Variable Temperature NMR Experiment Studying Restricted Bond Rotation

Abstract

Nuclear magnetic resonance (NMR) spectroscopy has become a cornerstone tool for studying a variety of molecular features in advanced chemistry laboratories. In this experiment, students prepare a pyrrole-β-amide via a one-pot, three-step synthesis and use NMR spectroscopy to confirm its molecular structure. In addition, variable temperature (VT) 1H NMR spectroscopy is used to study the dynamic amide bond, a functional group with restricted rotation about the C–N bond. The VT NMR data allow the students to determine the coalescence temperature associated with the slow equilibrium at room temperature and the corresponding energy barrier (ΔG⧧) for the restricted rotation about the C–N bond.