The guest ordering and dynamics in urea inclusion compounds studied by solid-state 1H and 13C MAS NMR spectroscopy

Urea inclusion compounds with different guest species were studied by 13C CP MAS and 1H MAS NMR spectroscopy. It is possible to arrange the asymmetric guest species in three different ways: head–head, head–tail and tail–tail. 13C CP MAS NMR studies indicate that the preference arrangement is determined by the interaction strength of the end functional groups. 13C relaxation experiments are used to study the dynamic properties of urea inclusion compounds. 13C relaxation studies on urea inclusion compounds with n-alkane or decanoic acid show that the 13C T1 and 13C T1ρ values exhibit the position dependence towards the center of the chain, indicating internal chain mobility. The analysis of variable-temperature 13C T1ρ experiments on urea inclusion compounds with hexadecane and pentadecane, for the first time, suggests that chain fluctuations and lateral motion of n-alkane guests may contribute to the 13C T1ρ relaxation.