Solid-state NMR characterization of drug-model molecules encapsulated in MCM-41silica

Abstract

In this contribution, we present a solid-state NMR approach to characterize drug-model molecules as ibuprofen, benzoic acid, and lauric acid, encapsulated in MCM-41 silicaand submitted to strong confinement effects. In particular, we show that by a careful choiceof the solid-state NMR sequences, it is possible to efficiently characterize these highlymobile molecules and their interactions with the pore surface. Thus, we demonstrate that13C NMR spectroscopy is a powerful tool to characterize and even quantifyentrapped and non-entrapped species by using either single-pulse excitation (SPE) orcross-polarization (CP). Whereas the standard {1H}-13C CPexperiment is of poor efficiency for mobile species, we show that 13Csignal-to-noise (S/N) ratio can be significantly improved through1H-13C cross-relaxation (namely, nuclear Overhauser effect,nOe) by using a 1H power-gated technique. The long transversal relaxationtimes [T2(1H) up to 22 ms] observed allow the setup ofJ-coupling-based experiments such as 2D {1H}-13 Cheteronuclear multiple-quantum coherence (HMQC) in order to fully characterize theencapsulated molecules. Thus, we demonstrate that the use of sequences derived fromsolution-state NMR such as these two latter experiments is highly efficient to characterizehighly mobile organic molecules trapped in mesopores. Finally, we show that1H spin diffusion-based experiments can give useful information on theproximities between trapped molecules and the silica surface. © 2009 IUPAC.