Search

It is necessary to quantitatively determine substituent effects to accurately elucidate reaction mechanisms in the field of organic chemistry. This paper reports that the molecular electrostatic potential (MESP) can be used as a general and versatile measure for the substituent effects in various chemical reactions by performing extensive density functional theory (DFT) calculations for more than 400 molecules, followed by statistical analyses. We observed a robust and linear correlation between the electrostatic potential and the substituent parameters for various cases of reactive systems, regardless of the DFT functionals, basis sets, and solvation models used. In addition, we statistically analysed the normality of the residuals from the linear regression to demonstrate that strong linear relationships hold universally, which indicates that the electrostatic potential can serve as a physically meaningful quantity for the predictive estimation of substituent effects. In contrast, conventionally used methods based on the charge deviation in the aromatic carbons, as computed using various charge analysis methods, (e.g., Hirshfeld charge analysis) do not demonstrate the statistical normality. Furthermore, we illustrate that MESP can be extensively adopted to strengthen the validity of the linear free energy relationships (LFERs) under various chemical conditions. The results revealed that the MESP shift derived by a functional group on a mono-substituted benzene ring is a strong predictor for the substituent effects on the electronic behaviours in chemical reactions; thus, it can serve as an alternative to other empirical parameters such as the Hammett or Swain–Lupton parameters, or the charge shift. [ABSTRACT FROM AUTHOR]

We investigate the charging phenomena of an electric double layer capacitor (EDLC) by conducting both equilibrium and non-equilibrium molecular dynamics (MD) simulations. A graphene electrode and 1-ethyl-3-methylimidazolium thiocyanate ([EMIM]+[SCN]−) ionic liquid were used as a system for the EDLC. We clarify the ionic layer structure and show that an abrupt change of the ionic layers leads to a high differential capacitance of the EDLC. The charging simulations reveal that the charging dynamics of the EDLC is highly dependent on the rearrangement of the ionic layer structure. Particularly, the electrode charge during the charging process is consistent with the perpendicular displacement of ionic liquid molecules. From this property, we analyze the contribution of each molecular ion to the electrode charge stored during charging. Charging of the EDLC is largely dependent on the desorption of the co-ions from the electrode rather than the adsorption of the counter-ions. In addition, the contribution of bulk ions to the charge stored in the EDLC is as important as that of ions adjacent to the electrode surface contrary to the conventional viewpoint. From these results, we identify the charging mechanism of the EDLC and discuss the relevance to experimental results. Our findings in the present study are expected to play an important role in designing an efficient EDLC with a novel perspective on the charging of the EDLC. [ABSTRACT FROM AUTHOR]

We numerically investigate the dynamic heterogeneity and its length scale found in coarse-grained ionic liquid model systems. In our ionic liquid model systems, cations are modeled as dimers with a positive charge, while anions are modeled as monomers with a negative charge, respectively. To study the effect of the charge distributions on the cations, two ionic liquid models with different charge distributions are used and the model with a neutral charge is also considered as a counterpart. To reveal the heterogeneous dynamics in the model systems, we examine spatial distributions of displacement and time distributions of exchange and persistence times. All the models show a significant increase of the dynamic heterogeneity as the temperature is lowered. The dynamic heterogeneity is quantified via the well-known four-point susceptibility, χ4(t), which measures the fluctuations of a time correlation function. The dynamic correlation length is calculated by fitting the dynamic structure factor, S4(k,t), with the Ornstein–Zernike form at the time scale at which the dynamic heterogeneity reaches the maximum value. The obtained time and length scales exhibit a power law relation at the low temperatures, similar to various supercooled liquid models. In particular, the charged model systems show unusual crossover behaviors which are not observed in the uncharged model system. We ascribe the crossover behavior to the enhanced cage effect caused by charges on the particles. [ABSTRACT FROM AUTHOR]

We study how dynamic heterogeneity in ionic liquids is affected by the length scale of structural relaxation and the ionic charge distribution by the molecular dynamics simulations performed on two differently charged models of ionic liquid and their uncharged counterpart. In one model of ionic liquid, the charge distribution in the cation is asymmetric, and in the other it is symmetric, while their neutral counterpart has no charge with the ions. It is found that all the models display heterogeneous dynamics, exhibiting subdiffusive dynamics and a nonexponential decay of structural relaxation. We investigate the lifetime of dynamic heterogeneity, τdh, in these systems by calculating the three-time correlation functions to find that τdh has in general a power-law behavior with respect to the structural relaxation time, τα, i.e., UGRAPHIC DISPLAY="INLINE" ID="UGT2" SRC="UGT2"/. Although the dynamics of the asymmetric-charge model is seemingly more heterogeneous than that of the symmetric-charge model, the exponent is found to be similar, ζdh≈ 1.2, for all the models studied in this work. The same scaling relation is found regardless of interactions, i.e., with or without Coulomb interaction, and it holds even when the length scale of structural relaxation is long enough to become the Fickian diffusion. This fact indicates that τdh is a distinctive time scale from τα, and the dynamic heterogeneity is mainly affected by the short-range interaction and the molecular structure. [ABSTRACT FROM AUTHOR]

We investigate the dynamic propensity in a coarse-grained model of a room-temperature ionic liquid via molecular dynamics simulations. Dynamic propensity is defined as the average of squared displacements for each ion during a given time interval over the isoconfigurational ensemble. As the temperature is lowered, distributions of the dynamic propensity develop fat tails at high values, indicating the presence of dynamic heterogeneity in the system. The increase in the heterogeneity for the cation is more evident than that for the anion, and a high propensity exhibits a large variance in the isoconfigurational ensemble, implying that dynamic propensity is related to ions' motions at a large length scale, rather than a direct measure of the individual ion dynamics. In addition, large non-Gaussian parameters observed for small dynamic propensities reveal intermittent dynamical behaviors of ions. In order to reveal the origin of the dynamic heterogeneity in a room-temperature ionic liquid, a possible correlation between the mobility and dynamic propensity is further probed. It is observed that spatial distributions of the dynamic propensity coincide with those of the mobility. The results suggest a possible connection between the structure and heterogeneous dynamics on large length scales. [ABSTRACT FROM AUTHOR]

The structure and dynamics of benzene inside and outside of single-walled carbon nanotubes (SWNTs) in the (n,n) armchair configuration are studied viamolecular dynamics computer simulations. Irrespective of the nanotube diameter, benzene molecules form cylindrical solvation shell structures on the outside of the nanotubes. Their molecular planes near the SWNTs in the first external solvation shell are oriented parallel to the nanotube surface, forming a π-stacked structure between the two. By contrast, the benzene distributions in the interior of the SWNTs are found to vary markedly with the nanotube diameter. In the case of the (7,7) and (8,8) nanotubes, internal benzene forms a single-file distribution, either in a vertex-to-vertex (n= 7) or face-to-face (n= 8) orientation between two neighboring molecules. Inside a slightly wider (9,9) nanotube channel, however, a cylindrical single-shell distribution of benzene arises. A secondary solvation structure, which begins to appear inside (10,10), develops into a full structure separate from the first internal solvation shell in (12,12). The ring orientation of internal benzene is generally parallel to the nanotube wall for n= 9–12, while it becomes either slanted with respect to (n= 7), or perpendicular to (n= 8), the nanotube axis. The confinement inside the small nanotube pores exerts a strong influence on the dynamics of benzene. Both translational and rotational dynamics inside SWNTs are slower and more anisotropic than in liquid benzene. It is also found that reorientational dynamics of internal benzene deviate dramatically from the rotational diffusion regime and change substantially with the nanotube diameter. [ABSTRACT FROM AUTHOR]