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1. Complex Organic Matter Synthesis on Siloxyl Radicals in the Presence of CO

   https://doi.org/10.3389/fchem.2020.621898  

   Fioroni, Marco ; DeYonker, Nathan J. ( February 2021 , Frontiers in Chemistry)

   null (Ed.)

   Heterogeneous phase astrochemistry plays an important role in the synthesis of complex organic matter (COM) as found on comets and rocky body surfaces like asteroids, planetoids, moons and planets. The proposed catalytic model is based on two assumptions: (a) siliceous rocks in both crystalline or amorphous states show surface-exposed defective centers such as siloxyl (Si-O•) radicals; (b) the second phase is represented by gas phase CO molecules, an abundant C 1 building block found in space. By means of quantum chemistry; (DFT, PW6B95/def2-TZVPP); the surface of a siliceous rock in presence of CO is modeled by a simple POSS (polyhedral silsesquioxane) where a siloxyl (Si-O•) radical is present. Four CO molecules have been consecutively added to the Si-O• radical and to the nascent polymeric CO (pCO) chain. The first CO insertion shows no activation free energy with ΔG 200 K = −21.7 kcal/mol forming the SiO-CO• radical. The second and third CO insertions show Δ G 200 K ‡ ≤ 10.5 kcal/mol. Ring closure of the SiO-CO-CO• (oxalic anhydride) moiety as well as of the SiO-CO-CO-CO• system (di-cheto form of oxetane) are thermodynamically disfavored. The last CO insertion shows no free energy of activation resulting in the stable five member pCO ring, precursor to 1,4-epoxy-1,2,3-butanone. Hydrogenation reactions of the pCO have been considered on the SiO oxygen or on the carbons and oxygens of the pCO chains. The formation of the reactive aldehyde SiO-CHO on the siliceous surface is possible. In principle, the complete hydrogenation of the (CO) 1−4 series results in the formation of methanol and polyols. Furthermore, all the SiO-pCO intermediates and the lactone 1,4-epoxy-1,2,3-butanone product in its radical form can be important building blocks in further polymerization reactions and/or open ring reactions with H (aldehydes, polyols) or CN (chetonitriles), resulting in highly reactive multi-functional compounds contributing to COM synthesis. 

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2. Ensemble Effects on Allylic Oxidation within Explicit Solvation Environments

   https://doi.org/10.1039/D1DT00785H  

   Hung M. Le, Mariano Guagliardo ( June 2021 , Dalton transactions)

   null (Ed.)

   Umbrella-sampling density functional theory molecular dynamics (DFT-MD) has been employed to study the full catalytic cycle of the allylic oxidation of cyclohexene using a Cu(II) 7-amino-6-((2- hydroxybenzylidene)amino)quinoxalin-2-ol complex in acetonitrile to create cyclohexenone and H2O as products. After the initial H-atom abstraction step, two different reaction pathways have been identified that are distinguished by the participation of alkyl hydroperoxide (referred to as the “open” cycle) versus the methanol side-product (referred to as the “closed” cycle) within the catalyst recovery process. Importantly, both pathways involve dehydrogenation and re-hydrogenation of the -NH2 group bound to the Cu-site - a feature that is revealed from the ensemble sampling of configurations of the reactive species that are stabilized within the explicit solvent environment of the simulation. Estimation of the energy span from the experimental turnover frequency yields an approximate value of 22.7 kcal/mol at 350 K. Whereas the closed cycle value is predicted to be 26.2 kcal/mol, the open cycle value at 16.5 kcal/mol. Both pathways are further consistent with the equilibrium between Cu(II) and Cu(III) than what has previously been observed. In comparison to prior static DFT calculations, the ensemble of of both solute and solvent configurations has helped to reveal a breadth of processes that underpin the full catalytic cycle yielding a more comprehensive understanding of the importance of radical reactions and catalysis recovery. 

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3. The reaction of thiourea and 1,3-dimethylthiourea towards organoiodines: oxidative bond formation and halogen bonding

   https://doi.org/10.1107/S205322962100869X  

   Peloquin, Andrew J. ; Ragusa, Arianna C. ; McMillen, Colin D. ; Pennington, William T. ( October 2021 , Acta Crystallographica Section C Structural Chemistry)

