An alternative NMR method to determine nuclear shielding anisotropies for molecules in liquid-crystalline solutions with 13C shielding anisotropy of methyl iodide as an example

Abstract

An alternative NMR method for determining nuclear shielding anisotropies in molecules is proposed. The method is quite simple, linear and particularly applicable for heteronuclear spin systems. In the technique, molecules of interest are dissolved in a thermotropic liquid crystal (LC) which is confined in a mesoporous material, such as controlled pore glass (CPG) used in this study. CPG materials consist of roughly spherical particles with a randomly oriented and connected pore network inside. LC Merck Phase 4 was confined in the pores of average diameter from 81 to 375 Å and LC Merck ZLI 1115 in the pores of average diameter 81 Å. In order to demonstrate the functionality of the method, the ¹³C shielding anisotropy of ¹³C-enriched methyl iodide, ¹³CH₃I, was determined as a function of temperature using one dimensional ¹³C NMR spectroscopy. Methane gas, ¹³CH₄, was used as an internal chemical shift reference. It appeared that methyl iodide molecules experience on average an isotropic environment in LCs inside the smallest pores within the whole temperature range studied, ranging from bulk solid to isotropic phase. In contrast, in the spaces in between the particles, whose diameter is approximately 150 µm, LCs behave as in the bulk. Consequently, isotropic values of the shielding tensor can be determined from spectra arising from molecules inside the pores at exactly the same temperature as the anisotropic ones from molecules outside the pores. Thus, for the first time in the solution state, shielding anisotropies can easily be determined as a function of temperature. The effects of pore size as well as of different LC media on the shielding anisotropy are examined and discussed.