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

The structural and dynamic properties of 1‐bromodecane in urea inclusion compounds investigated by solid‐state 1H, 13C and 2H NMR spectroscopy

“…Even across the phase transition temperature, with a slightly enlarged diameter of the urea channel, the dynamic properties of the guest molecules cannot alter dramatically. No discontinuity at the phase transition temperature can be observed in the T 1 and T 2 curves for di-or mono-bromoalkane/UICs [26,27] ; in contrast, a pronounced discontinuity occurs for n-alkane/UICs. [13] It is thus found that the dynamic properties of the guest molecules continuously change with the elevating temperature even crossing the phase transition temperature.…”

“…A similar discontinuity of the dynamic properties of the guest molecules at the temperature corresponding to the T 1Z minimum, as reflected by a discontinuity of the Arrhenius plots, is also observed for di-or mono-bromoalkane/UICs. [27,37] The phenomenon that the transition of the molecular behaviour of the guests is delayed to a higher temperature than the phase transition temperature was also observed in stearic acid/UICs. [54] Raman spectroscopy showed that there were abrupt discontinuities in the behaviour of the bands from urea at the temperature range of 232-236 K (only within an interval of 4 K), as a consequence of the phase transition centered at ∼234 K. In contrast, the Raman bands of the guest molecule stearic acid displayed a continuous change in the range from 223 to 253 K, i.e.…”

“…This phenomenon was also reported for other systems. [27,53] Attempts were made to overcome this discontinuity, for instance, by reducing the quadrupole coupling constant, varying the site population and using the distribution of the correlation time. However, none of these attempts were successful.…”

“…Similar observations were reported previously. [13,27] From the Arrhenius plot of the correlation times (Fig. 6), for the high-temperature phase an activation energy of 11 ± 0.6 kJ/mol was determined, whereas 35 ± 4 kJ/mol is derived for the lowtemperature phase, which correspond to the two different slopes of the high-and low-temperature phases separated by T 1Z and T 1Q minima at the T 1Z and T 1Q curves, respectively.…”

“…[13] However, this is not true for UICs with α, ω-dibromoalkanes, a dynamic discontinuity was postponed to a higher temperature than the phase transition temperature. [27,37] In order to get more insight into how the local structure and the dynamic properties of guest molecules in channels are affected by the nature of the guest molecules, in this work the asymmetric guest molecule 1-bromodecane in urea is investigated. Dynamic 2 H NMR spectroscopy, including variable temperature 2 H NMR line shape, spin-spin (T 2 ) and spin-lattice (T 1Z and T 1Q ) relaxation studies, is used to explore the dynamic properties of 1-bromodecane, selectively deuterated at the brominated methylene group.…”