Friday, May 14, 2010

J. Phys. Chem. C., v114, Issue 19

Solution State NMR Techniques Applied to Solid State Samples: Characterization of Benzoic Acid Confined in MCM-41

Thierry Azais*, Geoffrey Hartmeyer†, Sandrine Quignard, Guillaume Laurent and Florence Babonneau
J. Phys. Chem. C, 2010, 114 (19), pp 8884–8891
DOI: 10.1021/jp910622m

Abstract:In this paper we present an NMR methodology to characterize small organic molecules confined in mesoporous materials. In particular, we demonstrate that NMR techniques issued from solution state NMR are well suited to characterize benzoic acid encapsulated in hexagonally ordered mesoporous silica MCM-41 possessing two different averaged pore sizes (30 and 100 Å). As evidenced by differential scanning calorimetry, entrapped benzoic acid molecules are highly mobile at room temperature due to confinement effect and possess a glass phase transition temperature around −55 °C. Thus, the 13C NMR characterization of encapsulated molecules has to be adapted to that particular behavior. In particular, the cross-polarization technique traditionally used in solid state NMR to record 13C magic angle spinning (MAS) spectra is of poor efficiency due to weak 1H−13C dipolar interaction. Nevertheless, the presence of 1H−13C cross-relaxation phenomenon (nuclear Overhauser effect, NOE) allows us to record 13C spectra through power-gated techniques, routinely used in solution state NMR, in order to enhance the 13C signal through NOE. Furthermore, the long T2′(1H) values (up to 22 ms) are compatible with the setup of J-coupling-based experiments such as MAS refocused {1H}−13C INEPT experiments allowing us to characterize the sample through chemical bonds. These results combined with those of MAS 1H NOESY experiments lead us to distinguish unambiguously different benzoic acid populations within the large pore sample. Finally, we show that cooling down the samples at −35 °C diminishes the mobility and allows the reintroduction of the 1H−13C dipolar interaction. Thus, 2D MAS {1H}−13C HETCOR experiments can be performed at low temperature to explore spatial proximities.

Spin Canting of Maghemite Studied by 57Fe NMR and In-Field Mssbauer Spectrometry

T. Jean Daou†, Jean-Marc Greneche‡, Seong-Joo Lee§, Soonchil Lee§, Christophe Lefevre†, Sylvie Bgin-Colin† and Genevive Pourroy*†

J. Phys. Chem. C, 2010, 114 (19), pp 8794–8799
DOI: 10.1021/jp100726c

Abstract: Local probe techniques, 57Fe in-field Mssbauer, and 57Fe NMR spectrometries have been combined to describe the magnetic structure of maghemite nanoparticles of 39 (±5) nm in size and commercial maghemite. Maghemite nanoparticles were obtained from oxidation of magnetite nanoparticles. Commercial maghemite consists of nanostructured rods, and the size of crystalline domain fairly compares to that of nanoparticles. The structure of the two samples is a partially disordered spinel structure. Both local probe techniques agree that Fe magnetic moments of octahedral and tetrahedral sites are canted in both systems. It was concluded that the canting originates not only from surface effects but also from the bulk resulting from the disordered spinel structure and the frustrated cationic topology, giving rise to reversed Fe moments.

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