In connection with electrophilic substitution at the formylcarbon atom of orthoformamides"] we have studied the kinetic acidity of the formyl-hydrogen atom by means of HID exchange reactions. It is known that the H atom of chloroform can be exchanged for deuterium within a few hours in the presence of bases [21, whereas analogous experiments with orthoformic esters were negativer31. It was thus all the more surprising that, when dissolved in deuterioalcohols, most N,N-dialkylformamide acetals exchange their formyl proton for deuterium within a few minutes. The rate of exchange depends on the acidity of the alcohol used.
Overall velocity constants R+ (sec-1) [4] f o r H / D exchanse between amide acetals and deuterated alcohols. N,N-Diniethylformamide acetal
I R in ROD I R+
Dimethyl acetal Diethyl acetal Di-n-propyl acetal Diisopropyl acetal Di-n-botyl acetalThe exchange reactions could be followed by N M R spectroscopy, using the diminishing CH-proton signal and the increasing intensity of the OH-proton signal of the alcohol formed. We assume that the exchange occurs by way of an ylide ( I ) .
Es wird die Lichtabsorption von oktaedrisch koordiniertem Cr3+ nach isomorphem Einbau in oxydische Wirtsgitter untersucht. Als Wirtsgitter dienten MgAl2O4, Al2O3 sowie die Perowskite LaAlO3, LaGaO3 und YAlO3. Eine Aufweitung des Wirtsgitters durch isomorphen Einbau steigender Mengen von Cr3+ oder Ga3+ an Stelle von Al3+ bewirkt eine Rotverschiebung der beiden Hauptabsorptionsbanden, bedingt durch eine parallel gehende Schwächung des Kristallfeldes am Ort des farbgebenden Cr3+. Anzeichen dafür, daß eine Verzerrung des Koordinationsoktaeders mit Cr3+ als Koordinationszentrum ebenfalls eine Rotverschiebung der beiden Absorptionsbanden zur Folge hat, sind vorhanden.
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