1H NMR linewidths of small organic guest molecules physisorbed on different mesoporous silicas

Abstract

The excessive linewidths of 1H NMR spectra of solids is a longstanding difficulty. Organic molecules (crystalline at room temperatures) of different chemical characters (camphor, menthol, HMB, HMTA, biphenyl, terphenyl, anthracene, perylene, etc.) have been confined or physisorbed in regular (MCM-41, SBA-15) and irregular (aerogel) silica mesopores. Unexpectedly, steadily decreasing proton NMR linewidths have been observed upon increasing pore sizes. Comparison of the static 1H NMR spectra of the confined and “free” bulk molecules showed an impressive resolution improvement (a factor of 5–10). Static 1H NMR and 2H magic angle spinning (MAS) spectra pointed out the increased molecular mobility as origin. In a qualitative approach the possible reasons of mobility have been thoroughly discussed. The increased mobility seems to be easiest achievable in aerogel and so, in the future, this extreme light mesoporous silica nanomaterial (MSN) may be considered as “active” filling material in solid state NMR spectroscopy. Since sensitivity is not an issue in 1H MAS measurements the proposal - simultaneous dilution of the dense 1H–1H spin network and increasing the molecular mobility of the molecules of interest by confinement or physisorption in/on silica mesopores - may find applications, especially in laboratories where high magnetic fields and state of art ultra-fast MAS rotation are not available.