Contact Ion Pair Formation between Hard Acids and Soft Bases in Aqueous Solutions Observed with 2DIR Spectroscopy Zheng Sun, Wenkai Zhang, Minbiao Ji, † Robert Hartsock, and Kelly J. Gaffney* PULSE Institute, SLAC National Accelerator Laboratory, Stanford University, Stanford, California 94305, United States * S Supporting Information 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. ■ INTRODUCTION How water solvates ions and mitigates the interaction between charged species in solution has wide-ranging implications in chemistry, biology, and environmental science. The propensity for ions to form contact ion pairs in solution, in particular, the propensity for ions to bind to charged side chains at protein surfaces, has been hypothesized to explain the influence of ions on the solubility of proteins, an effect categorized by the Hö fmeister series. 1-5 Collins has postulated a Law of Matching Water Affinities to explain the influence of ions on protein solvation. 3,4 The proposal of Collins states that only ions with similar charge densities form contact ion pairs. The Law of Matching Water Affinities conforms to the theory of hard-soft Lewis acid-base chemistry, often termed the Pearson acid- base concept. 6,7 While Collins has demonstrated that many thermodynamic properties of aqueous ionic solutions can be correlated with his Law of Matching Water Affinities, more direct conformation has been lacking. Determining the validity of the Law of Matching Water Affinities requires the robust determination of contact ion pair (CIP) formation in aqueous solution, but clear identification of CIP formation has proven to be challenging. A variety of spectroscopic methods have been utilized to study the equilibrium and dynamical properties of aqueous ionic solutions. 8-19 To date, dielectric relaxation spectroscopy has been the most useful technique for studying the interaction of ions in solution, providing information about the association and rotation of CIPs, solvent-separated ion pairs (SIPs), and doubly solvent-separated ion pairs (2SIP). 8-14 This sensitivity also makes analysis challenging because the contribution of each ion pair configuration exhibits complex, overlapping lineshapes. Time-resolved IR spectroscopy of molecular anions provides an alternative approach to characterizing CIP equilibria in aqueous solution for molecular ions. The IR spectra of a variety of anions shift measurably when they form a CIP with alkali and alkaline earth metal cations in polar solvents. 20-27 Of these molecular anions, thiocyanate (SCN¯), isocyanate (OCN¯), and azide (N 3 ¯) provide the most tractable IR spectroscopy, particularly for time-resolved measurements. 19,22,24,25,28-32 We have used the NCS - anion to study contact ion pairing in water because of the long excited-state lifetime of the CN-stretch vibration. 28,33 The sensitivity of the nitrile stretch frequency to variations in the nitrile chemical and electrostatic environment has been used in a variety of prior experiments. 34,35 Under certain circumstances, the CN-stretch frequency provides a probe of the local electrostatic environment. 34,36 Stronger intermolecular interactions, such as CIP formation or hydro- gen-bond formation to the nitrile group, lead to shifts in the CN-stretch frequency that cannot be accounted for with the Stark effect. 37,38 For the specific case of CIP formation between Mg 2+ or Ca 2+ and NCS - , bonding of the M 2+ cation to the nitrogen atom in the NCS - anion induces an electronic polarization that shifts electron density from the CS bond to the CN bond, leading to an increase in the CN-stretch frequency. 24 This electronic polarization induced by CIP stabilizes the CN triple-bond resonance structure of thio- cyanate, an alternative means of explaining the increase in the CN-stretch frequency. In aqueous solution, the water- thiocyanate interaction also perturbs the CN-stretch frequency and generates an inhomogeneous broadened spectrum. Accurate description of the CN-stretch absorption spectrum requires modeling the distribution of cation and water solvation Special Issue: Michael D. Fayer Festschrift Received: April 5, 2013 Revised: July 24, 2013 Published: July 29, 2013 Article pubs.acs.org/JPCB © 2013 American Chemical Society 15306 dx.doi.org/10.1021/jp4033854 | J. Phys. Chem. B 2013, 117, 15306-15312