Pure and Applied Chemistry

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 silica and submitted to strong confinement effects. In particular, we show that by a careful choice of the solid-state NMR sequences, it is possible to efficiently characterize these highly mobile molecules and their interactions with the pore surface. Thus, we demonstrate that ¹³C NMR spectroscopy is a powerful tool to characterize and even quantify entrapped and non-entrapped species by using either single-pulse excitation (SPE) or cross-polarization (CP). Whereas the standard {¹H}-¹³C CP experiment is of poor efficiency for mobile species, we show that ¹³C signal-to-noise (S/N) ratio can be significantly improved through ¹H-¹³C cross-relaxation (namely, nuclear Overhauser effect, nOe) by using a ¹H power-gated technique. The long transversal relaxation times [T₂(¹H) up to 22 ms] observed allow the set-up of J-coupling based experiments such as 2D {¹H}-¹³C heteronuclear multiple-quantum coherence (HMQC) in order to fully characterize the encapsulated molecules. Thus, we demonstrate that the use of sequences derived from solution-state NMR such as these two latter experiments is highly efficient to characterize highly mobile organic molecules trapped in mesopores. Finally, we show that ¹H spin diffusion-based experiments can give useful informations on the proximities between trapped molecules and the silica surface.