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.

Synthesis and characterization of four N-acylhydrazones as potential O,N,O donors for Cu2+: An experimental and theoretical study

“…Afterward, using compound 4a as a representative, 1 H-NMR experiments at different temperatures were carried out, which allowed to determine that coalescence of the duplicated signals was observed (Figure 10). Again, this result supported the hypothesis of two rotational isomers: the energy required to more quickly overcome the rotational barrier is reached at higher temperatures, causing rapid interconversion between the two conformers, which is not detectable on the NMR time scale [38]. Another remarkable feature derived from the experiments, which agrees with the hypothesis is that the reversibility of the interconversion was confirmed when the experimental temperature was returned to 27°C.…”

Cy-clo-tetrakis(μ-diphenylphosphido)-1,5-bis(tri-tert-butylphosphine-tetracopper(I)

“…A coalescing of the alkyl peaks was observed upon increasing the temperature from 25 °C to 45 °C. This behavior is consistent with hindered bond rotation, slow conformational change or a derivative speciation of 1 in solution under rapid exchange on the NMR time scale[19].Similarly, 31 P NMR spectra of 1 have features that show evidence of dynamic behavior. A spectrum obtained at 25 °C initially appears to have two broad singlets, but closer inspection by enlarging the peaks reveals multiplets at  = -19.5, 60.1, and 62.3 that integrate in a 1 : 0.15 : 1 ratio.…”

Cyclo-Tetrakis(μ-diphenylphosphido)-1,5-bis(tri-tert-butylphosphine)-Tetracopper

“…The tert-butyl substituents in the 1 H spectrum of 1 are split into several overlapping multiplets centered around δ = 1.11 and 1.28. To confirm that the multiple alkyl peaks were features of 1 in solution, and not impurities, a variable temperature (VT) NMR experiment was performed in which 1 H NMR spectra were taken at 25 • C, 35 • C, and 45 • C. A coalescing of the alkyl peaks was observed upon increasing the temperature from 25 • C to 45 • C. This behavior is consistent with hindered bond rotation, slow conformational change or a derivative speciation of 1 in solution under rapid exchange on the NMR time scale [19].…”