Search

A new two-dimensional NMR carbon-proton chemical shift correlation experiment, the MAS-J-HMQC experiment, is proposed for natural abundance rotating solids. The magnetization transfer used to obtain the correlations is based on scaler heteronuclear J couplings. The 2D map provides through-bend chemical shift correlations between directly bonded proton-carbon pairs in a way similar to that in corresponding high-resolution liquid-state experiments. The transfer through J coupling is shown to be efficient and more selective than those based on heteronuclear dipolar couplings. The experiment, which works at high MAS spinning frequencies, yields the unambiguous assignment of the proton resonances. The experiment is demonstrated on several organic compounds.

International audience; Selective functionalization of methane at moderate temperature is of crucial economic, environmental, and scientific importance. Here, we report that methane reacts with heteropolyacids (HPAs) chemisorbed on silica to produce acetic acid under soft conditions. Specially, when chemisorbed on silica, H4SiW12O40, H3PW12O40, H4SiMo12O40, and H3PMo12O40 activate the primary C-H bond of methane at room temperature and atmospheric pressure. With these systems, acetic acid is produced directly from methane, in a single step, in the absence of Pd and without adding CO. Extensive surface characterization by solid-state NMR spectroscopy, IR spectroscopy, cyclic voltammetry, and X-ray photoelectron spectroscopy suggests that C-H activation of methane is triggered by the protons in the HPA-silica interface with concerted reduction of the Keggin cage, leading to water formation and hydration of the interface. This is the simplest and mildest way reported to date to functionalize methane.

WOS:000447953600032; Dynamic nuclear polarization (DNP) solid-state nuclear magnetic resonance (NMR) has developed into an invaluable tool for the investigation of a wide range of materials. However, the sensitivity gain achieved with many polarizing agents suffers from an unfavorable field and magic angle spinning (MAS) frequency dependence. We present a series of new hybrid biradicals, soluble in organic solvents, that consist of an isotropic narrow electron paramagnetic resonance line radical, alpha,gamma-isdiphenylene-beta-phenylallyl (BDPA), tethered to a broad line nitroxide. By tuning the distance between the two electrons and the substituents at the nitroxide moiety, correlations between the electron-electron interactions and the electron spin relaxation times on one hand and the DNP enhancement factors on the other hand are established. The best radical in this series has a short methylene linker and bears bulky phenyl spirocyclohexyl ligands. In a 1.3 mm prototype DNP pr obe, it yields enhancements of up to 185 at 18.8 T (800 MHz H-1 resonance frequency) and 40 kHz MAS. We show that this radical gives enhancement factors of over 60 in 3.2 mm sapphire rotors at both 18.8 and 21.1 T (900 MHz H-1 resonance frequency), the highest magnetic field available today for DNP. The effect of the rotor size and of the microwave irradiation inside the MAS rotor is discussed. Finally, we demonstrate the potential of this new series of polarizing agents by recording high field Al-27 and Si-29 DNP surface enhanced NMR spectra of amorphous aluminosilicates and O-17 NMR on silica nanoparticles.

The feasibility of spectral editing techniques making use of heteronuclear scalar couplings in solid-state CPMAS NMR is demonstrated for powder samples exhibiting line widths of several tens of hertz such as ordinary amino acids. These simple and robust experiments are insensitive to molecular motion, they work at high MAS spinning rates, and yield unambiguous identification of carbon types in natural abundance samples in a manner directly analogous to liquid-state NMR. Examples are given for some amino acids and for cholesteryl acetate.

We show how high-resolution NMR spectra can be obtained for solids for which the spectra are normally broadened due to structural disorder. The method relies on correlations in the chemical shifts between pairs of coupled spins. It is found experimentally that there are strong correlations in the chemical shifts between neighboring spins in both phosphorus-31 and carbon-13 spectra. These correlations can be exploited not only to provide resolution in two-dimensional spectra, but also to yield ``chains'' of correlated chemical shifts, constituting a valuable new source of structural information for disordered materials.