engleski

1H NMR Study of Gelation Process by Stilbene Oxalamide Compound

N-L-Leucine methyl ester-N'-[4-(4'-dodecyloxy)-trans-stilbene]-oxamide acts as an efficient low molecular weight organic gelator of various organic solvents. Thermoreversible and stable organogels are preferably formed by intermolecular hydrogen bonding additionally stabilized by  - stacking and lipophilic interactions. In order to get an insight in to organizational motifs of the molecular self-assembly in polar as well as in apolar aromatic solvents, concentration and temperature dependent 1H NMR spectra of the gelling substance were obtained in dimethylsulfoxide and toluene. Concentration increase leading to aggregation and gel formation results in broad peaks with unresolved J-coupling, which is characteristic of molecules dissolved in entrapped solvent [1]. Signals of the assembled molecules are not observed due to the suppressed mobility of the gelator molecule organized in rigid gel network and to long correlation times. On the other hand, temperature increase causes an increase in proton peak intensity implying a collapse of the gel network and, finally, dissolution of the fibrous aggregates. Using an internal standard, concentration of dissolved gelator, cd, is determined and the dissolution equilibrium constant, Kd, evaluated [2]. From the plot of lnKd versus 1/T, the dissolution enthalpy,  Hd, and entropy,  Sd, are estimated. Moreover, temperature induced chemical shift changes reveal intermolecular interactions involved in gel fibers formation. An up-field trend of NH protons following the gel dissolution indicates hydrogen bonding between oxalamide fragments of the molecules, whereas a down-field shift of the aromatic protons confirms  - stacking of stilbene moieties.

gelation; 1H NMR spectroscopy; stilbenes

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