Ligand Exchange Dynamics in Aqueous Solution Studied with 2DIR Spectroscopy Sungnam Park, †,‡ Minbiao Ji, †,§ and Kelly J. Gaffney* ,† PULSE Institute for Ultrafast Energy Science, SLAC National Accelerator Laboratory, Stanford UniVersity, Stanford, California 94305, Department of Chemistry, Korea UniVersity, Seoul, 136-701, Republic of Korea, and Department of Physics, Stanford UniVersity, Stanford, California 94305 ReceiVed: January 28, 2010; ReVised Manuscript ReceiVed: March 24, 2010 We have used time-resolved multidimensional vibrational spectroscopy, generally termed 2DIR spectroscopy, to study the equilibrium dynamics of ligand exchange in an aqueous solution containing 3.4 M Mg(ClO 4 ) 2 and 1.2 M NaSCN. The sensitivity of the CN stretching frequency of thiocyanate (SCN - ) to contact ion pair formation with Mg 2+ ions generates distinct spectroscopic signatures for the MgNCS + contact ion pair and the free SCN - . We have utilized 2DIR spectroscopy to successfully resolve the interconversion between these thiocyanate configurations and measured the MgNCS + contact ion pair dissociation time constant to be 52 ( 10 ps. We attribute the observed dynamics to perchlorate-thiocyanate anion exchange in the first solvation shell of the Mg 2+ cation. Magnesium ions in this concentrated ionic solution will be coordinated by water molecules, as well as perchlorate and thiocyanate ions. While prior studies have observed microsecond residence times for water ligands in the first coordination sphere of Mg 2+ , our study represents the first experimental observation of anion exchange in the first solvent shell of the Mg 2+ cation. We have also used orientational relaxation and spectral diffusion dynamics to quantify the dynamical distinctions between the free anion and the anion in the contact ion pair. I. Introduction The variable reactivity of a chemical substrate in the presence or absence of a solvent, a heterogeneous catalyst, or an enzymatic protein highlights the significant impact the reaction environment has on chemical dynamics. While the influence of reaction environment has been extensively investigated for photochemical reactions, dynamical studies of equilibrium chemical dynamics remains a novel area of investigation 1-4 with significant opportunity to complement and enhance molecular dynamics simulations of equilibrium chemical dynamics. 5-8 Ligand exchange in solution represents a fundamental solvent- controlled reaction. 7,9 While the separation between ligand and metal ion provides the intuitive reaction coordinate, 5,6,10 fluctua- tions in the solvent structure surrounding the ligand-metal ion pair explicitly influence the solution phase reaction mechanism. 7 Molecular dynamics (MD) simulations provide the most direct route to mechanistic understanding of thermal reaction dynamics in ionic solutions, but the 20 to 30 ps durations of ab initio molecular dynamics simulations 11-16 limit the accessible con- formational dynamics and classical simulations depend upon the empirical force fields utilized to perform the MD simulations. 5-10,17 Given these limitations, benchmark dynamical measurements provide a critical complement to the MD simulation studies. A variety of spectroscopic methods have been utilized to study the equilibrium and dynamical properties of ionic solutions. 18-32 Dielectric and ultrasonic relaxation spectroscopy have proven particularly useful for studying the dynamic properties of ionic solutions, providing information about the association and rotation of contact ion pairs (CIP), solvent separated ion pairs (SIP), and doubly solvent separated ion pairs (2SIP). 21,23,29-31 This sensitivity also makes analysis challenging, since the contribution of each ion pair configuration to the spectrum overlap and the dynamics of ion pair rotation and dissociation can be difficult to distinguish. This has proven to be particularly true for ion pairing in aqueous solutions, 33 where the rate of ion pair dissociation has generally been observed for strongly associating ion pairs. 21,29,34 These measurements have observed ion pair dissociation on the nanosecond to microsecond time scale and have been generally insensitive to the picosecond time scale dynamics accessible to molecular dynamics simulations. 5-8 Time-resolved IR spectroscopy of molecular anions provides an alternative approach to characterizing ionic equilibria and dynamics in solution. 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, 35-39 but the spectra do not distinguish among SIP, 2SIP, and free anions. Of these molecular anions, thiocyanate (SCN - ), isocyanate (OCN - ), and azide (N 3 - ) provide the most tractable IR spectroscopy, particularly for time-resolved measurements. 32,39,40 Due to the longer excited state lifetime of the antisymmetric CN stretch of SCN - , 40,41 we have used this anion to study ion pairing in water. For aqueous solutions of Mg 2+ and SCN - , the MgNCS + contact ion pair (CIP) has a CN stretch frequency of 2110 cm -1 , while all other SCN - ionic configurations lead to absorption at 2068 cm -1 . Prior studies of concentrated aqueous MgSO 4 solutions indicate that the concentration of free ionic species will greatly exceed the SIP and 2SIP concentrations in the solution we have investigated. 33 The dominance of free ions does not reflect the mean distance between ions, since the majority of ions will be within two solvation shells of a counterion, but rather indicates the ions move independently. These distinct vibrational transition energies for the CIP and the free anion configurations provide the opportunity to use * To whom correspondence should be addressed. E-mail: kgaffney@ slac.stanford.edu. † PULSE Institute for Ultrafast Energy Science, SLAC National Ac- celerator Laboratory, Stanford University. ‡ Department of Chemistry, Korea University. § Department of Physics, Stanford University. J. Phys. Chem. B 2010, 114, 6693–6702 6693 10.1021/jp100833t  2010 American Chemical Society Published on Web 04/28/2010