Thiocyanate as a Local Probe of Ultrafast Structure and Dynamics in Imidazolium-Based Ionic Liquids: Water-Induced Heterogeneity and Cation-Induced Ion Pairing

Ren, Zhe; Brinzer, Thomas; Dutta, Samrat; Garrett-Roe, Sean

doi:10.1021/jp512851v

“…50,56,61,88,90,95,102,126,[134][135][136][137] They are also known to be dynamically heterogeneous, 138-142 chemical 143,144 and spectroscopic 139,[145][146][147][148][149] signatures of such heterogeneity are ubiquitous in the literature. More recently a different manifestation of heterogeneity has been observed when probing solute diffusion.…”

Section: Introductionmentioning

confidence: 99%

Modern Room Temperature Ionic Liquids, a Simple Guide to Understanding Their Structure and How It May Relate to Dynamics

Araque

¹

,

Hettige

²

,

Margulis

³

2015

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Modern room temperature ionic liquids are structurally defined by symmetries on different length scales. Polar-apolar alternation defines their nanoscale structural heterogeneity, whereas positive-negative charge alternation defines short length scale order. Much progress has been made in the past few years as it pertains to the theoretical interpretation of X-ray scattering experiments for these liquids. Our group has contributed to the development of theoretical interpretation guidelines for the analysis of their structure function. Perhaps less well developed is our understanding of how transport and dynamics in general couple to the very unique structure of ionic liquids which are often dynamically and structurally heterogeneous. This article attempts to present our most current understanding of ionic liquid structure in general and its coupling to transport and dynamics in minimally technical terms for the benefit of the broadest audience.

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“…One potential explanation is that the vibrational relaxation rate is frequency dependent, which has been observed in imidazolium ionic liquids. 37 For SCN in EAN, if the vibrational relaxation rate is higher at low frequency, the line will shift and narrow as population is removed from the low frequency part of the band. At each ω 1 , T 1 was determined by fitting a single exponential to the integrated area of the GSB/SE gaussian fit to the isotropic spectrum (to eliminate the contribution of orienational relaxation) as a function of t 2 .…”

Section: Multiple Subensembles In Eanmentioning

confidence: 99%

“…24 Early MD simulations noted few linear hydrogen bonds with water 25 and many arrangements of dipolar character, while more recent MD simulations of SeCN -26 found an average of 3-4 hydrogen bonds, with one of them being an axial (linear) hydrogen bond. Ultrafast infrared spectroscopy has used the ν 3 mode of pseudo-halide anions to investigate ultrafast vibrational dynamics of water and other polar solvents, 23,[27][28][29][30][31][32] concentrated ion solutions, [33][34][35] ILs, [36][37][38][39][40] supported IL membranes, 41 and colloid emulsions. 42 Furthermore, a combination of MD simulations and ultrafast infrared spectroscopy developed a spectroscopic map of SeCNin D 2 O that describes the frequency dependence of the nitrile stretch to a hydrogen bonding environment.…”

Section: Introductionmentioning

confidence: 99%

An Ultrafast Vibrational Study of Dynamical Heterogeneity in the Protic Ionic Liquid Ethyl-Ammonium Nitrate

Johnson

¹

,

Parker²,

Donaldson³

et al. 2019

Preprint

Self Cite

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Using ultrafast two-dimensional infrared spectroscopy (2D-IR), a vibrational probe (thiocyanate, SCN-) was used to investigate the hydrogen bonding network of a molten salt (ethyl-ammonium nitrate, EAN) compared to that of H2O. The 2D-IR experiments were performed in both parallel () and perpendicular () conditions along with temperature dependence for both EAN (14-130 ˚C) and H2O (5-89 ˚C). With polarization control, the frequency fluctuation correlation function can be separated into structural and rotational components. An Arrhenius analysis lead to independent activation energies for the isotropic, and anisotropic signals, structural spectral diffusion, and rotational induced spectral diffusion. The rotational Eas were similar for both EAN and H2O suggesting that SCN- is experiencing a similar jump model, i.e. where hydrogen bond reorientation is dominated by large angular jumps stemming from molecular rotation dynamics. The frequency pre-factors, however, were different where the more rigid and vicious EAN had a slower rate. Furthermore, the 2D-IR anisotropy and vibrational relaxation rates of EAN are frequency dependent, revealing that SCN- experiences two subensembles. We suggest that the sub-ensemble with a faster rotational timescale correlates to SCN- with more, weaker hydrogen bonds, and the sub-ensemble with a slower rotational timescale correlates to SCN- with a stronger, more directional hydrogen bond.

