, this work illustrates the potential of the approach for the elucidation of supramolecular structures in electrolyte solutions. Ion pairs between sodium and acetate are evidenced by IR spectroscopy in solution with the help of gas phase data and simulations.