Solid-State NMR Studies on the Guest Ordering and Dynamics in α,ω-Dibromoalkane/Urea Inclusion Compounds

Abstract

Solid-state nuclear magnetic resonance (NMR) spectroscopy is utilized to study the molecular behavior of 1,10-dibromodecane and 1,11-dibromoundecane in their urea inclusion compounds. The guest dynamics and conformational order are explored by 13C cross polarization magic-angle spinning (CP/MAS) and 1H MAS NMR spectroscopy which confirm an all-trans conformation of the guest chains. Dynamic 2H NMR experiments are carried out on two guest molecules selectively deuterated at both end groups. A quantitative analysis of the experimental data, obtained from variable-temperature line shape, spin–spin and spin–lattice relaxation measurements, shows that both guest molecules undergo similar motions within the investigated temperature range between 100 and 298 K. The combination of nondegenerate 6-site (or 3-site) rotational jumps and small-angle overall chain wobbling provides an appropriate motional model for the guest motions in these compounds. It is found that the populations of the jump sites exhibit a characteristic temperature dependence, although a discontinuity is missing at the solid–solid phase transition. The same holds for the guest motions which also remain unaffected by the change of the urea lattice structure. Rather, a discontinuity of the guest dynamics at about 30 and 10 degrees above the corresponding solid–solid phase transition is observed for 1,10-dibromodecane and 1,11-dibromoundecane in urea, respectively. Likewise, there is no clear evidence for an odd–even effect due to the change of the guest chain length on the molecular properties of the present inclusion compounds. As a general result, it is concluded that the intermolecular interactions in the present materials are stronger than in n-alkane/urea inclusion compounds.