22280-51-9Relevant academic research and scientific papers
Large molecular weight nitroxide biradicals providing efficient dynamic nuclear polarization at temperatures up to 200 K
Zagdoun, Alexandre,Casano, Gilles,Ouari, Olivier,Schwarzwaelder, Martin,Rossini, Aaron J.,Aussenac, Fabien,Yulikov, Maxim,Jeschke, Gunnar,Coperet, Christophe,Lesage, Anne,Tordo, Paul,Emsley, Lyndon
supporting information, p. 12790 - 12797 (2013/09/23)
A series of seven functionalized nitroxide biradicals (the bTbK biradical and six derivatives) are investigated as exogenous polarization sources for dynamic nuclear polarization (DNP) solid-state NMR at 9.4 T and with ca. 100 K sample temperatures. The impact of electron relaxation times on the DNP enhancement (ε) is examined, and we observe that longer inversion recovery and phase memory relaxation times provide larger ε. All radicals are tested in both bulk 1,1,2,2-tetrachloroethane solutions and in mesoporous materials, and the difference in ε between the two cases is discussed. The impact of the sample temperature and magic angle spinning frequency on ε is investigated for several radicals each characterized by a range of electron relaxation times. In particular, TEKPol, a bulky derivative of bTbK with a molecular weight of 905 g·mol-1, is presented. Its high-saturation factor makes it a very efficient polarizing agent for DNP, yielding unprecedented proton enhancements of over 200 in both bulk and materials samples at 9.4 T and 100 K. TEKPol also yields encouraging enhancements of 33 at 180 K and 12 at 200 K, suggesting that with the continued improvement of radicals large ε may be obtained at higher temperatures.
Oxoammonium Salts. 5. A New Synthesis of Hindered Piperidines Leading to Unsymmetrical TEMPO-Type Nitroxides. Synthesis and Enantioselective Oxidations with Chiral Nitroxides and Chiral Oxoammonium Salts
Ma, Zhenkun,Huang, Qingtao,Bobbitt, James M.
, p. 4837 - 4843 (2007/10/02)
A new synthesis of unsymmetrical 2,2,6,6-tetraalkyl-4-piperidones from acetonin (2,2,4,4,6-pentamethyl-2,3,4,5-tetrahydropyrimidine) and several ketones is described.When the ketone was a naturally occurring optically active ketone, the piperidones were optically active.The piperidones were converted to unsymmetrical TEMPO-type nitroxides and chiral nitroxides.The optically active nitroxides were used as catalysts for oxidations or converted to chiral oxoammonium salts.The structures of the chiral compounds were determined by 2D (1)H and (13)C NMR, and the cyclic voltammetric properties of the various nitroxides were measured.Several other pyrrolidine oxoammonium salts were prepared, and both types were used as oxidizing agents.Preliminary results of chiral oxidations are presented.
