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Bacterial cellulose ionogels (BCIGs) represent a new class of material comprising a significant content of entrapped ionic liquid (IL) within a porous network formed from crystalline cellulose microfibrils. BCIGs suggest unique opportunities in separations, optically active materials, solid electrolytes, and drug delivery due to the fact that they can contain as much as 99% of an IL phase by weight, coupled with an inherent flexibility, high optical transparency, and the ability to control ionogel cross-sectional shape and size. To allow for the tailoring of BCIGs for a multitude of applications, it is necessary to better understand the underlying principles of the mesoscopic confinement within these ionogels. Toward this, we present a study of the structural, relaxation, and diffusional properties of the ILs, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf2N]) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([bmpy][Tf2N]), using 1H and 19F NMR T1 relaxation times, rotational correlation times, and diffusion ordered spectroscopy (DOSY) diffusion coefficients, accompanied by molecular dynamics (MD) simulations. We observed that the cation methyl groups in both ILs were primary points of interaction with the cellulose chains and, while the pore size in cellulose is rather large, [emim]+ diffusion was slowed by ∼2-fold, whereas [Tf2N]− diffusion was unencumbered by incorporation in the ionogel. While MD simulations of [bmpy][Tf2N] confinement at the interface showed a diffusion coefficient decrease roughly 3-fold compared to the bulk liquid, DOSY measurements did not reveal any significant changes in diffusion. This suggests that the [bmpy][Tf2N] alkyl chains dominate diffusion through formation of apolar domains. This is in contrast to [emim][Tf2N] where delocalized charge appears to preclude apolar domain formation, allowing interfacial effects to be manifested at a longer range in [emim][Tf2N]. [ABSTRACT FROM AUTHOR]

Sulfur crosslinked polyisoprene (rubber) is used in important material components for a number of technical tasks (e.g., in tires and sealings). If mechanical stress, like tension or shear, is applied on these material components, the sulfur crosslinks suffer from homolytic bond breaking. In thiswork, we have simulated the bond breaking mechanism of sulfur crosslinks between polyisoprene chains using Car- Parrinello molecular dynamic simulations and investigated the maximum forces which can be resisted by the crosslinks. Small model systems with crosslinks formed by chains of N = 1 to N = 6 sulfur atoms have been simulated with the slow growth-technique, known from the literature. The maximum force can be thereby determined from the calculated energies as a function of strain (elongation). The stability of the crosslink under strain is quantified in terms of the maximum force that can be resisted by the system before the crosslink breaks. As shown by our simulations, this maximum force decreases with the sulfur crosslink length N in a step like manner. Our findings indicate that in bridges with N = 1, 2, and 3 sulfur atoms predominantly, carbon-sulfur bonds break, while in crosslinks with N > 3, the breaking of a sulfur-sulfur bond is the dominant failure mechanism. The results are explained within a simple chemical bond model, which describes how the delocalization of the electrons in the generated radicals can lower their electronic energy and decrease the activation barriers. It is described which of the double bonds in the isoprene units are involved in the mechanochemistry of crosslinked rubber. [ABSTRACT FROM AUTHOR]

The N‐heterocyclic carbene organocatalytic reactivity of the 1‐ethyl‐3‐methylimidazolium acetate ionic liquid was investigated on the model reaction between this solvent and anisaldehyde. The formation of carbenes by a proton transfer from the cation to the anion was compared to a direct reaction mechanism, in which the proton transfer and the C–C bond formation between catalyst and substrate occurs in a single elementary step. Interestingly, the two reaction mechanisms show a much smaller difference in activation energies than those observed for analogous catalytic systems with neutral bases, showing that the mechanism might switch from one to the other at different temperatures or with different substrates. In this particular case, however, the direct reaction mechanism, avoiding free carbenes in the solution, is apparently more feasible. Based on the detailed analysis of this reaction path, the earlier contradictions between theory and experiments can be resolved, resulting in a consistent mechanistic picture for the related processes. Additionally, we show on the example of a platinum surface that introducing metal probes into the liquid may induce carbene‐like reactions, as the formation of a strong coordinative bond between the carbene and a platinum atom at the surface is highly exothermic, shifting the acid‐base equilibrium considerably. NHC‐like reactions of imidazolium acetate ionic liquids have been observed in the last decade. Does this presume the presence of free NHCs, or is there an alternative reaction mechanism? Through an inspection of the contradicting former results, and via further calculations and simulations the disagreements can be explained, and resolved. [ABSTRACT FROM AUTHOR]

In this work the mechanism of the aldehyde umpolung reactions, catalyzed by azolium cations in the presence of bases, was studied through computational methods. Next to the mechanism established by Breslow in the 1950s that takes effect through the formation of a free carbene, we have suggested that these processes can follow a concerted asynchronous path, in which the azolium cation directly reacts with the substrate, avoiding the formation of the carbene intermediate. We hereby show that substituting the azolium cation, and varying the base or the substrate do not affect the preference for the concerted reaction mechanism. The concerted path was found to exhibit low barriers also for the reactions of thiamine with model substrates, showing that this path might have biological relevance. The dominance of the concerted mechanism can be explained through the specific structure of the key transition state, avoiding the liberation of the highly reactive, and thus unstable carbene lone pair, whereas activating the substrate through hydrogen-bonding interactions. Polar and hydrogen-bonding solvents, as well as the presence of the counterions of the azolium salts facilitate the reaction through carbenes, bringing the barriers of the two reaction mechanisms closer, in many cases making the concerted path less favorable. Thus, our data show that by choosing the exact components in a reaction, the mechanism can be switched to occur with or without carbenes., (© 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

In this work, we assess the fundamental aspects of mutual miscibility of solvents by studying the mixing of two potential candidates, methanol and n-dodecane, for nonaqueous solvent extraction. To do so, 1 H NMR spectroscopy and molecular dynamics simulations are used jointly. The NMR spectra show that good phase separation can be obtained by adding LiCl and that the addition of a popular extractant (tri- n-butyl phosphate) yields the opposite effect. It is also demonstrated that in a specific case the poor phase separation is not due to the migration of n-dodecane into the more polar phase, but due to the transfer of the extractant into it, which is especially relevant when considering industrial applications of solvent extraction. With the aid of molecular dynamics simulations, explanations of this behavior are given. Specifically, an increase of all hydrogen-bond lifetimes is found to be consequent to the addition of LiCl which implies an indirect influence on the methanol liquid structure, by favoring a stronger hydrogen-bond network. Therefore, we found that better phase separation is not directly due to the presence of LiCl, but due to the "hardening" of the hydrogen-bond network.

Invited for the cover of this issue are Sascha Gehrke and Oldamur Hollóczki at the University of Bonn. The image depicts how N-heterocyclic carbenes exchange hydrogen bonding partners in solution. The "breakup" with the old partner happens either before or after forming a connection with the new. Read the full text of the article at 10.1002/chem.201802286., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Azolium cations are widely employed in organocatalysis to catalyse highly valuable synthetic processes in the presence of a base. These reactions are called "N-heterocyclic carbene catalysis", based on the assumption that they are initiated by the formation of a free carbene through deprotonation, which can then react with the substrates and thereby affect their reactivity to obtain the desired products. However, we herein provide evidence that an electrophilic aromatic substitution mechanism is energetically more favourable, in which the azolium cation reacts directly with the substrate, avoiding the formation of the free carbene in solution., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)