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Boosted molecular mobility during common chemical reactions

Science

During a chemical reaction, the reorganization of solvent molecules not directly in contact with reactants and products is normally viewed as a simple diffusion response. Wang et al. studied molecular diffusion in six common reactions—including the copper-catalyzed click reaction and the Diels-Alder reaction—with pulsed-field gradient nuclear magnetic resonance. They observed a boost in mobility relative to Brownian diffusion that was stronger for the catalyzed reactions that were studied. The mobilities for the click reaction were verified with a microfluidic gradient method. They argue that energy release produces transient translational motion of reacting centers that mechanically perturbs solvent molecules. Science , this issue p. [537][1] Mobility of reactants and nearby solvent is more rapid than Brownian diffusion during several common chemical reactions when the energy release rate exceeds a threshold. Screening a family of 15 organic chemical reactions, we demonstrate the largest boost for catalyzed bimolecular reactions, click chemistry, ring-opening metathesis polymerization, and Sonogashira coupling. Boosted diffusion is also observed but to lesser extent for the uncatalyzed Diels-Alder reaction, but not for substitution reactions SN1 and SN2 within instrumental resolution. Diffusion coefficient increases as measured by pulsed-field gradient nuclear magnetic resonance, whereas in microfluidics experiments, molecules in reaction gradients migrate “uphill” in the direction of lesser diffusivity. This microscopic consumption of energy by chemical reactions transduced into mechanical motion presents a form of active matter. [1]: /lookup/doi/10.1126/science.aba8425


A synthetic polymer system with repeatable chemical recyclability

Science

The development of chemically recyclable polymers offers a solution to the end-of-use issue of polymeric materials and provides a closed-loop approach toward a circular materials economy. However, polymers that can be easily and selectively depolymerized back to monomers typically require low-temperature polymerization methods and also lack physical properties and mechanical strengths required for practical uses. We introduce a polymer system based on γ-butyrolactone (GBL) with a trans-ring fusion at the α and β positions. Such trans-ring fusion renders the commonly considered as nonpolymerizable GBL ring readily polymerizable at room temperature under solvent-free conditions to yield a high–molecular weight polymer. The polymer has enhanced thermostability and can be repeatedly and quantitatively recycled back to its monomer by thermolysis or chemolysis.


Ionic liquids assist in vacuum

Science

Thin Films Molecular layer deposition, an analog of atomic layer deposition, alternates self-limiting reactions to grow materials such as polymers. However, the vacuum conditions in practice generally limit the choices to polymers in which the barrier to reaction is low without solvent assistance, such as those with thionyl or acyl backbones. Shi and Bent show that an ionic liquid, 1-ethyl-3-methylimidazolium, can create a solvation environment for the Friedel-Crafts reaction in vacuum. This ionic liquid did not evaporate at reaction temperatures, wetted a silicon substrate, and formed a eutectic with the AlCl3 catalyst. The authors alternated deposition of isophthaloyl dichloride and diphenyl ether along with the catalyst to grow polyetherketoneketone thin films at a rate of about 5.5 angstroms per reaction cycle. ACS Nano 10.1021/acsnano.0c09329 (2021).


Ontologies and Representations of Matter

AAAI Conferences

We carry out a comparative study of the expressive power of different ontologies of matter in terms of the ease with which simple physical knowledge can be represented. In particular, we consider five ontologies of models of matter: particle models, fields, two ontologies for continuous material, and a hybrid model. We evaluate these in terms of how easily eleven benchmark physical laws and scenarios can be represented.


[Perspective] Rethinking the SN2 reaction

Science

The SN2 nucleophilic substitution reaction, X RY XR Y, is a paradigm reaction in organic chemistry (1). The modern understanding of the SN2 reaction mechanism is based on work of Hughes and Ingold (2), who proposed that the nucleophile (X) approaches the carbon atom that bears the leaving group (Y). As a result, the bond between the carbon atom and the leaving group becomes weakened. As this bond breaks and a new bond forms between the nucleophile and the carbon atom, the configuration of the carbon atom is inverted. Analyses of gas-phase reaction rates led to the suggestion of a potential energy surface (PES) with two wells connected by a central barrier transition state (3).