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.