show abstract

“…One potential explanation is that the vibrational relaxation rate is frequency dependent, which has been observed in imidazolium ionic liquids. 37 For SCN in EAN, if the vibrational relaxation rate is higher at low frequency, the line will shift and narrow as population is removed from the low frequency part of the band. At each ω 1 , T 1 was determined by fitting a single exponential to the integrated area of the GSB/SE gaussian fit to the isotropic spectrum (to eliminate the contribution of orienational relaxation)…”

Section: Multiple Subensembles In Eanmentioning

confidence: 99%

“…24 Early MD simulations noted few linear hydrogen bonds with water 25 and many arrangements of dipolar character, while more recent MD simulations of SeCN -26 found an average of 3-4 hydrogen bonds, with one of them being an axial (linear) hydrogen bond. Ultrafast infrared spectroscopy has used the ν 3 mode of pseudo-halide anions to investigate ultrafast vibrational dynamics of water and other polar solvents, 23,[27][28][29][30][31][32] concentrated ion solutions, [33][34][35] ILs, [36][37][38][39][40] supported IL membranes, 41 and colloid emulsions. 42 Furthermore, a combination of MD simulations and ultrafast infrared spectroscopy developed a spectroscopic map of SeCNin D 2 O that describes the frequency dependence of the nitrile stretch to a hydrogen bonding environment.…”

Section: Introductionmentioning

confidence: 99%

An Ultrafast Vibrational Study of Dynamical Heterogeneity in the Protic Ionic Liquid Ethyl-Ammonium Nitrate

Johnson¹,

Parker²,

Donaldson³

et al. 2019

Preprint

Self Cite

0

View full text Add to dashboard Cite

Using ultrafast two-dimensional infrared spectroscopy (2D-IR), a vibrational probe (thiocyanate, SCN-) was used to investigate the hydrogen bonding network of a molten salt (ethyl-ammonium nitrate, EAN) compared to that of H2O. The 2D-IR experiments were performed in both parallel () and perpendicular () conditions along with temperature dependence for both EAN (14-130 ˚C) and H2O (5-89 ˚C). With polarization control, the frequency fluctuation correlation function can be separated into structural and rotational components. An Arrhenius analysis lead to independent activation energies for the isotropic, and anisotropic signals, structural spectral diffusion, and rotational induced spectral diffusion. The rotational Eas were similar for both EAN and H2O suggesting that SCN- is experiencing a similar jump model, i.e. where hydrogen bond reorientation is dominated by large angular jumps stemming from molecular rotation dynamics. The frequency pre-factors, however, were different where the more rigid and vicious EAN had a slower rate. Furthermore, the 2D-IR anisotropy and vibrational relaxation rates of EAN are frequency dependent, revealing that SCN- experiences two subensembles. We suggest that the sub-ensemble with a faster rotational timescale correlates to SCN- with more, weaker hydrogen bonds, and the sub-ensemble with a slower rotational timescale correlates to SCN- with a stronger, more directional hydrogen bond.

show abstract

Thiocyanate as a Local Probe of Ultrafast Structure and Dynamics in Imidazolium-Based Ionic Liquids: Water-Induced Heterogeneity and Cation-Induced Ion Pairing

Cited by 66 publications

References 120 publications

Modern Room Temperature Ionic Liquids, a Simple Guide to Understanding Their Structure and How It May Relate to Dynamics

Modern Room Temperature Ionic Liquids, a Simple Guide to Understanding Their Structure and How It May Relate to Dynamics

An Ultrafast Vibrational Study of Dynamical Heterogeneity in the Protic Ionic Liquid Ethyl-Ammonium Nitrate

An Ultrafast Vibrational Study of Dynamical Heterogeneity in the Protic Ionic Liquid Ethyl-Ammonium Nitrate

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