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

Youngah Kwon, Sungnam Park

Research output: Contribution to journalArticle

3 Citations (Scopus)

Abstract

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
Volume17
Issue number37
DOIs
Publication statusPublished - 2015 Aug 13

Fingerprint

Complexation
acetonitrile
Infrared spectroscopy
Spectrum Analysis
infrared spectroscopy
Ions
Nitriles
Vibration
nitriles
Stretching
ions
Spectroscopy
Entropy
Fourier Transform Infrared Spectroscopy
vibration
Molecules
Hot Temperature
spectroscopy
time constant
molecules

ASJC Scopus subject areas

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

Cite this

Complexation dynamics of CH<inf>3</inf>SCN and Li<sup>+</sup> in acetonitrile studied by two-dimensional infrared spectroscopy. / Kwon, Youngah; Park, Sungnam.

In: Physical Chemistry Chemical Physics, Vol. 17, No. 37, 13.08.2015, p. 24193-24200.

Research output: Contribution to journalArticle

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abstract = "Ion-molecule complexation dynamics were studied with CH3SCN and Li+ in acetonitrile by vibrationally probing the nitrile stretching vibration of CH3SCN. The nitrile stretching vibration of CH3SCN has a long lifetime (T1 = ∼90 ps) and its frequency is significantly blue-shifted when CH3SCN is bound with Li+ ions to form a CH3SCN⋯Li+ complex in acetonitrile. Such spectral properties enable us to distinguish free CH3SCN and the CH3SCN⋯Li+ complex in solutions and measure their dynamics occurring on hundred picosecond timescales. For the complexation between CH3SCN and Li+ in acetonitrile, the change in enthalpy (ΔH = -7.17 kJ mol-1) and the change in entropy (ΔS = -34.4 J K-1 mol-1) 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 CH3SCN and the CH3SCN⋯Li+ complex. Especially, the orientational relaxation of the CH3SCN⋯Li+ complex was found to be almost 3 times slower than those of free CH3SCN because Li+ ions strongly interact with the neighboring solvents. Most importantly, the complexation dynamics of CH3SCN and Li+ 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.",
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