Sundaram Balasubramanian

Refined potential model for atomistic simulations of ionic liquid [bmim][PF6].

Refined parameters of an atomistic interaction potential model for the room temperature ionic liquid 1-n-butyl,3-methylimidazolium hexafluorophosphate are presented. Classical molecular dynamics simulations have been carried out to validate this fully flexible all-atom model. It predicts the density of the liquid at different temperatures between 300 and 500 K within 1.4% of the experimental value. Intermolecular radial distribution functions and the spatial distribution functions obtained from the new model are in close agreement with ab initio simulations. The calculated diffusion coefficients of ions and the surface tension of the liquid agree well with experiment.

Hydrogen-bond dynamics near a micellar surface: origin of the universal slow relaxation at complex aqueous interfaces

The dynamics of hydrogen bonds among water molecules themselves and with the polar head groups (PHG) at a micellar surface have been investigated by long molecular dynamics simulations. The lifetime of the hydrogen bond between a PHG and a water molecule is found to be much longer than that between any two water molecules, and is likely to be a general feature of hydrophilic surfaces of organized assemblies. Analyses of individual water trajectories suggest that water molecules can remain bound to the micellar surface for more than 100 ps. The activation energy for such a transition from the bound to a free state for the water molecules is estimated to be about 3.5 kcal/mol.

Are there stable ion-pairs in room-temperature ionic liquids? Molecular dynamics simulations of 1-n-butyl-3-methylimidazolium hexafluorophosphate.

Molecular dynamics simulations with an all-atom model were carried out to study the ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF(6)]. Analysis was carried out to characterize a number of structural and dynamic properties. It is found that the hydrogen bonds are weaker than expected, as indicated by their short lifetimes, which is due to the fast rotational motion of anions. Transport properties such as ion diffusion coefficients and ionic conductivity were also measured on the basis of long trajectories, and good agreement was obtained with experimental results. The phenomenon that electrical conductivity of ionic liquids deviates from the Nernst-Einstein relation was well reproduced in our work. On the basis of our analysis, we suggest that this deviation results from the correlated motion of cations and anions over time scales up to nanoseconds. In contrast, we find no evidence for long-lived ion-pairs migrating together.

Nanoscale organization in room temperature ionic liquids: a coarse grained molecular dynamics simulation study

A model to perform coarse grained molecular dynamics simulations of room temperature ionic liquids of the family 1--alkyl-3-methylimidazolium hexafluorophosphate has been developed. Large scale simulations of ionic liquids with butyl, heptyl, and decyl side chains have been carried out. Calculated structure factors demonstrate intermediate range ordering in these liquids. The spatial correlations between anions are shown to dominate the neutron or X-ray scattering at low wave vectors. Ionic liquids with long side chains exhibit a bicontinuous morphology, one region consisting of polar moieties and the other of non-polar, alkyl tails.

Dynamics in a room-temperature ionic liquid: a computer simulation study of 1,3-dimethylimidazolium chloride.

The transport properties and solvation dynamics of model 1,3-dialkylimidazolium chloride melt at 425 K is studied using molecular-dynamics simulations. Long trajectories of a large system have been generated and quantities such as the self-diffusion coefficient of ions, shear viscosity, and ionic conductivity have been calculated. Interestingly, the diffusion of the heavier cation is found to be faster than the anion, in agreement with experiment. The interaction model is found to predict a higher viscosity and lower electrical conductivity compared to experimental estimates. Analysis of the latter calculations points to correlated ion motions in this melt. The solvation time correlation function for dipolar and ionic probes studied using equilibrium simulations exhibits three time components, which include an ultrafast (subpicosecond) part as well as one with a time constant of around 150 ps. The ultrafast solvent relaxation is ascribed to the rattling of anions in their cage, while the slow component could be related to the reorientation of the cations as well as to ion diffusion.

