Solute Rotation in Ionic Liquids: Size, Shape, and Electrostatic Effects

Christopher A. Rumble, Caleb Uitvlugt, Brian Conway, Mark Maroncelli

Research output: Contribution to journalArticle

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Abstract

Herein are reported temperature-dependent measurements and molecular dynamics simulations designed to investigate the effects of molecular size, shape, and electrostatics on rotational dynamics in ionic liquids. Experiments were performed in the representative ionic liquid 1-butyl-3-methylimadazolium tetrafluoroborate ([Im41][BF4]) and simulations in the generic ionic liquid model ILM2 as well as a more detailed representation of [Im41][BF4]. 2H longitudinal spin relaxation times (T1) were measured for deuterated versions of 1,4-dimethylbenzene, 1-cyano-4-methylbenzene, and 1,4-dimethylpyridinium between 296 and 337 K. Fluorescence anisotropy measurements were made on the larger solutes 9,10-dimethylanthracene, 9-cyano-10-methylanthracence, and 9,10-dimethylacridnium between 240 and 292 K. Both experiment and simulation showed the nonpolar solutes rotate ∼2-fold faster than their dipolar and charged counterparts. The rotational correlation functions measured in fluorescence experiments are significantly nonexponential and can be fit to stretched exponential functions having stretching exponents 0.4 ≤ β ≤ 0.8, with β decreasing with decreasing temperature. Rotational correlation times in both the NMR and fluorescence experiments conform approximately to the hydrodynamic expectation τrot ∞ (η/T)p with p ≃ 1, and observed times are reasonably close to slip hydrodynamic predictions. Simulations, even with the idealized ILM2 solvent model, are in semiquantitative agreement with experiment when compared on the basis of equal values of ηT-1. When rotational diffusion coefficients (Di) rather than correlation times were considered, much larger departures from hydrodynamic predictions are found in many cases (p ∼ 0.5 and Di ≫ slip predictions). Rotational van Hove functions and trajectory analyses reveal the importance of large-angle jumps about some axes, even in the larger solutes.

Original languageEnglish (US)
Pages (from-to)5094-5109
Number of pages16
JournalJournal of Physical Chemistry B
Volume121
Issue number19
DOIs
StatePublished - May 18 2017

Fingerprint

Ionic Liquids
Ionic liquids
Electrostatics
solutes
electrostatics
hydrodynamics
liquids
fluorescence
Hydrodynamics
Fluorescence
slip
simulation
predictions
Experiments
exponential functions
Xylenes
trucks
Exponential functions
diffusion coefficient
relaxation time

All Science Journal Classification (ASJC) codes

  • Physical and Theoretical Chemistry
  • Surfaces, Coatings and Films
  • Materials Chemistry

Cite this

Rumble, Christopher A. ; Uitvlugt, Caleb ; Conway, Brian ; Maroncelli, Mark. / Solute Rotation in Ionic Liquids : Size, Shape, and Electrostatic Effects. In: Journal of Physical Chemistry B. 2017 ; Vol. 121, No. 19. pp. 5094-5109.
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abstract = "Herein are reported temperature-dependent measurements and molecular dynamics simulations designed to investigate the effects of molecular size, shape, and electrostatics on rotational dynamics in ionic liquids. Experiments were performed in the representative ionic liquid 1-butyl-3-methylimadazolium tetrafluoroborate ([Im41][BF4]) and simulations in the generic ionic liquid model ILM2 as well as a more detailed representation of [Im41][BF4]. 2H longitudinal spin relaxation times (T1) were measured for deuterated versions of 1,4-dimethylbenzene, 1-cyano-4-methylbenzene, and 1,4-dimethylpyridinium between 296 and 337 K. Fluorescence anisotropy measurements were made on the larger solutes 9,10-dimethylanthracene, 9-cyano-10-methylanthracence, and 9,10-dimethylacridnium between 240 and 292 K. Both experiment and simulation showed the nonpolar solutes rotate ∼2-fold faster than their dipolar and charged counterparts. The rotational correlation functions measured in fluorescence experiments are significantly nonexponential and can be fit to stretched exponential functions having stretching exponents 0.4 ≤ β ≤ 0.8, with β decreasing with decreasing temperature. Rotational correlation times in both the NMR and fluorescence experiments conform approximately to the hydrodynamic expectation τrot ∞ (η/T)p with p ≃ 1, and observed times are reasonably close to slip hydrodynamic predictions. Simulations, even with the idealized ILM2 solvent model, are in semiquantitative agreement with experiment when compared on the basis of equal values of ηT-1. When rotational diffusion coefficients (Di) rather than correlation times were considered, much larger departures from hydrodynamic predictions are found in many cases (p ∼ 0.5 and Di ≫ slip predictions). Rotational van Hove functions and trajectory analyses reveal the importance of large-angle jumps about some axes, even in the larger solutes.",
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Solute Rotation in Ionic Liquids : Size, Shape, and Electrostatic Effects. / Rumble, Christopher A.; Uitvlugt, Caleb; Conway, Brian; Maroncelli, Mark.

