Mapping of Guest Localization in Mesoporous Silica Particles by Solid-State NMR and Ab Initio Modeling: New Insights into Benzoic Acid and p-Fluorobenzoic Acid Embedded in MCM-41 via Ball Milling

Abstract

We present a novel strategy for mapping the localization of guest molecules (GMs) in mesoporous materials by combining mechanochemistry with solid-state nuclear magnetic resonance (ssNMR) spectroscopy. To this end, we consider model guest–host systems of benzoic acid (BA) and para-fluorobenzoic acid (4-FBA) embedded in mesoporous MCM-41 material and examine the recently proposed loading (MeLo) procedure for efficient encapsulation of molecular species. Application of high-resolution NMR experiments (1H and 19F NMR) has allowed detection of a multimodal distribution of the spectral signals ascribed to embedded GMs. This peculiarity reflects the presence of distinct molecular ensembles subjected to intrinsically different local environments and exhibiting different dynamical behavior. Furthermore, a considerable fraction of an amorphous phase has been found as a byproduct of the ball-milling. The stability of the phase mixture was further checked by subjecting the samples to chemical and physical stimuli, and a detailed interpretation of the NMR data was corroborated by theoretical calculations. To this end, we have undertaken a challenge to predict the NMR spectra of the GMs@MCM-41 using advanced ab initio molecular dynamics (AIMD) simulations, providing an accurate and exhaustive analysis of the NMR spectra. On the basis of the ab initio modeling validated against the experimental results, we find that the multimodal signal distribution reflects the level of the pore filling, and can be ascribed to the presence of both interface and fluid molecular species trapped inside the pores. This has been confirmed for both BA@MCM-41 and 4-FBA@MCM-41 systems. The presence of the third, amorphous fraction can be linked to interstitial space between randomly ordered crystallites, pointing at the importance of the external surface in further stabilization of encapsulated materials. Altogether, a consistent experimental and theoretical methodology has been presented, paving the way for a more accurate analysis of complex nanoconfined systems.