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.
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