In: Journal of Physical Chemistry B, Vol. 121, No. 19, 18.05.2017, p. 5094-5109.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Solute Rotation in Ionic Liquids

T2 - Size, Shape, and Electrostatic Effects

AU - Rumble, Christopher A.

AU - Uitvlugt, Caleb

AU - Conway, Brian

AU - Maroncelli, Mark

PY - 2017/5/18

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N2 - Herein are reported temperature-dependent measurements and molecular dynamics simulations designed to investigate the effects of molecular size, shape, and electrostatics on rotational dynamics in ionic liquids. Experiments were performed in the representative ionic liquid 1-butyl-3-methylimadazolium tetrafluoroborate ([Im41][BF4]) and simulations in the generic ionic liquid model ILM2 as well as a more detailed representation of [Im41][BF4]. 2H longitudinal spin relaxation times (T1) were measured for deuterated versions of 1,4-dimethylbenzene, 1-cyano-4-methylbenzene, and 1,4-dimethylpyridinium between 296 and 337 K. Fluorescence anisotropy measurements were made on the larger solutes 9,10-dimethylanthracene, 9-cyano-10-methylanthracence, and 9,10-dimethylacridnium between 240 and 292 K. Both experiment and simulation showed the nonpolar solutes rotate ∼2-fold faster than their dipolar and charged counterparts. The rotational correlation functions measured in fluorescence experiments are significantly nonexponential and can be fit to stretched exponential functions having stretching exponents 0.4 ≤ β ≤ 0.8, with β decreasing with decreasing temperature. Rotational correlation times in both the NMR and fluorescence experiments conform approximately to the hydrodynamic expectation τrot ∞ (η/T)p with p ≃ 1, and observed times are reasonably close to slip hydrodynamic predictions. Simulations, even with the idealized ILM2 solvent model, are in semiquantitative agreement with experiment when compared on the basis of equal values of ηT-1. When rotational diffusion coefficients (Di) rather than correlation times were considered, much larger departures from hydrodynamic predictions are found in many cases (p ∼ 0.5 and Di ≫ slip predictions). Rotational van Hove functions and trajectory analyses reveal the importance of large-angle jumps about some axes, even in the larger solutes.

AB - Herein are reported temperature-dependent measurements and molecular dynamics simulations designed to investigate the effects of molecular size, shape, and electrostatics on rotational dynamics in ionic liquids. Experiments were performed in the representative ionic liquid 1-butyl-3-methylimadazolium tetrafluoroborate ([Im41][BF4]) and simulations in the generic ionic liquid model ILM2 as well as a more detailed representation of [Im41][BF4]. 2H longitudinal spin relaxation times (T1) were measured for deuterated versions of 1,4-dimethylbenzene, 1-cyano-4-methylbenzene, and 1,4-dimethylpyridinium between 296 and 337 K. Fluorescence anisotropy measurements were made on the larger solutes 9,10-dimethylanthracene, 9-cyano-10-methylanthracence, and 9,10-dimethylacridnium between 240 and 292 K. Both experiment and simulation showed the nonpolar solutes rotate ∼2-fold faster than their dipolar and charged counterparts. The rotational correlation functions measured in fluorescence experiments are significantly nonexponential and can be fit to stretched exponential functions having stretching exponents 0.4 ≤ β ≤ 0.8, with β decreasing with decreasing temperature. Rotational correlation times in both the NMR and fluorescence experiments conform approximately to the hydrodynamic expectation τrot ∞ (η/T)p with p ≃ 1, and observed times are reasonably close to slip hydrodynamic predictions. Simulations, even with the idealized ILM2 solvent model, are in semiquantitative agreement with experiment when compared on the basis of equal values of ηT-1. When rotational diffusion coefficients (Di) rather than correlation times were considered, much larger departures from hydrodynamic predictions are found in many cases (p ∼ 0.5 and Di ≫ slip predictions). Rotational van Hove functions and trajectory analyses reveal the importance of large-angle jumps about some axes, even in the larger solutes.

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