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

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 angstrom). 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 degrees C. Thus, the (13)C 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 (13)C magic angle spinning (MAS) spectra is of poor efficiency due to weak (1)H-(13)C dipolar interaction. Nevertheless, the presence of (1)H-(13)C cross-relaxation phenomenon (nuclear Overhauser effect, NOE) allows us to record (13)C spectra through power-gated techniques, routinely used in solution state NMR, in order to enhance the (13)C signal through NOE. Furthermore, the long T(2)'((1)H) values (up to 22 ms) are compatible with the setup of J-coupling-based experiments such as MAS refocused {(1)H} -(13)C INEPT experiments allowing us to characterize the sample through chemical bonds. These results combined with those of MAS (1)H 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 degrees C diminishes the mobility and allows the reintroduction of the (1)H-(13)C dipolar interaction. Thus, 2D MAS {(1)H}-(13)C HETCOR experiments can be performed at low temperature to explore spatial proximities.