5 research outputs found
Thiocyanate as a Local Probe of Ultrafast Structure and Dynamics in Imidazolium-Based Ionic Liquids: Water-Induced Heterogeneity and Cation-Induced Ion Pairing
Ultrafast
two-dimensional infrared spectroscopy (2D-IR) of thiocyanate
([SCN]<sup>−</sup>) in 1-butyl-3-methylimidazolium bisÂ(trifluoromethylsulfonyl)Âimide
([C<sub>4</sub>C<sub>1</sub>im]Â[NTf<sub>2</sub>]) and 1-butyl-2,3-dimethylimidazolium
bisÂ(trifluoromethylsulfonyl)Âimide ([C<sub>4</sub>C<sub>1</sub>C<sub>1</sub><sup>2</sup>im]Â[NTf<sub>2</sub>]) ionic liquids probes local structure and dynamics as a function
of the water content, solute counterion, and solute concentration.
The 2D-IR spectra of the water-saturated ionic liquids resolve two
distinct kinds of dynamics. This dynamical heterogeneity is explained
as two subensembles, one with and one without a water molecule in
the first solvation shell. When the countercation is K<sup>+</sup>, ion pairs between K<sup>+</sup> and [SCN]<sup>−</sup> that
persist for >100 ps are detected by long-lasting vibrational frequency
correlations. The observed dynamics are invariant to [SCN]<sup>−</sup>concentration, which indicates that the [SCN]<sup>−</sup> does
not cluster in ionic liquid solution. Taken together, these results
are consistent with a picture of thiocyanate as a local probe that
can interrogate ultrafast structure and dynamics at a small spatial
scale in ionic liquids
Enthalpic Driving Force for the Selective Absorption of CO<sub>2</sub> by an Ionic Liquid
Molecular dynamics (MD) simulations
validated against two-dimensional
infrared (2D-IR) measurements of CO<sub>2</sub> in an imidazolium-based
ionic liquid have revealed new insights into the mechanism of CO<sub>2</sub> solvation. The first solvation shell around CO<sub>2</sub> has a distinctly quadrupolar structure, with strong negative charge
density around the CO<sub>2</sub> carbon atom and positive charge
density near the CO<sub>2</sub> oxygen atoms. When CO<sub>2</sub> is
modeled without atomic charges (thus removing its strong quadrupole
moment), its solvation shell weakens and changes significantly into
a structure that is similar to that of N<sub>2</sub> in the same liquid.
The solvation shell of CO<sub>2</sub> evolves more quickly when its
quadrupole is removed, and we find evidence that solvent cage dynamics
is measured by 2D-IR spectroscopy. We also find that the solvent cage
evolution of N<sub>2</sub> is similar to that of CO<sub>2</sub> with
no atomic charges, implying that the weaker quadrupole of N<sub>2</sub> is responsible for its higher diffusion and lower absorption in
ionic liquids
Modeling Carbon Dioxide Vibrational Frequencies in Ionic Liquids: II. Spectroscopic Map
The
primary challenge for connecting molecular dynamics (MD) simulations
to linear and two-dimensional infrared measurements is the calculation
of the vibrational frequency for the chromophore of interest. Computing
the vibrational frequency at each time step of the simulation with
a quantum mechanical method like density functional theory (DFT) is
generally prohibitively expensive. One approach to circumnavigate
this problem is the use of spectroscopic maps. Spectroscopic maps
are empirical relationships that correlate the frequency of interest
to properties of the surrounding solvent that are readily accessible
in the MD simulation. Here, we develop a spectroscopic map for the
asymmetric stretch of CO<sub>2</sub> in the 1-butyl-3-methylimidazolium
hexafluorophosphate ([C<sub>4</sub>C<sub>1</sub>im]Â[PF<sub>6</sub>]) ionic liquid (IL). DFT is used to compute the vibrational frequency
of 500 statistically independent CO<sub>2</sub>-[C<sub>4</sub>C<sub>1</sub>im]Â[PF<sub>6</sub>] clusters extracted from an MD simulation.
