Dynamics of IonAssembly in Solution: 2DIR Spectroscopy Study of LiNCS in Benzonitrile

ABSTRACT The solute-solvent interaction dictates the equilibrium structure of ionic assemblies in solution, ranging from free ions, ionpairs, ion-pair dimers, and ion-pair tetramers to nanoparticles and ionic crystals. We use two-dimensional infrared (2DIR) spectroscopic measurements of the antisymmetric CN stretch of thiocyanate (NCS-) to study theequilibriumchemical exchangebetween the spectrallydistinct ion pair and the ion-pair dimer in benzonitrile. We observe this chemical exchange to occur with a 36 ( 4 ps time constant. Our measurement indicates that 2DIR will provide a useful tool for investigating the dynamics of ion assembly in solution. SECTION Kinetics, Spectroscopy T he solvent-mediated interaction of ions has wide-ranging impact in chemistry, biology, and materialsscience.1-6 The solvation of ions influences ion mobi-lity, the folding of proteins, ionic assembly, and crystal nuclea-tion. Understanding the physicochemical phenomena that determine the mobility of lithium cations in aprotic solvents has particular technological significance due to the impor-tance of lithium salt electrolytes in secondary lithium io