Thematic Unit for Excellence Computational Materials Science

Salt Lake, India

Thematic Unit for Excellence Computational Materials Science

Salt Lake, India
SEARCH FILTERS
Time filter
Source Type

Das S.,se National Center For Basic Science | Biswas R.,se National Center For Basic Science | Biswas R.,Thematic Unit for Excellence Computational Materials Science | Mukherjee B.,Thematic Unit for Excellence Computational Materials Science
Journal of Physical Chemistry B | Year: 2015

Here we report results from our molecular dynamics simulations on orientational relaxation and hydrogen bond dynamics of molten acetamide. Signatures for orientational jumps have been detected with jump barrier estimated to be ∼0.7 kBT. Simulated orientational relaxations indicate deviations from hydrodynamics and this deviation has been ascribed to the detected orientational jumps. Simulated free energy surfaces obtained at various distances between the rotating acetamide and its initial and final H-bond acceptors have been found to be symmetric double-well in nature at the transition state. H-bond relaxation times obtained from our simulations corroborate well with the time scales associated with the jump and waiting time distributions, suggesting an interrelationship between jump dynamics and H-bond fluctuations. Jump angle distributions are asymmetric and depict long tails extending to large angles. (Graph Presented). © 2014 American Chemical Society.


Das S.,se National Center For Basic Science | Biswas R.,se National Center For Basic Science | Biswas R.,Thematic Unit for Excellence Computational Materials Science | Mukherjee B.,Thematic Unit for Excellence Computational Materials Science
Journal of Physical Chemistry B | Year: 2015

All-atom molecular dynamics simulations have been carried out to investigate orientation jumps of acetamide molecules in three different ionic deep eutectics made of acetamide (CH3CONH2) and lithium salts of bromide (Br-), nitrate (NO3-) and perchlorate (ClO4-) at approximately 80:20 mole ratio and 303 K. Orientational jumps have been dissected into acetamide-acetamide and acetamide-ion catagories. Simulated jump characteristics register a considerable dependence on the anion identity. For example, large angle jumps are relatively less frequent in the presence of NO3- than in the presence of the other two anions. Distribution of jump angles for rotation of acetamide molecules hydrogen bonded (H-bonded) to anions has been found to be bimodal in the presence of Br- and is qualitatively different from the other two cases. Estimated energy barrier for orientation jumps of these acetamide molecules (H-bonded to anions) differ by a factor of ∼2 between NO3- and ClO4-, the barrier height for the latter being lower and ∼0.5kBT. Relative radial and angular displacements during jumps describe the sequence ClO4- > NO3- > Br- and follow a reverse viscosity trend. Jump barrier for acetamide-acetamide pairs reflects weak dependence on anion identity and remains closer to the magnitude (∼0.7kBT) found for orientation jumps in molten acetamide. Jump time distributions exhibit a power law dependence of the type, P(tjump) ∞ A(tjumpτ)-β, with both β and τ showing substantial anion dependence. The latter suggests the presence of dynamic heterogeneity in these systems and supports earlier conclusions from time-resolved fluorescence measurements. (Graph Presented). © 2015 American Chemical Society.


Das S.,se National Center For Basic Science | Biswas R.,se National Center For Basic Science | Biswas R.,Thematic Unit for Excellence Computational Materials Science | Mukherjee B.,Thematic Unit for Excellence Computational Materials Science
Journal of Chemical Physics | Year: 2016

The paper reports a detailed simulation study on collective reorientational relaxation, cooperative hydrogen bond (H-bond) fluctuations, and their connections to dielectric relaxation (DR) in deep eutectic solvents made of acetamide and three uni-univalent electrolytes, lithium nitrate (LiNO3), lithium bromide (LiBr), and lithium perchlorate (LiClO4). Because cooperative H-bond fluctuations and ion migration complicate the straightforward interpretation of measured DR timescales in terms of molecular dipolar rotations for these conducting media which support extensive intra- and inter-species H-bonding, one needs to separate out the individual components from the overall relaxation for examining the microscopic origin of various timescales. The present study does so and finds that reorientation of ion-complexed acetamide molecules generates relaxation timescales that are in sub-nanosecond to nanosecond range. This explains in molecular terms the nanosecond timescales reported by recent giga-Hertz DR measurements. Interestingly, the simulated survival timescale for the acetamide-Li+ complex has been found to be a few tens of nanosecond, suggesting such a cation-complexed species may be responsible for a similar timescale reported by mega-Hertz DR measurements of acetamide/potassium thiocyanate deep eutectics near room temperature. The issue of collective versus single particle relaxation is discussed, and jump waiting time distributions are determined. Dependence on anion-identity in each of the cases has been examined. In short, the present study demonstrates that assumption of nano-sized domain formation is not required for explaining the DR detected nanosecond and longer timescales in these media. © 2016 Author(s).


PubMed | se National Center For Basic Science and Thematic Unit for Excellence Computational Materials Science
Type: Journal Article | Journal: The Journal of chemical physics | Year: 2016

The paper reports a detailed simulation study on collective reorientational relaxation, cooperative hydrogen bond (H-bond) fluctuations, and their connections to dielectric relaxation (DR) in deep eutectic solvents made of acetamide and three uni-univalent electrolytes, lithium nitrate (LiNO3), lithium bromide (LiBr), and lithium perchlorate (LiClO4). Because cooperative H-bond fluctuations and ion migration complicate the straightforward interpretation of measured DR timescales in terms of molecular dipolar rotations for these conducting media which support extensive intra- and inter-species H-bonding, one needs to separate out the individual components from the overall relaxation for examining the microscopic origin of various timescales. The present study does so and finds that reorientation of ion-complexed acetamide molecules generates relaxation timescales that are in sub-nanosecond to nanosecond range. This explains in molecular terms the nanosecond timescales reported by recent giga-Hertz DR measurements. Interestingly, the simulated survival timescale for the acetamide-Li(+) complex has been found to be a few tens of nanosecond, suggesting such a cation-complexed species may be responsible for a similar timescale reported by mega-Hertz DR measurements of acetamide/potassium thiocyanate deep eutectics near room temperature. The issue of collective versus single particle relaxation is discussed, and jump waiting time distributions are determined. Dependence on anion-identity in each of the cases has been examined. In short, the present study demonstrates that assumption of nano-sized domain formation is not required for explaining the DR detected nanosecond and longer timescales in these media.


PubMed | se National Center For Basic Science, Ruhr University Bochum and Thematic Unit for Excellence Computational Materials Science
Type: Journal Article | Journal: The Journal of chemical physics | Year: 2016

A combined experimental (mid- and far-infrared FTIR spectroscopy and THz time domain spectroscopy (TTDS) (0.3-1.6 THz)) and molecular dynamics (MD) simulation technique are used to understand the evolution of the structure and dynamics of water in its binary mixture with 1,2-dimethoxy ethane (DME) over the entire concentration range. The cooperative hydrogen bond dynamics of water obtained from Debye relaxation of TTDS data reveals a non-monotonous behaviour in which the collective dynamics is much faster in the low X

Loading Thematic Unit for Excellence Computational Materials Science collaborators
Loading Thematic Unit for Excellence Computational Materials Science collaborators