Study on conformations of n-alkanes in urea inclusion compounds using CH dipolar switching-angle sample spinning nuclear magnetic resonance and MM2 molecular mechanics calculations

Abstract

The CH dipolar spectra of n-heptane, n-octane, n-nonane, and n-decane in urea inclusion compounds were separately obtained for the α-, β-, and inner-CH2 and CH3 groups using the SASS (switching-angle sample spinning) NMR method. The CH dipolar coupling constant for the CH2 group decreases with the increasing distance from the center of the molecule and with the decreasing chain length. MM2 molecular mechanics calculations suggested that the included molecule can have various conformations, and provided their geometries and populations. The experimental results could be reproduced by using the calculated parameters and assuming the occurrence of dynamical disorder of various conformations in addition to rotation of the included molecule around the long axis. This is the first evidence for the existence of conformations other than all-anti in the included n-alkane molecule; it has long been taken to be in the all-anti conformation. Previous studies which were explained by the all-anti conformation are reinterpreted by dynamical disorder of various conformations.