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Keywords = hypervalent silicon

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10 pages, 1757 KB  
Article
Ab Initio MD Study of the Mechanism of Carbonization of Si(001) Surfaces with Methane at High Temperatures
by Dobromir A. Kalchevski, Dimitar V. Trifonov, Stefan K. Kolev, Hristiyan A. Aleksandrov, Dimitar A. Dimov, Valentin N. Popov and Teodor I. Milenov
Coatings 2025, 15(4), 427; https://doi.org/10.3390/coatings15040427 - 4 Apr 2025
Cited by 1 | Viewed by 1033
Abstract
This study employs ab initio metadynamics to simulate the carbonization of Si(001) surfaces with chemical vapor deposition at a temperature of 1423 K. We reveal the complete reaction mechanism, including the beginning of silicon carbide crystal formation. The existence of surficial native oxide [...] Read more.
This study employs ab initio metadynamics to simulate the carbonization of Si(001) surfaces with chemical vapor deposition at a temperature of 1423 K. We reveal the complete reaction mechanism, including the beginning of silicon carbide crystal formation. The existence of surficial native oxide is incorporated into the theoretical model. The mechanism determination includes clarification of all intermediate products and transition states. The free-energy surface of the reaction chain has been found. Carbonization initiates with alkylated surface products and continues with consecutive dehydrogenation steps. Carbon is integrated in the volume, near the crystal surface, only if no covalent interactions with hydrogen atoms remain. The native oxide was not found to prohibit the process of carbonization. The oxygen atoms have certain surface mobility at high temperatures. It was revealed that hypervalency of carbon atoms is possible in transition state structures. The theoretical activation free energy of the rate-determining step was found to be only 166 kJ/mol. This work sheds light on the advantage of the practical use of Si(001) substrates for the synthesis of silicon carbide and Si-O-C glasses using direct carbonization via chemical vapor deposition. We also aim to enable more methodical designs of future synthetic routes and better-informed decisions for experimental investigations. Full article
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11 pages, 1688 KB  
Article
Novel Chiral Bis-Phosphoramides as Organocatalysts for Tetrachlorosilane-Mediated Reactions
by Sergio Rossi, Marco Ziliani, Rita Annunziata and Maurizio Benaglia
Molecules 2017, 22(12), 2181; https://doi.org/10.3390/molecules22122181 - 8 Dec 2017
Cited by 2 | Viewed by 5770
Abstract
The formation of novel chiral bidentate phosphoroamides structures able to promote Lewis base-catalyzed Lewis acid-mediated reactions was investigated. Two different classes of phosphoroamides were synthetized: the first class presents a phthalic acid/primary diamine moiety, designed with the aim to perform a self-assembly recognition [...] Read more.
The formation of novel chiral bidentate phosphoroamides structures able to promote Lewis base-catalyzed Lewis acid-mediated reactions was investigated. Two different classes of phosphoroamides were synthetized: the first class presents a phthalic acid/primary diamine moiety, designed with the aim to perform a self-assembly recognition process through hydrogen bonds; the second one is characterized by the presence of two phosphoroamides as side arms connected to a central pyridine unit, able to chelate SiCl4 in a 2:1 adduct. These species were tested as organocatalysts in the stereoselective allylation of benzaldehyde and a few other aromatic aldehydes with allyl tributyltin in the presence of SiCl4 with good results. NMR studies confirm that only pyridine-based phosphoroamides effectively coordinate tetrachlorosilane and may lead to the generation of a self-assembled entity that would act as a promoter of the reaction. Although further work is necessary to clarify and confirm the formation of the hypothesized adduct, the study lays the foundation for the design and the synthesis of chiral supramolecular organocatalysts. Full article
(This article belongs to the Collection Recent Advances in Organocatalysis)
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