Complexation dynamics of CH₃SCN and Li⁺ in acetonitrile studied by two-dimensional infrared spectroscopy

Ion-molecule complexation dynamics were studied with CH₃SCN and Li⁺ in acetonitrile by vibrationally probing the nitrile stretching vibration of CH₃SCN. The nitrile stretching vibration of CH₃SCN has a long lifetime (T₁ = ∼90 ps) and its frequency is significantly blue-shifted when CH₃SCN is bound with Li⁺ ions to form a CH₃SCN⋯Li⁺ complex in acetonitrile. Such spectral properties enable us to distinguish free CH₃SCN and the CH₃SCN⋯Li⁺ complex in solutions and measure their dynamics occurring on hundred picosecond timescales. For the complexation between CH₃SCN and Li⁺ in acetonitrile, the change in enthalpy (ΔH = -7.17 kJ mol^-1) and the change in entropy (ΔS = -34.4 J K^-1 mol^-1) were determined by temperature-dependent FTIR experiments. Polarization-controlled infrared pump-probe (IR PP) spectroscopy was used to measure the population decay and orientational dynamics of free CH₃SCN and the CH₃SCN⋯Li⁺ complex. Especially, the orientational relaxation of the CH₃SCN⋯Li⁺ complex was found to be almost 3 times slower than those of free CH₃SCN because Li⁺ ions strongly interact with the neighboring solvents. Most importantly, the complexation dynamics of CH₃SCN and Li⁺ in acetonitrile were successfully measured in real time by 2DIR spectroscopy for the first time and the dissociation and association time constants were directly determined by using the two-species exchange kinetic model. Our experimental results provide a comprehensive overview of the ion-molecule complexation dynamics in solutions occurring under thermal equilibrium conditions.