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(ClO4)2 and 1.2 M NaSCN. The sensitivity of the CN stretching frequency of thiocyanate (SCN−) to contact ion pair formation with Mg2+ 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 Mg2+ 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 Mg2+, our study represents the first experimental observation of anion exchange in the first solvent shell of the Mg2+ 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.
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