Browsing by Author "Yang, Xiaorong"

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Solid state NMR studies of the guest molecules in urea inclusion compounds
(2007) Yang, Xiaorong; Mueller, Klaus (Prof. Dr.)
Urea inclusion compounds are suitable to study the dynamics of the guest molecules and the interactions between guest and guest as well as guest and host molecules under very confined spatial conditions. The understanding of the functional group interactions and the molecular dynamics can provide useful information for practical applications. 1,10-Dibromodecane and 1,11-dibromoundecane/urea inclusion compounds The structural features of urea inclusion compounds with the guest molecules 1,10-dibromodecane and 1,11-dibromoundecance were examined by solid-state 13C CP MAS and 1H MAS NMR spectroscopy. The comparison of the 13C and 1H chemical shifts of the guest species in urea and solution shows downfield shifts upon enclathration for both samples with 1,10-dibromodecane and 1,11-dibromoundecane, which supports an almost all-trans conformation of the guests in urea. The dynamic properties of urea inclusion compounds with the guest molecules 1,10-dibromodecane and 1,11-dibromoundecance selectively deuterated at both end groups, were studied by means of dynamic 2H NMR spectroscopy. Variable temperature line shapes, spin-spin relaxation and spin-lattice relaxation data were obtained between 100 K and room temperature. By examining the various motional models, it is shown that 2H NMR data can at best be reproduced by a superposition of a non-degenerate 3-site or 6-site jump model and an overall chain wobbling motion with a maximal wobbling angle of 24° at room temperature. 1,6-Dibromohexane/urea inclusion compounds The structural and dynamic properties of 1,6-dibromohexane in urea were explored by solid-state NMR spectroscopy. 13C CP/MAS NMR spectra show unusual large shifts up to about 5 ppm, which is attributed to the conformation change of the guests from the co-existence of the gauche and trans conformers in solution to exclusive gauche conformational states in urea. Variable temperature 2H NMR studies were performed in a temperature range between 220 and 320 K on two samples with non-deuterated urea, and 1,6-dibromohexane deuterated either at carbons C-1/C-6 (positions) or at carbons C-2/C-5 (-positions). The quantitative analysis of the variable temperature 2H NMR line shape experiments and spin-lattice relaxation studies supports the proposed non-degenerate 2-site jump model which describes mutual exchange between the two gauche conformers within the urea matrix. It is concluded that the gauche-gauche exchange process is the dominant source for 2H and 13C spin-lattice relaxation of 1,6-dibromohexane in urea. Analysis of the T1Z and T1Q data provides the correlation times for this motional process. The activation energy of about 20 kJ/mol for the low temperature range is rather large, and reflects considerable spatial constraints imposed by the surrounding urea matrix. Above room temperature, a lower activation energy of about 9 kJ/mol along with a discontinuous change for the mobility is observed which is attributed to the gain in spatial freedom. In general, compared with its long-chain analogues, 1,6-dibromohexane guest molecules in urea experience slower and more hindered motions. 1-bromodecane /urea inclusion compounds The structural and dynamic properties of 1-bromodecane guest molecules in urea were investigated by solid-state NMR spectroscopy. 13C CP/MAS NMR studies of 1-bromodecane/urea inclusion compounds reveal that there is no preference for a particular arrangement of the chain ends, i.e. no preference for head-head, head-tail or tail-tail arrangement. The dynamic properties of the 1-bromodecane guests were examined by 2H NMR spectroscopy including line shape, spin-spin (T2) and spin-lattice relaxation (T1Z and T1Q studies). The quantitative analysis of the experimental data shows that a non-degenerate 3-site or 6-site jump process can describe the motion of the guests in the low temperature phase while the dynamic characteristics of the guests in the high temperature phase are described by a degenerate 3-site or 6-site jump model. An activation energy of 11 kJ/mole is derived for the high temperature phase and 35 kJ/mole for the low temperature phase, which correspond to the two different slopes of the high and low temperature phases separated by T1Z and T1Q minima at the T1Z and T1Q curves, respectively. For the guest motion, a small angle fluctuation, which gives rise to a reduction of the quadrupole coupling constant in 2H NMR spectra, is also considered in the whole temperature range investigated. MAS NMR studies on selected urea inclusion compounds Urea inclusion compounds with different guest species were investigated by 13C CP MAS and 1H MAS NMR experiments. 13C NMR spectra of 1-fluorodecane/urea show that the methyl group gives rise to two peaks reflecting different end-group environments. However, for carboxylic acid, the resonances of the CH3 or COOH end-group of decanoic acid display a single peak, which is attributed to the formation of exclusively head-head dimers in the urea channels via hydrogen bonding. Urea inclusion compounds with hexadecane and pentadecane in urea were investigated by 13C CP MAS and 1H MAS NMR experiments. The comparison of the 13C NMR chemical shifts of the guest species in urea and in solution shows a downfield shift for the guest species in urea, and these chemical shift alterations are again traced back to conformational changes. 13C relaxation studies show that the 13C T1 and 13C T1values exhibit a distinct position dependence. They both reflect internal chain mobility. From the semi-quantitative 13C T1 analysis of urea inclusion compounds with hexadecane and pentadecane, correlation times of ca. 10-6 s are obtained. It is argued that chain fluctuations and lateral motion of n-alkane guests may contribute to the 13C T1relaxation, although a final proof is still missing.