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

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.