Molecular mobility depending on chain length and thermally induced molecular motion of n-alkane/urea inclusion compounds

Abstract

Solid-state high resolution 13C NMR was used to analyze the end group conformation and molecular mobility of n-alkanes in a urea host as a function of the carbon number of the n-alkane. It was shown that the chemical shift of the inner methylenes could be interpreted by the γ- gauche effect. Of further interest is our finding that the chemical shift of 3-methylene is independent of both chain length and temperature, a result indicating that the torsional rotation of the bond ω 3 between the 4-methylene and 5-methylene carbons is so inhibited that there is little gauche conformation. The chemical shift of the inner methylenes indicated a different tendency between the even- and the odd-numbered n-alkanes. The fact that the signals of the even-numbered n-alkanes were observed at a comparatively more upfield location than those of the odd-numbered ones indicated that the even-numbered n-alkane had a higher molecular mobility and tended to adopt a more gauche conformation. The decomposition temperature obtained by thermal analysis also suggested a difference between the even- and odd-numbered n-alkanes. The decomposition temperature of the even-numbered n-alkane/urea inclusion compounds was a little lower than that of the odd-numbered ones, a disparity corresponding to the higher molecular mobility of the n-alkane in the urea host. The spin-lattice relaxation time ( T 1C) increased with increasing chain length for chains with less than the 14 carbon atoms but reached a constant value for all longer chains. This result is completely different from that for the n-alkane crystal, which gave a longer T 1C depending on the chain length, and can be explained by a reduced intermolecular interaction between the n-alkane and the urea host. Clearly, T 1C measurements can be applied to confirm the formation of inclusion compounds. However, the different T 1C values between the methyl, 2-, 3-, and inner methylene carbons indicates that the n-alkane molecule does not rotate so fast to provide the same molecular motion. When the temperature was raised, the chemical shift of the inner methylene carbons moved upfield due to the γ- gauche effect; this shift correlated with a higher molecular mobility at the chain end.