Effect of ion-ligand binding on ion pairing dynamics studied by two-dimensional infrared spectroscopy

Cation-specific ion pairing dynamics between M⁺ (M = Ag or Cu) and SCN^- in N,N-dimethylthioformamide (DMTF) are studied by probing the nitrile (CN) stretching vibration. The SCN^- ion, which is an ambidentate ligand, readily associates with cations to form two different types of contact ion pairs (CIPs) (i.e., M-SCN or M-NCS) and its CN stretching frequency is significantly blue-shifted so that free SCN^- and CIPs can be well-distinguished in the FTIR spectra. Interestingly, Ag⁺ ions prefer the formation of Ag-SCN in DMTF (Ag⁺ + SCN^- ⇋ Ag-SCN) but Cu⁺ ions form Cu-NCS (Cu⁺ + SCN^- ⇋ Cu-NCS). We have studied the effect of ion-ligand binding on the ion pairing equilibria and dynamics in great detail by using FTIR, IR pump-probe (IR PP), and two-dimensional infrared (2DIR) spectroscopy combined with quantum chemical calculations. First, our quantum chemical calculations corroborate that Ag-SCN and Cu-NCS of the two possible CIP configurations (M-SCN or M-NCS) are energetically stable and favored in DMTF. Second, the thermodynamic properties (ΔH and ΔS) of ion pairing equilibria are determined by temperature-dependent FTIR experiments. Finally, IR PP and 2DIR experiments are used to measure the association and dissociation rate constants. The ion pairing dynamics between Cu⁺ and SCN^- are found to occur on much faster timescales than those between Ag⁺ and SCN^-. Our current results provide important insights into understanding the effect of ion-ligand binding on the ion pairing equilibria and dynamics in polar solvents.