Evidence in support of levitation effect as the reason for size dependence of ionic conductivity in water: a molecular dynamics simulation

We report extensive molecular dynamics simulations of (i) model ions in water at high concentrations as a function of the size and charge of the ion as well as (ii) realistic simulation of Cl- and Br- ions at low concentrations in water at room temperature. We also analyze existing experimental data in light of the results obtained here. The halide ion simulations have been carried out using the interaction potentials of Koneshan et al. (J. Phys. Chem. B 1998, 102, 4193). We compute structural and dynamical properties of ions in water and explore their variation with size and charge of the ion. We find that ions of certain intermediate sizes exhibit a maximum in self-diffusivity in agreement with previous experimental measurements and computer simulations. We analyze molecular dynamics trajectories in light of the previous understanding of the levitation effect (LE) and the recent suggestion that ionic conductivity has its origin in LE (J. Phys. Chem. B 2005, 109, 8120). We report the distribution of void and neck radii that exist amidst water. Our analysis suggests that the ion with maximum self-diffusivity is characterized by a lower activation energy and a single-exponential decay of F(s)(k,t). The behavior of these and other related quantities of the ion with maximum self-diffusivity are characteristic of the anomalous regime of the LE. The simulation results of Br- and Cl- ions in water also yield results in agreement with the predictions of LE. A plot of experimental conductivity data in the literature for alkali ions in water by Kay and Evans (J. Phys. Chem. 1966, 70, 2325) also yields a lower activation energy for the ion with maximum conductivity in excellent agreement with the LE. To the best of our knowledge, none of the existing theories predict a lower activation energy for the ion with maximum conductivity.     

Reorganization energy of electron transfer in viscous solvents above the glass transition

We present a molecular-dynamics study of the solvent reorganization energy of electron transfer in supercooled water. We observe a sharp decrease of the reorganization energy at a temperature identified as the temperature of structural arrest due to cage effect as discussed by the mode coupling theory. Both the heat capacity and dielectric susceptibility of the pure water show sharp drops at about the same temperature. This temperature also marks the onset of the enhancement of translational diffusion relative to rotational relaxation signaling the breakdown of the Stokes-Einstein relation. The change in the reorganization energy at the transition temperature reflects the dynamical arrest of the slow, collective relaxation of the solvent related to Debye relaxation of the solvent dipolar polarization.     

Electronic structure, ionization potential, and electron affinity of the enzyme cofactor (6R)-5,6,7,8-tetrahydrobiopterin in the gas phase, solution, and protein environments

(6R)-5,6,7,8-Tetrahydrobiopterin (BH(4)) is a key cofactor involved in the electron transfer to the P(450) heme of nitric oxide synthase. We calculated the electronic structure of the neutral, cationic, and anionic forms of BH(4) in the gas phase, in solution (both dielectric and explicit water), and in the protein environment using density functional theory (B3LYP/6-31+G(d,p)). Subsequently, we derived the ionization potential (IP) and electron affinity (EA) of the cofactor in these chemical environments. We found that the electronic structure of BH(4) is susceptible to the presence of an external electric field and that conformational changes in the structure of BH(4) alone do not affect its electronic structure significantly. In the gas phase, water, and protein environments neutral BH(4) is the most stable species, while in the dielectric environment the anion becomes the most stable species. The IP of BH(4) in the protein environment is about half of that in the gas phase, and its EA is about 5 times smaller than that in the gas phase. Our results indicate that changes in the external electric field created by moving charged amino acid residues around BH(4) may lead to configurations that have the BH(4) ion as stable as or more stable than the neutral form, thus facilitating the electron transfer.     

Solvent reorganization entropy of electron transfer in polar solvents

We report the results of molecular dynamics simulations of the solvent reorganization energy of intramolecular electron transfer in a charge-transfer molecule dissolved in water and acetonitrile at varying temperatures. The simulations confirm the prediction of microscopic solvation theories of a positive reorganization entropy in polar solvents. The results of simulations are analyzed in terms of the splitting of the reorganization entropy into the contributions from the solute-solvent interaction and from the alteration of the solvent structure induced by the solute. These two contributions mutually cancel each other, resulting in the reorganization entropy amounting to only a fraction of each component.     

Submanifolds of generalized $$(k,\mu )$$-space-forms

In this paper, we have studied submanifolds especially, totally umbilical submanifolds of generalized \((k,\mu )\)-space-forms. We have found a necessary and sufficient condition for such submanifolds to be either invariant or anti-invariant. It is also shown that every totally umbilical submanifold of a generalized \((k,\mu )\)-space-form is a pseudo quasi-Einstein manifold.     

Additive Function-on-Function Regression

We study additive function-on-function regression where the mean response at a particular time point depends on the time point itself, as well as the entire covariate trajectory. We develop a computationally efficient estimation methodology based on a novel combination of spline bases with an eigenbasis to represent the trivariate kernel function. We discuss prediction of a new response trajectory, propose an inference procedure that accounts for total variability in the predicted response curves, and construct pointwise prediction intervals. The estimation/inferential procedure accommodates realistic scenarios, such as correlated error structure as well as sparse and/or irregular designs. We investigate our methodology in finite sample size through simulations and two real data applications. Supplementary material for this article is available online.     

Asymmetric synthesis of hexapropionate synthons by sequential enantiotopic group selective enolization of meso diketones

Meso 1,9-diketones (six to seven stereocenters) are readily obtained by stepwise or simultaneous two-directional aldol reactions of tetrahydro-4H-thiopyran-4-one with a thiopyran-derived aldehyde or dialdehyde. Enantioselective enolizations of these diketones with the lithium amide from (R,R)-bis(1-phenylethyl)amine occur with simultaneous kinetic resolution to give the mono-TMS enol ethers in >90% yields (BORSM) and >95% ee. The products are applicable to polypropionate synthesis. [reaction: see text]     

Conducting polymer based hybrid structure as transparent and flexible field electron emitter

An efficient cathode material with high transparency (93%) based on conducting polymer poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and single wall carbon nanotubes (SWCNTs) has been developed for the fabrication of highly transparent and flexible field electron emitters (FEE). This kind of material showed superior field emission (FE) performance with very high current density (10^–3A/cm²) at very low electric field. The FE performance of the hybrid materials was dramatically improved compared to either SWCNTs and PEDOT:PSS. Thus the hybrid structures of conducting polymer and SWCNTs might be a good choice for use as a cathode material to enhance the FE performance and for potential application in future portable displays. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ultrasound promoted asymmetric transfer hydrogenation of ketones using Ru(II)arene/amino alcohol catalyst system

Asymmetric transfer hydrogenation of ketones using Ru(II)arene/amino alcohol catalyst system proceeds with significant rate enhancement by ultrasound promotion. Comparison of the silent reactions carried out at 25 degrees C with reactions under sonochemical activation at 25 degrees C clearly showed enhancement in catalytic activity by 5-10 times without significantly affecting the enantioselectivity.