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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. 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)

   Abstract 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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3. Low pKa Phosphido-Boranes Capture Carbon Dioxide with Exceptional Strength: DFT Predictions Followed by Experimental Validation

   Riassti, A; Muskgrave, C; Yeboah, A; Prokofjevs, A; Goddard, W (October 2024, ACS omega)

   We have developed a class of phosphido-boranes (BoPh’s) with formula X+[R2PBH3–] that bind CO2 with exceptional strength (ΔG = −8.2 to −24.0 kcal/mol) at ambient conditions. We use quantum mechanics (QM) to determine how the choice of electron-donating versus electron-withdrawing ligand impacts the CO2 binding strength, in the presence of a donating borane moiety. We also examine the role of the cation in CO2 binding, finding that the ion position relative to the bound CO2 dramatically alters binding strength. We find that the BoPh with two ethyl ligands Li[Et2PBH3] leads to ΔG = −24.0 kcal/mol upon CO2 binding while Li[Ph2PBH3] leads to ΔG = −12.8 kcal/mol. We synthesized the BoPh with two phenyl ligands Li[Ph2PBH3] to validate the QM-predicted stability and predicted pKa. 

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4. 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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5. Enhancement of tetrel bond involving tetrazole-TtR ₃ (Tt = C, Si; R = H, F). Promotion of SiR ₃ transfer by a triel bond

   https://doi.org/10.1039/D2CP04194D  

   Wu, Qiaozhuo; Xie, Xiaoying; Li, Qingzhong; Scheiner, Steve (November 2022, Physical Chemistry Chemical Physics)

   When attached to a tetrazole, a TtR 3 group (Tt = C, Si; R = H, F) engages in a Tt⋯N tetrel bond (TtB) with the Lewis base NCM (M = Li, Na). MP2/aug-cc-pVTZ calculations find that the Si⋯N TtB is rather strong, more than 20 kcal mol −1 for SiH 3 , and between 46 and 53 kcal mol −1 for SiF 3 . The C⋯N TtBs are relatively weaker, less than 8 kcal mol −1 . All of these bonds are intensified when a BH 3 or BF 3 molecule forms a triel bond to a N atom of the tetrazole ring, particularly for the C⋯N TtB, up to 11 kcal mol −1 . In these triads, the SiR 3 group displaces far enough along the line toward the base that it may be thought of as half transferred. 

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