Resumen de NMR Determination of Hydrogen Bond Thermodynamics in a Simple Diamide: A Physical Chemistry Experiment
Janine G. Morton, Candice L. Joe, Massiel C. Stolla, Sophia R. Koshland, Casey H. Londergan, Mark H. Schofield
Variable temperature NMR spectroscopy is used to determine the ΔH° and ΔS° of hydrogen bond formation in a simple diamide. In this two- or three-day experiment, students synthesize N,N′-dimethylmalonamide, dimethylsuccinamide, dimethylglutaramide, or dimethyladipamide from methylamine and the corresponding diester (typically in 50% recrystallized yield) and record NMR spectra at temperatures between 200 and 313 K. Solutions of N-methylacetamide in concentrations between 1.00 mM and 13.1 M (neat) are prepared and their NMR spectra recorded to determine the chemical shift of the amide proton in the hydrogen-bonded (δB) and nonbonded (δN) limits. By using these data, the equilibrium constants, ΔH°, and ΔS° for the conversion of an open-chain diamide to a cyclic structure with an intramolecular hydrogen bond are determined.
