Investigation of conformational changes of organic molecules sorbed in zeolites by proton magnetic resonance spectroscopy

Abstract

An appreciable increase in spectral resolution in the 1H nuclear magnetic resonance (NMR) spectra of small organic molecules adsorbed in well-crystallized zeolites is obtained by the application of the magic angle spinning (MAS) technique, which allows to observe slight changes in the proton chemical shifts and to measure well-resolved two-dimensional (2D) NMR spectra. The behavior of 1-butene adsorbed in NaX zeolite is studied by analyzing proton chemical shifts at various temperatures and measuring intramolecular proton–proton distances by means of 1H NOESY NMR spectroscopy. Drastic changes in the NMR line shape of the protons belonging to the terminal group =CH2 are found at varying temperatures (230–340 K) and pore filling factors (between ∼0.05 and 4 molecules per large cavity). There is clear evidence that 1-butene molecules interacting with the exchangeable Na+ cations in NaX zeolites reveal a change in the conformation in comparison to those in the bulk phase.