15 research outputs found
Conformational Entropy as a Means to Control the Behavior of Poly(diketoenamine) Vitrimers In and Out of Equilibrium.
Control of equilibrium and non-equilibrium thermomechanical behavior of poly(diketoenamine) vitrimers is shown by incorporating linear polymer segments varying in molecular weight (MW) and conformational degrees of freedom into the dynamic covalent network. While increasing MW of linear segments yields a lower storage modulus at the rubbery plateau after softening above the glass transition (Tg ), both Tg and the characteristic time of stress relaxation are independently governed by the conformational entropy of the embodied linear segments. Activation energies for bond exchange in the solid state are lower for networks incorporating flexible chains; the network topology freezing temperature decreases with increasing MW of flexible linear segments but increases with increasing MW of stiff segments. Vitrimer reconfigurability is therefore influenced not only by the energetics of bond exchange for a given network density, but also the entropy of polymer chains within the network
A communal catalogue reveals Earth's multiscale microbial diversity
Our growing awareness of the microbial world's importance and diversity contrasts starkly with our limited understanding of its fundamental structure. Despite recent advances in DNA sequencing, a lack of standardized protocols and common analytical frameworks impedes comparisons among studies, hindering the development of global inferences about microbial life on Earth. Here we present a meta-analysis of microbial community samples collected by hundreds of researchers for the Earth Microbiome Project. Coordinated protocols and new analytical methods, particularly the use of exact sequences instead of clustered operational taxonomic units, enable bacterial and archaeal ribosomal RNA gene sequences to be followed across multiple studies and allow us to explore patterns of diversity at an unprecedented scale. The result is both a reference database giving global context to DNA sequence data and a framework for incorporating data from future studies, fostering increasingly complete characterization of Earth's microbial diversity.Peer reviewe
A communal catalogue reveals Earthâs multiscale microbial diversity
Our growing awareness of the microbial worldâs importance and diversity contrasts starkly with our limited understanding of its fundamental structure. Despite recent advances in DNA sequencing, a lack of standardized protocols and common analytical frameworks impedes comparisons among studies, hindering the development of global inferences about microbial life on Earth. Here we present a meta-analysis of microbial community samples collected by hundreds of researchers for the Earth Microbiome Project. Coordinated protocols and new analytical methods, particularly the use of exact sequences instead of clustered operational taxonomic units, enable bacterial and archaeal ribosomal RNA gene sequences to be followed across multiple studies and allow us to explore patterns of diversity at an unprecedented scale. The result is both a reference database giving global context to DNA sequence data and a framework for incorporating data from future studies, fostering increasingly complete characterization of Earthâs microbial diversity
Electrostatic Basis for Enantioselective BrĂžnsted-Acid-Catalyzed Asymmetric Ring Openings of <i>meso</i>-Epoxides
Computational studies of three chiral
phosphoric-acid-catalyzed
asymmetric ring-openings of <i>meso</i>-epoxides show that
the enantioselectivity of these reactions stems from favorable electrostatic
interactions of the preferred transition state with the phosphoryl
oxygen of the catalyst. The 3,3âČ-aryl substituents of the catalysts,
which are vital for enantioselectivity, serve primarily to create
a narrow binding groove that restricts the substrate orientations
within the chiral electrostatic environment of the phosphoric acid.
This electrostatic, enzyme-like mode of stereoinduction appears to
be general for these reactions and suggests a complementary means
of achieving stereoinduction in chiral phosphoric acid catalysis.
Finally, examination of the mechanism for subsequent reactions in
Listâs organocatalytic cascade for the synthesis of ÎČ-hydroxythiols
(Monaco, M. R.; PreÌvost, S.; List, B. <i>J. Am. Chem.
