Contact Ion Pair Formation between Hard Acids and Soft Bases in Aqueous Solutions Observed with 2DIR Spectroscopy

Abstract

The interaction of charged species in aqueous solution has important implications for chemical, biological, and environmental processes. We have used 2DIR spectroscopy to study the equilibrium dynamics of thiocyanate chemical exchange between free ion (NCS(-)) and contact ion pair configurations (MNCS(+)), where M(2+) = Mg(2+) or Ca(2+). Detailed studies of the influence of anion concentration and anion speciation show that the chemical exchange observed with the 2DIR measurements results from NCS(-) exchanging with other anion species in the first solvation shell surrounding Mg(2+) or Ca(2+). The presence of chemical exchange in the 2DIR spectra provides an indirect, but robust, determinant of contact ion pair formation. We observe preferential contact ion pair formation between soft Lewis base anions and hard Lewis acid cations. This observation cannot be easily reconciled with Pearson's acid-base concept or Collins' Law of Matching Water Affinities. The anions that form contact ion pairs also correspond to the ions with an affinity for water and protein surfaces, so similar physical and chemical properties may control these distinct phenomena.

Aqueous Mg2+and Ca2+Ligand Exchange Mechanisms Identified with 2DIR Spectroscopy

“…Being a time domain measurement primarily sensitive to nearest neighbor interactions, 2DIR spectroscopy provides a new perspective to the study of ion-ion interaction in aqueous solution15,38 that complements prior studies and has the potential to provide incisive tests of molecular dynamics studies [16][17][18]25. Our 2DIR measurements show clear evidence for exchange between the free thiocyanate and the CIP configurations, but these dynamics can originate from five different processes: transient heating 15 energy exchange,23 and three distinct chemical exchange reactions.…”

Water-Mediated Ion Pairing: Occurrence and Relevance

“…96 The occurrence of ion pairing and clustering in aqueous solutions can also be inferred from concentration-dependent viscosity and molecular rotation measurements. 14,97,99,102,103 In molecular rotation measurements, reorientation of the water molecules or the ions is probed via dynamics of the anisotropy of the nonlinear signal resulting from excitation of the hydroxyl stretch vibration of the water molecules or a vibrational mode of the ions, for example, the CN stretch of SCN − . Decay of the anisotropy directly reflects reorientation of water molecules or ions in the solution.…”

Phosphate–Magnesium Ion Interactions in Water Probed by Ultrafast Two-Dimensional Infrared Spectroscopy

“…Two-dimensional infrared (2D-IR) spectroscopy in combination with theoretical work has provided detailed insight into the dynamics and strength of phosphate-water interactions. 2D-IR studies of other solvated ions have mapped solvation and chemical exchange dynamics. − …”