   By varying the halogen-bond-donor molecule, 11 new halogen-bonding cocrystals involving thiourea or 1,3-dimethylthiourea were obtained, namely, 1,3-dimethylthiourea–1,2-diiodo-3,4,5,6-tetrafluorobenzene (1/1), C 6 F 4 I 2 ·C 3 H 8 N 2 S, 1 , thiourea–1,3-diiodo-2,4,5,6-tetrafluorobenzene (1/1), C 6 F 4 I 2 ·CH 4 N 2 S, 2 , 1,3-dimethylthiourea–1,3-diiodo-2,4,5,6-tetrafluorobenzene (1/1), C 6 F 4 I 2 ·C 3 H 8 N 2 S, 3 , 1,3-dimethylthiourea–1,3-diiodo-2,4,5,6-tetrafluorobenzene–methanol (1/1/1), C 6 F 4 I 2 ·C 3 H 8 N 2 S·CH 4 O, 4 , 1,3-dimethylthiourea–1,3-diiodo-2,4,5,6-tetrafluorobenzene–ethanol (1/1/1), C 6 F 4 I 2 ·C 3 H 8 N 2 S·C 2 H 6 O, 5 , 1,3-dimethylthiourea–1,4-diiodo-2,3,5,6-tetrafluorobenzene (1/1), C 6 F 4 I 2 ·C 3 H 8 N 2 S, 6 , 1,3-dimethylthiourea–1,3,5-trifluoro-2,4,6-triiodobenzene (1/1), C 6 F 3 I 3 ·C 3 H 8 N 2 S, 7 , 1,3-dimethylthiourea–1,1,2,2-tetraiodoethene (1/1), C 6 H 16 N 4 S 2 ·C 2 I 4 , 8 , [(dimethylamino)methylidene](1,2,2-triiodoethenyl)sulfonium iodide–1,1,2,2-tetraiodoethene–acetone (1/1/1), C 5 H 8 I 3 N 2 S + ·I − ·C 3 H 6 O·C 2 I 4 , 9 , 2-amino-4-methyl-1,3-thiazol-3-ium iodide–1,1,2,2-tetraiodoethene (2/3), 2C 4 H 7 N 2 S + ·2I − ·3C 2 I 4 , 10 , and 4,4-dimethyl-4 H -1,3,5-thiadiazine-3,5-diium diiodide–1,1,2,2-tetraiodoethene (2/3), 2C 5 H 12 N 4 S 2+ ·4I − ·3C 2 I 4 , 11 . When utilizing the common halogen-bond-donor molecules 1,2-, 1,3-, and 1,4-diiodotetrafluorobenzene, as well as 1,3,5-trifluoro-2,4,6-triiodobenzene, bifurcated I...S...I interactions were observed, resulting in the formation of isolated rings, chains, and sheets. Tetraiodoethylene (TIE) provided I...S...I cocrystals as well, but further yielded a sulfonium-containing product through the reaction of the S atom with TIE. This particular sulfonium motif is the first of its kind to be structurally characterized, and is stabilized in the solid state through a three-dimensional I...I halogen-bonding network. Thiourea reacted with acetone in the presence of TIE to provide two novel heterocyclic products, again stabilized in the solid state through I...I halogen bonding. 

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4. Thermodynamic stabilization of crystalline silicon carbide polymer‐derived ceramic fibers

   https://doi.org/10.1002/ces2.10153  

   Leonel, Gerson J. ; Mujib, Shakir Bin ; Singh, Gurpreet ; Navrotsky, Alexandra ( July 2022 , International Journal of Ceramic Engineering & Science)

   Three crystalline SiC fibers were studied: Tyranno, Hi‐Nicalon, and Sylramic. Thermodynamic stability of the SiC fibers was determined by high temperature oxide melt solution calorimetry. Results shed light on the thermodynamic penalty or benefit associated with microstructural modification of the ceramic fibers, and how energetics correlate to mechanical properties. Enthalpies of formation from components (SiC, SiO₂, Si₃N₄, and C, ∆H°_f,comp) for Tyranno, Hi‐Nicalon, and Sylramic are −12.05 ± 8.71, −58.75 ± 6.93, and −71.10 ± 8.71 kJ/mol Si, respectively. The microstructure in Sylramic offers the greatest stabilizing effect, thus resulting in its much more exothermic enthalpy of formation relative to elements and crystalline components. In contrast, the microstructure in Tyranno offers the least stabilization. The thermodynamic stability of the fibers increases with increasing mixed bonding (Si bonded to both C and O). From mechanical testing, Young's moduli of Tyranno, Hi‐Nicalon, and Sylramic are 112, 205, and 215 GPa, respectively. Greater thermodynamic stability is correlated with a higher Young's modulus.

    

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5. Cascade Ring Strain Release Polymerization of Cyclohexene Oxide and Derivatives Using a Mono(μ-Alkoxo)bis(alkylaluminum) Initiator

   https://doi.org/10.1021/acs.macromol.3c00473  

   Pedretti, Benjamin J. ; Zhu, Congzhi ; Watanabe, Hironobu ; Aoshima, Sadahito ; Lynd, Nathaniel A. ( July 2023 , Macromolecules)

   Cyclohexene oxide (CHO) is a useful building block for the synthesis of novel materials and is a model substrate for polymerization catalyst development. The driving force for CHO polymerization is derived from its bicyclic structure, which combines the release of the enthalpy from epoxide ring-opening (ca. −15 kcal/mol) and a twist-chair-to-chair conformation shift in the cyclohexane ring (ca. −5 kcal/mol) upon enchainment. The lack of regio-defined functional handles attached to the CHO monomer limits the ability to both pre- and post-functionalize the resultant materials and establish structure–property relationships, which reduces the versatility of currently accessible materials. We report the synthesis of two series of CHO derivatives with butyl, allyl, and halogen substituents in the α and β positions relative to the epoxide ring. Adding substituents to the CHO ring was found to affect polymerization kinetics, with 4-substituted (β) CHO being more reactive than 3-substituted (α) CHO analogs when initiated with a mono(μ-alkoxo)bis(alkylaluminum) pre-catalyst. Polymer thermal properties depended on substituent location and identity. Halogenated CHO rings were most reactive and produced the highest glass transition temperatures in the resultant polymers (up to 105 °C). Density functional theory revealed a possible mechanistic explanation consistent with the observed differences in polymerization rate for the 3- and 4-substituted CHOs derived from a combination of steric and thermodynamic considerations. 

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