Real‐Time Probing of Ion Pairing Dynamics with 2DIR Spectroscopy

Abstract

Chemical exchange two-dimensional infrared (2DIR) spectroscopy is applied to investigate ion pairing dynamics occurring on picosecond timescales. SeCN(-) ion is used as a vibrational probe. The SeCN(-) ion dissolved in N,N-dimethyl formamide (DMF) has a sufficiently long vibrational lifetime and can form a contact ion pair with Li(+) ion in DMF. The CN stretch frequency of the contact ion pair is significantly blue-shifted from that of free SeCN(-) so the free SeCN(-) ion can be spectrally distinct from the contact ion pair in DMF. Therefore, we were able to directly monitor the ion pairing dynamics of Li(+) and SeCN(-) in real time by using ultrafast 2DIR spectroscopy. As a result, we have determined the dissociation time constant of the LiSeCN contact ion pair to be 420±40 ps.

Applications of two-dimensional infrared spectroscopy

“…28–33 In the discussion that follows, we highlight a few examples in order to illustrate the information available from 2D IR spectroscopy.…”

“…2D IR spectroscopy has been similarly applied to characterize the ligand exchange and solvation dynamics of a wide variety of ion pairs. 28, 39, 40 …”

Acid–base equilibrium dynamics in methanol and dimethyl sulfoxide probed by two-dimensional infrared spectroscopy

“…2DIR spectroscopy, which is an ultrafast vibrational analog of twodimensional nuclear magnetic resonance (2D NMR), has been extensively used to elucidate fundamentally important molecular processes, such as solute-solvent complexation, 1,2 carbon-carbon rotation, 3 hydrogen-bond switching dynamics in aqueous solutions, 4-8 ligand exchange, 9 ion pair dimerization, 10 ion pairing dynamics in electrolyte solutions, 11,12 intra-and intermolecular vibrational population dynamics [13][14][15][16][17][18][19] and structural dynamics in solutions. Under thermal equilibrium conditions, an acid (HA) dissociates into a proton (H + ) and its conjugate base (A À ) while the conjugate base is protonated to form HA.…”

“…The vibrational lifetimes (T 1,HN 3 = 6.0 ps and T 1,N 3 À = 3.0 ps) and orientational relaxation times (t or,HN 3 = 4.2 ps and t or,N 3 À = 10.5 ps) for HN 3 and N 3 À were experimentally measured by polarizationcontrolled IR PP spectroscopy as shown in Fig. 11,12 The analytical solution for the two-state exchange kinetic model was fitted to the plot of the peak volumes against T w as shown in Fig. Additionally, the cross-peaks grow due to the deprotonation and protonation reactions.…”

“…Additionally, the cross-peaks grow due to the deprotonation and protonation reactions. 11,12 From the two-state exchange kinetic analyses, the deprotonation and protonation rate constants were determined to be k depro = (180 AE 20 ps) À1 and k pro 0 = k depro /K a 0 = (80 AE 10 ps) À1 , respectively, and the exchange rate constant obtained was k ex = k depro + k pro 0 = (55 AE 10 ps) À1 , which corresponds to the rate constant of the cross-peak growth in T w -dependent 2DIR spectra. The analytical solutions represent the time-dependent changes of the concentrations of individual peaks and can be directly used to determine the deprotonation and protonation rate constants.…”