Complexation dynamics of CH<inf>3</inf>SCN and Li<sup>+</sup> in acetonitrile studied by two-dimensional infrared spectroscopy

Youngah Kwon, Sungnam Park

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Ion-molecule complexation dynamics were studied with CH<inf>3</inf>SCN and Li<sup>+</sup> in acetonitrile by vibrationally probing the nitrile stretching vibration of CH<inf>3</inf>SCN. The nitrile stretching vibration of CH<inf>3</inf>SCN has a long lifetime (T<inf>1</inf> = ∼90 ps) and its frequency is significantly blue-shifted when CH<inf>3</inf>SCN is bound with Li<sup>+</sup> ions to form a CH<inf>3</inf>SCN⋯Li<sup>+</sup> complex in acetonitrile. Such spectral properties enable us to distinguish free CH<inf>3</inf>SCN and the CH<inf>3</inf>SCN⋯Li<sup>+</sup> complex in solutions and measure their dynamics occurring on hundred picosecond timescales. For the complexation between CH<inf>3</inf>SCN and Li<sup>+</sup> in acetonitrile, the change in enthalpy (ΔH = -7.17 kJ mol<sup>-1</sup>) and the change in entropy (ΔS = -34.4 J K<sup>-1</sup> mol<sup>-1</sup>) 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 CH<inf>3</inf>SCN and the CH<inf>3</inf>SCN⋯Li<sup>+</sup> complex. Especially, the orientational relaxation of the CH<inf>3</inf>SCN⋯Li<sup>+</sup> complex was found to be almost 3 times slower than those of free CH<inf>3</inf>SCN because Li<sup>+</sup> ions strongly interact with the neighboring solvents. Most importantly, the complexation dynamics of CH<inf>3</inf>SCN and Li<sup>+</sup> 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.

Original languageEnglish
Pages (from-to)24193-24200
Number of pages8
JournalPhysical Chemistry Chemical Physics
Issue number37
Publication statusPublished - 2015 Aug 13


ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Physics and Astronomy(all)

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