Soc</i>. <b>2014</b>, <i>136</i>, 16982) explains
the requirement for elevated temperatures for the latter steps in
the cascade sequence, as well as the lack of reactivity of five-membered
cyclic epoxides in this transformation
Competing Noncovalent Interactions Control the Stereoselectivity of Chiral Phosphoric Acid Catalyzed Ring Openings of 3âSubstituted Oxetanes
The
noncovalent interactions responsible for enantioselectivity
in organocatalytic oxetane ring openings were quantified using density
functional theory (DFT) computations. Data show that the mode of stereoinduction
in these systems differs markedly for different substituted oxetanes,
highlighting the challenge of developing general stereochemical models
for such reactions. For oxetanes monosubstituted at the 3-position,
the enantioselectivity is primarily due to differential CH···Ï
interactions between the mercaptobenzothiazole nucleophile and the
aromatic backbone of the catalyst. This can be contrasted with 3,3-disubstituted
oxetanes, for which interactions between an oxetane substituent and
the phosphoric acid functionality and/or the anthryl groups of the
catalyst become more important. The former effects are particularly
important in the case of 3-OH-substituted oxetanes. Overall, these
reactions demonstrate the diversity of competing noncovalent interactions
that control the stereoselectivity of many phosphoric acid catalyzed
reactions
Enantioselectivity in Catalytic Asymmetric Fischer Indolizations Hinges on the Competition of ÏâStacking and CH/Ï Interactions
Computational
analyses of the first catalytic asymmetric Fischer indolization (<i>J. Am. Chem. Soc</i>. <b>2011</b>, <i>133</i>, 18534) reveal that enantioselectivity arises from differences in
hydrogen bonding and CH/Ï
interactions between the substrate and catalyst in the operative transition
states. This selectivity occurs despite strong Ï-stacking interactions
that reduce the enantioselectivity
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Elucidating Solvation Structures for Rational Design of Multivalent Electrolytes-A Review.
Fundamental molecular-level understanding of functional properties of liquid solutions provides an important basis for designing optimized electrolytes for numerous applications. In particular, exhaustive knowledge of solvation structure, stability, and transport properties is critical for developing stable electrolytes for fast-charging and high-energy-density next-generation energy storage systems. Accordingly, there is growing interest in the rational design of electrolytes for beyond lithium-ion systems by tuning the molecular-level interactions of solvate species present in the electrolytes. Here we present a review of the solvation structure of multivalent electrolytes and its impact on the electrochemical performance of these batteries. A direct correlation between solvate species present in the solution and macroscopic properties of electrolytes is sparse for multivalent electrolytes and contradictory results have been reported in the literature. This review aims to illustrate the current understanding, compare results, and highlight future needs and directions to enable the deep understanding needed for the rational design of improved multivalent electrolytes
Elucidating Solvation Structures for Rational Design of Multivalent Electrolytes-A Review.
Fundamental molecular-level understanding of functional properties of liquid solutions provides an important basis for designing optimized electrolytes for numerous applications. In particular, exhaustive knowledge of solvation structure, stability, and transport properties is critical for developing stable electrolytes for fast-charging and high-energy-density next-generation energy storage systems. Accordingly, there is growing interest in the rational design of electrolytes for beyond lithium-ion systems by tuning the molecular-level interactions of solvate species present in the electrolytes. Here we present a review of the solvation structure of multivalent electrolytes and its impact on the electrochemical performance of these batteries. A direct correlation between solvate species present in the solution and macroscopic properties of electrolytes is sparse for multivalent electrolytes and contradictory results have been reported in the literature. This review aims to illustrate the current understanding, compare results, and highlight future needs and directions to enable the deep understanding needed for the rational design of improved multivalent electrolytes
Noncovalent Interactions in Organocatalysis and the Prospect of Computational Catalyst Design
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Enhanced Stability of the Carba- closo-dodecaborate Anion for High-Voltage Battery Electrolytes through Rational Design.
Future energy applications rely on our ability to tune liquid intermolecular interactions and achieve designer electrolytes with highly optimized properties. In this work, we demonstrate rational, combined experimental-computational design of a new carba- closo-dodecaborate-based salt with enhanced anodic stability for Mg energy storage applications. We first establish, through a careful examination using a range of solvents, the anodic oxidation of a parent anion, the carba- closo-dodecaborate anion at 4.6 V vs Mg0/2+ (2.0 vs Fc0/+), a value lower than that projected for this anion in organic solvent-based electrolytes and lower than weakly associating bis(trifluoromethylsulfonyl)imide and tetrafluoroborate anions. Solvents such as acetonitrile, 3-methylsulfolane, and 1,1,1,3,3,3-hexafluoroisopropanol are shown to enable the direct measurement of carba- closo-dodecaborate oxidation, where the resultant neutral radical drives passive film formation on the electrode. Second, we employ computational screening to evaluate the impact of functionalization of the parent anion on its stability and find that replacement of the carbon-vertex proton with a more electronegative fluorine or trifluoromethyl ligand increases the oxidative stability and decreases the contact-ion pair formation energy while maintaining reductive stability. This predicted expansion of the electrochemical window for fluorocarba- closo-dodecaborate is experimentally validated. Future work includes evaluation of the viability of these derivative anions as efficient and stable carriers for energy storage as a function of the ionic transport through the resulting surface films formed on candidate cathodes