2,233 research outputs found
Unraveling the Wacker Oxidation Mechanisms
Full text linkThe mechanisms for the aqueous PdCl2 mediated olefin oxidation reaction (the Wacker process) have been studied with density functional theory, with special emphasis on determining competitive pathways that explain the product distribution's dependence on reaction conditions. Surprisingly, our results indicate that the previously suggested inner-sphere rate-determining step for this process is incompatible with the experimental observations. We describe three key steps, all with barriers between 22.7 and 23.3 kcal/mol. These results, together with literature experimental data, were used to construct a model that explains the observations in the Wacker process. We find that the rate-determining step under low [Cl^-] conditions is not hydroxypalladation as generally believed, but intermolecular isomerization after a lower-energy water-catalyzed internal nucleophilic attack. The pathway under high [Cl^-] leading to anti-addition aldehyde products is only accessible when CuCl_2 is available to selectively stabilize associative chloride exchange. The controversial switch in mechanisms is caused by both this selective stabilization from CuCl_2, and the prerequisite dissociation of Cl^- prior to internal attack. Finally, we suggest that the previously published rate expression for the Wacker process under high [Cl^-] is incomplete and should be replaced with a two-term expression, featuring one term first-order and one term second- (or higher) order in [CuCl_2]
Unraveling the Wacker Oxidation Mechanisms
No full textThe mechanisms for the aqueous PdCl2 mediated olefin oxidation reaction (the Wacker process) have been studied with density functional theory, with special emphasis on determining competitive pathways that explain the product distribution's dependence on reaction conditions. Surprisingly, our results indicate that the previously suggested inner-sphere rate-determining step for this process is incompatible with the experimental observations. We describe three key steps, all with barriers between 22.7 and 23.3 kcal/mol. These results, together with literature experimental data, were used to construct a model that explains the observations in the Wacker process. We find that the rate-determining step under low [Cl-] conditions is not hydroxypalladation as generally believed, but intermolecular isomerization after a lower-energy water-catalyzed internal nucleophilic attack. The pathway under high [Cl-] leading to anti-addition aldehyde products is only accessible when CuCl2 is available to selectively stabilize associative chloride exchange. The controversial switch in mechanisms is caused by both this selective stabilization from CuCl2, and the prerequisite dissociation of Cl- prior to internal attack. Finally, we suggest that the previously published rate expression for the Wacker process under high [Cl-] is incomplete and should be replaced with a two-term expression, featuring one term first-order and one term second- (or higher) order in [CuCl2]
Accentuation of Jonas Rėza's Psalter of 1625
No full textStraipsnyje trumpai apžvelgiama dabartinių kalbų kirčio ženklų istorija – nuo Antikos kalbininko Aristofano Bizantiečio žymėtų akūto, gravio ir cirkumflekso iki Mažvydo Katekizme pažymėto į riestinį cirkumfleksą panašaus ženklo, Baltramiejaus Vilento raštų, D. Kleino gramatikos, J. Rėzos psalmyno ,,Psalteras Dowido“ kirčio ženklų. Išsamiau straipsnyje analizuojamas 1625 m. J. Rėzos psalmyno kirčiavimas, iš graikų perimti kirčio ženklai, paties autoriaus įsivestas kirčio ženklas. Straipsnyje taip pat aptariama Rėzos psalmyne vartotų kirčio ženklų funkcijos, kirčio ženklų vartojimo įvairavimas, sąsajos tarp psalmyno kirčiavimo ir D. Kleino gramatikos Reikšminiai žodžiai: Akūtas; Gravis; Cirkumfleksas; Psalmynas; Lietuvių kalbos istorija; KirčiavimasThis article gives a brief overview of the history of the accent marks of languages from Antiquity linguist Aristophanes of Byzantium marked the acute accent, grave accent and circumflex accent until the sign similar to a tilde-shaped circumflex marked in Mažvydas’ Catechism, and accent signs of Baltramiejus Vilentas’ writings, Daniel Klein‘s grammer, and Jonas Rhesa’s Psalter of David. The article gives a comprehensive analysis of the accentuation made by Jonas Rhesa in the psalter, accent marks taken from Greek, and an accent mark developed by the author himself. The article also discusses the functions of the accent marks used in Rhesa’s psalter, the variation of the usage of accent marks and the interaction between the accentuation of the psalter and D. Klein’s grammer
Kernel computations from large-scale random features obtained by optical processing units
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