4 research outputs found
Tricationic Ionic Liquids: Structural and Dynamical Properties via Molecular Dynamics Simulations
Three imidazolium-based
linear tricationic ionic liquids (LTILs)
have been simulated to study their structural and dynamical properties
and obtain a fundamental understanding of the molecular basis of the
microscopic and macroscopic properties of their bulk liquid phase.
The effects of temperature and alkyl chain length on the physiochemical,
transport, and structural properties of these LTILs have been investigated.
A nonpolarizable all-atom force field, which is a refined version
of the Canongia Lopes and Paudua force field, was adopted for the
simulations. Densities, mean square displacements, self-diffusivities,
viscosities, electrical conductivities, and transference numbers have
been presented for various ions from MD simulations. The detailed
microscopic structures have been discussed in terms of radial distribution
functions and spatial distribution functions. The results show that,
similar to that in monocationic and dicationic ILs (MILs and DILs,
respectively), the anions are mainly organized around the imidazolium
rings. The diffusion coefficients of the studied LTILs are smaller
than those of both MILs and DILs, with comparable viscosities. Unlike
those of MILs and DILs, the diffusion coefficients of the cations
and anions of the studied LTILs increase with an increase in the length
of the alkyl chain between the rings for LTIL-1 and LTIL-2 but then
decrease for LTIL-3, which is in a good agreement with the trend of
viscosity data. The calculated transference numbers show that, similar
to that in MILs and DILs, cations have a major role in carrying electric
current in LTILs, but this role increases from MILs to LTILs
Evaluating the Effects of Geometry and Charge Flux in Force Field Modeling
We apply a model
for analyzing the importance of conformational
charge flux to 11 molecules with the R–(CH<sub>2</sub>)<sub><i>n</i></sub>–R structure (R = Cl, F, OH, SH, COOH,
CONH<sub>2</sub>, and NH<sub>2</sub> and <i>n</i> = 4–6).
Atomic charges were obtained by fitting to results from density functional
theory calculations using the HLY procedure, and their geometry dependence
is decomposed into contributions from changes in bond lengths, bond
angles, and torsional angles. The torsional degrees of freedom are
the main contribution to the conformational dependence of atomic charges
and molecular dipole moments, but indirect effects due to changes
in bond distances and angles account for ∼15% of the variations.
While the magnitude of charge flux and geometry effects have been
found to be independent of the number of internal degrees of freedom,
the nature of the R- group has a moderate influence. The indirect
effects are comparable for all of the R-groups and are approximately
one-half the magnitude of the corresponding effects in peptide models.
However, the magnitudes are different, yet the relative importance
of geometry and charge flux effects are completely similar to those
of the peptide models, which suggests that modeling the charge flux
effects for changes in bond lengths, bond angles, and torsional angles
should be considered for developing improved force fields
Evaluating the Effects of Geometry and Charge Flux in Force Field Modeling
We apply a model
for analyzing the importance of conformational
charge flux to 11 molecules with the R–(CH<sub>2</sub>)<sub><i>n</i></sub>–R structure (R = Cl, F, OH, SH, COOH,
CONH<sub>2</sub>, and NH<sub>2</sub> and <i>n</i> = 4–6).
Atomic charges were obtained by fitting to results from density functional
theory calculations using the HLY procedure, and their geometry dependence
is decomposed into contributions from changes in bond lengths, bond
angles, and torsional angles. The torsional degrees of freedom are
the main contribution to the conformational dependence of atomic charges
and molecular dipole moments, but indirect effects due to changes
in bond distances and angles account for ∼15% of the variations.
While the magnitude of charge flux and geometry effects have been
found to be independent of the number of internal degrees of freedom,
the nature of the R- group has a moderate influence. The indirect
effects are comparable for all of the R-groups and are approximately
one-half the magnitude of the corresponding effects in peptide models.
However, the magnitudes are different, yet the relative importance
of geometry and charge flux effects are completely similar to those
of the peptide models, which suggests that modeling the charge flux
effects for changes in bond lengths, bond angles, and torsional angles
should be considered for developing improved force fields
Probing the Importance of Charge Flux in Force Field Modeling
We analyze the conformational
dependence of atomic charges and
molecular dipole moments for a selection of ∼900 conformations
of peptide models of the 20 neutral amino acids. Based on a set of
reference density functional theory calculations, we partition the
changes into effects due to changes in bond distances, bond angles,
and torsional angles and into geometry and charge flux contributions.
This allows an assessment of the limitations of fixed charge force
fields and indications for how to design improved force fields. The
torsional degrees of freedom are the main contribution to conformational
changes of atomic charges and molecular dipole moments, but indirect
effects due to change in bond distances and angles account for ∼25%
of the variation. Charge flux effects dominate for changes in bond
distances and are also the main component of the variation in bond
angles, while they are ∼25% compared to the geometry variations
for torsional degrees of freedom. The geometry and charge flux contributions
to some extent produce compensating effects