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

International audienceIn solution, an ion pair consists of a positive ion (cation, z+) and a negative ion (anion, z-) bonded together by the attractive electrostatic forces between them, forming a supramolecular neutral complex. Several types of ion pairs coexist: contact (CIP), solvent-shared (SIP) and solvent-separated (SSIP), in equilibrium with free ions (FI); their distribution depends on many parameters, and especially the ion concentration. Ion pairing occurs frequently in environments that are naturally rich in ions like seawater, inorganic atmospheric aerosol particles, or living organisms, and thus is of central importance in understanding chemical and biological processes in solution. Ion pairing is suspected to be involved in many ion specific effects, but their characterization as well as their properties still remain mostly to be understood despite the numerous experiments carried out in solution during the past decades. In addition, theoretical chemistry approaches often struggle to establish a quantitative picture of ion pairing. A striking example of the difficulties encountered is illustrated by a recent set of studies on carboxylate anions paired with alkali metal cations which have led to inconsistent conclusions.[1,2] Relying on IR spectroscopic experiments recently developed in our group on isolated neutral ion pairs,[3] we investigated carboxylate anions paired to alkali metal cations by using an original approach where gas phase results were used as an input to refine high level quantum chemistry calculations in solution, leading to an unprecedented level of accuracy in vibrational frequency prediction. First, gas phase experiments focused on a series of isolated contact ion pairs ([M+, Ph-CH2-COO-] with M= Li, Na, K, Rb, Cs) for which UV and conformer-selective IR spectra were recorded. These experiments provided vibrational frequencies of the carboxylate stretch enabling us to assess the accuracy of mode-dependent scaled harmonic frequency calculations at the RI-B97-D3/dhf-TZVPP level. This level of calculation was then employed on large water clusters embedding a free acetate ion or its CIPs or SIPs with a sodium cation to obtain the individual vibrational spectra of these species in solution. These theoretical results show that the stretching modes of carboxylate are sensitive to both SIP and CIP formation. FT-IR spectroscopy confirms this finding since solutions of increasing concentrations exhibit spectral evolutions 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.[4