When the map was tested on 500 different CO<sub>2</sub>-[C<sub>4</sub>C<sub>1</sub>im]Â[PF<sub>6</sub>] clusters, the correlation coefficient
between the benchmark frequencies and the predicted frequencies was <i>R</i> = 0.94, and the root-mean-square error was 2.7 cm<sup>–1</sup>. The calculated distribution of frequencies also
agrees well with experiment. The spectroscopic map required information
about the CO<sub>2</sub> angle, the electrostatics of the surrounding
solvent, and the Lennard-Jones interaction between the CO<sub>2</sub> and the IL. The contribution of each term in the map was investigated
using symmetry-adapted perturbation theory calculations
Modeling Carbon Dioxide Vibrational Frequencies in Ionic Liquids: I. <i>Ab Initio</i> Calculations
This
work elucidates the molecular binding mechanism of CO<sub>2</sub> in
[C<sub>4</sub>C<sub>1</sub>IM]Â[PF<sub>6</sub>] ionic liquid
(IL) and its interplay with the CO<sub>2</sub> asymmetric stretch
frequency ν<sub>3</sub>, and establishes computational protocols
for the reliable construction of spectroscopic maps for simulating
ultrafast 2D-IR data of CO<sub>2</sub> solvated in ILs. While charge
transfer drives the static frequency shift between <i>different</i> ionic liquids (J. Chem.
Phys. 2015, 142, 212425), we find here that electrostatic and Pauli repulsion
effects dominate the dynamical frequency shift between different geometries
sampled from the finite-temperature dynamics <i>within</i> a single ionic liquid. This finding is also surprising because dispersion
interactions dominate the CO<sub>2</sub>–IL interaction energies,
but are comparably constant across different geometries. An important
practical consequence of this finding is that density functional theory
is expected to be sufficiently accurate for constructing potential
energy surfaces for CO<sub>2</sub> in [C<sub>4</sub>C<sub>1</sub>IM]Â[PF<sub>6</sub>], as needed for accurate anharmonic calculations to construct
a reliable spectroscopic map. Similarly, we established appropriate
computational and chemical models for treating the extended solvent
environment. We found that a QM/MM treatment including at least 2
cation-ion pairs at the QM level and at least 32 pairs at the MM level
is necessary to converge vibrational frequencies to within 1 cm<sup>–1</sup>. Using these insights, this work identifies a computational
protocol as well as a chemical model necessary to construct accurate
spectroscopic maps from first principles
Modeling Carbon Dioxide Vibrational Frequencies in Ionic Liquids: I. <i>Ab Initio</i> Calculations
This
work elucidates the molecular binding mechanism of CO<sub>2</sub> in
[C<sub>4</sub>C<sub>1</sub>IM]Â[PF<sub>6</sub>] ionic liquid
(IL) and its interplay with the CO<sub>2</sub> asymmetric stretch
frequency ν<sub>3</sub>, and establishes computational protocols
for the reliable construction of spectroscopic maps for simulating
ultrafast 2D-IR data of CO<sub>2</sub> solvated in ILs. While charge
transfer drives the static frequency shift between <i>different</i> ionic liquids (J. Chem.
Phys. 2015, 142, 212425), we find here that electrostatic and Pauli repulsion
effects dominate the dynamical frequency shift between different geometries
sampled from the finite-temperature dynamics <i>within</i> a single ionic liquid. This finding is also surprising because dispersion
interactions dominate the CO<sub>2</sub>–IL interaction energies,
but are comparably constant across different geometries. An important
practical consequence of this finding is that density functional theory
is expected to be sufficiently accurate for constructing potential
energy surfaces for CO<sub>2</sub> in [C<sub>4</sub>C<sub>1</sub>IM]Â[PF<sub>6</sub>], as needed for accurate anharmonic calculations to construct
a reliable spectroscopic map. Similarly, we established appropriate
computational and chemical models for treating the extended solvent
environment. We found that a QM/MM treatment including at least 2
cation-ion pairs at the QM level and at least 32 pairs at the MM level
is necessary to converge vibrational frequencies to within 1 cm<sup>–1</sup>. Using these insights, this work identifies a computational
protocol as well as a chemical model necessary to construct accurate
spectroscopic maps from first principles