Molecular Mechanics and Variable-Temperature 1H NMR Studies on N,N-Diethyl-m-toluamide. An Undergraduate NMR and Molecular Modeling Experiment

• Bruce L. Jensen ^[1] ; Raymond C. FortJr. ^[1]
  1. [1] University of Maine

     University of Maine

     Town of Orono, Estados Unidos

     
• Localización: Journal of chemical education, ISSN 0021-9584, Vol. 78, Nº 4, 2001, pág. 538
• Idioma: inglés
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• Resumen

  • A combination of molecular modeling and variable-temperature NMR experiments is used to analyze the barrier to rotation about the amide bond of N,N-diethyl-m-toluamide (DEET). This approach utilizes the ability of computers to calculate the potential energy of a set of conformations obtained from a dihedral drive around the N-CO bond. The results of this experiment demonstrate a substantial barrier of 15.9 kcal/mol. These data are applied to a set of 1H NMR spectra taken over a range of temperatures from 9 to 85 °C. At very low temperatures the conformation is "locked" and the spectrum displays two sets of triplets and two sets of quartets for the two nonequivalent ethyl groups. However, at high temperature the rapid rotation about the amide linkage produces only one quartet and one triplet, characteristic of two indistinguishable ethyl groups. The experiment offers students hands-on experience with two important laboratory instruments and allows for both qualitative and quantitative analysis of the data. This experiment is scheduled to coincide with lecture discussion of NMR spectroscopy, after the fundamentals of bond rotation have been presented.