Carbon–Carbon Bond Formation by Activation of CH₃F on Alumina

The reaction of CH₃F on partially dehydroxylated Al₂O₃ yields isobutene and higher hydrocarbons along with dimethyl ether. In a combined experimental and computational study, we show that the reaction starts through the initial formation of a methoxy intermediate observed experimentally and formed via an SN₂ type process between surface O atoms and the CH₃F molecule, which undergoes C–F activation on the most acidic Lewis site of the 110 termination of the Al₂O₃ surface. Coordination of an additional CH₃F molecule on adjacent Al Lewis acid sites generates methyl cation-like species, which abstract a hydride from the adjacent initially generated surface methoxy species, yielding a “AlOCH₂⁺” oxonium-like transient structure and CH₄. Both species can further react, forming the first carbon–carbon bond by producing an ethoxy intermediate in a process very exoergic and with accessible energy barriers. This ethoxy intermediate can further react in successive exoergic steps with additional “activated” CH₃F on Al sites leading to higher alkoxy species, which are finally liberated as isobutene and higher hydrocarbons

Large Molecular Weight Nitroxide Biradicals Providing Efficient Dynamic Nuclear Polarization at Temperatures up to 200 K

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

Solid-Phase Polarization Matrixes for Dynamic Nuclear Polarization from Homogeneously Distributed Radicals in Mesostructured Hybrid Silica Materials

Mesoporous hybrid silica–organic materials containing homogeneously distributed stable mono- or dinitroxide radicals covalently bound to the silica surface were developed as polarization matrixes for solid-state dynamic nuclear polarization (DNP) NMR experiments. For TEMPO-containing materials impregnated with water or 1,1,2,2-tetrachloroethane, enhancement factors of up to 36 were obtained at ∼100 K and 9.4 T without the need for a glass-forming additive. We show that the homogeneous radical distribution and the subtle balance between the concentration of radical in the material and the fraction of radicals at a sufficient inter-radical distance to promote the cross-effect are the main determinants for the DNP enhancements we obtain. The material, as well as an analogue containing the poorly soluble biradical bTUrea, is used as a polarizing matrix for DNP NMR experiments of solutions containing alanine and pyruvic acid. The analyte is separated from the polarization matrix by simple filtration