Flexible and Rigid Amine-Functionalized Microporous Frameworks Based on Different Secondary Building Units: Supramolecular Isomerism, Selective CO2 Capture, and Catalysis

We report the synthesis, structural characterization, and porous properties of two isomeric supramolecular complexes of ([Cd(NH2 bdc)(bphz)0.5 ]⋅DMF⋅H2 O}n (NH2 bdc=2-aminobenzenedicarboxylic acid, bphz=1,2-bis(4-pyridylmethylene)hydrazine) composed of a mixed-ligand system. The first isomer, with a paddle-wheel-type Cd2 (COO)4 secondary building unit (SBU), is flexible in nature, whereas the other isomer has a rigid framework based on a μ-oxo-bridged Cd2 (μ-OCO)2 SBU. Both frameworks are two-fold interpenetrated and the pore surface is decorated with pendant -NH2 and NN functional groups. Both the frameworks are nonporous to N2 , revealed by the type II adsorption profiles. However, at 195 K, the first isomer shows an unusual double-step hysteretic CO2 adsorption profile, whereas the second isomer shows a typical type I CO2 profile. Moreover, at 195 K, both frameworks show excellent selectivity for CO2 among other gases (N2 , O2 , H2 , and Ar), which has been correlated to the specific interaction of CO2 with the -NH2 and NN functionalized pore surface. DFT calculations for the oxo-bridged isomer unveiled that the -NH2 group is the primary binding site for CO2 . The high heat of CO2 adsorption (ΔHads =37.7 kJ mol(-1) ) in the oxo-bridged isomer is realized by NH2 ⋅⋅⋅CO2 /aromatic π⋅⋅⋅CO2 and cooperative CO2 ⋅⋅⋅CO2 interactions. Further, postsynthetic modification of the -NH2 group into -NHCOCH3 in the second isomer leads to a reduced CO2 uptake with lower binding energy, which establishes the critical role of the -NH2 group for CO2 capture. The presence of basic -NH2 sites in the oxo-bridged isomer was further exploited for efficient catalytic activity in a Knoevenagel condensation reaction.

Modified nonequilibrium molecular dynamics for fluid flows with energy conservation

The nonequilibrium molecular dynamics generated by the SLLOD algorithm [so called due to its association with the DOLLS tensor algorithm (D. J. Evans and G. P. Morriss, Statistical Mechanics of Nonequilibrium Liquids (Academic, New York, 1990)] for fluid flow is considered. It is shown that, in the absence of time-dependent boundary conditions (e.g., shearing boundary conditions via explicit cell dynamics or Lees–Edwards boundary conditions), a conserved energy, H exists for the equations of motion. The phase space distribution generated by SLLOD dynamics can be explicitly derived from H. In the case of a fluid confined between two immobile boundaries undergoing planar Couette flow, the phase space distribution predicts a linear velocity profile, a fact which suggests the flow is field driven rather than boundary driven. For a general flow in the absence of time-dependent boundaries, it is shown that the SLLOD equations are no longer canonical in the laboratory momenta, and a modified form of the SLLOD dy...

What Molecular Features Govern the Mechanism of Supramolecular Polymerization

An understanding of the mechanisms of supramolecular polymerization from a molecular point of view is lacking. Several reports in the literature on the mechanism exhibited by different classes of molecules are examined in an attempt to correlate the molecular features to the aggregation pathway followed. It is proposed that long-range interactions between oligomers could lead to their cooperative growth. The lack thereof leads to isodesmicity.

The effects of pressure on structural and dynamical properties of associated liquids: Molecular dynamics calculations for the extended simple point charge model of water

Constant temperature and constant pressure molecular dynamics calculations have been performed on the extended simple point charge (SPC/E) model water to understand the effects of pressure on its structural and dynamical properties. The effect of pressure is to increase the presence of interstitial water molecules, which is shown to be responsible for the experimentally observed increase in the first x-ray diffraction peak and a broadening of the OOO∧ bond angle distribution. These structural changes lead to weaker hydrogen bonding and the observed minimum in the rotational correlation time as a function of pressure. The weak maximum in the measured translational diffusion coefficient at around 1.5 kbar was not reproduced. The temperature of the density maximum was estimated to occur at 245±4 K, which suggests that structural changes with increasing pressure and decreasing temperature are not quantitatively reproduced by the SPC/E model.

A molecular dynamics study of the mixed alkali effect in silicate glasses

Mixed alkali silicate glasses have been studied in detail by the molecular dynamics method in order to understand the microscopic origin of the mixed alkali effect (MAE). Subtle structural variations in the local environment of alkali ions are observed as a function of the inter-alkali ratio. The MAE has been observed in the variation of self-diffusion coefficients and thus in the electrical conductivity. Cage vibration frequencies of the alkali ions remain unaffected in the mixed alkali regime. Van Hove correlation functions have been employed to study the nature of transport of the alkali ions. Results indicate that alkali ions preferentially jump only to like-ion sites which have been rationalized in terms of their site energies. The MAE has been attributed to blockage of preferred migration paths in interalkali compositions.