H. Friebolin

The conformers of a nitrosamine and a carboxamide: Comparison of NMR line shape and equilibration methods☆

Activation parameters for the interconversion of conformers IA and IB of neopentylbenzylnitrosamine and of rotamers IIA and IIB of N,2,4,6-tetramethyl-N-benzylbenzamide were determined by two techniques: Equilibration of pure IB (or IIA, respectively) and subsequent measurement of 1H-NMR line shapes with the same sample. Ea- and k0-values obtained by the two methods agree well within error limits, thus confirming the accuracy of rate determinations using NMR line shapes. Chemical shifts Δν, line widths bE, and equilibrium constants K were extrapolated to those temperatures at which they could not be measured directly. Line shape parameters were used for the evaluation of spectra. This method is shown to be preferable to the complete line shape analysis if the line broadening by field inhomogeneities is not taken into account by a convolution.

Konformative beweglichkeit flexibler ringsysteme. Untersuchungen mit hilfe der protonenresonanz-spektroskopie—VI. Mitteilung: Eine erweiterung und verfeinerung des verfahrens zur bestimmung der aktivierungsenergie

Abstract The methods to evaluate the lifetime of conformative molecular states by measurementsof the temperature dependence of proton resonance, have been refined and extended. For this purpose, the apparent line width is divided into two components, the temperature dependence of each being studied. It is shown, that also that part of the total line width which is not influenced by the exchange, the “Eigenbreite”, depends on temperature. The temperature dependence of the “Eigenbreite” has to be considered when using the various methods for the determination of τ, especially when the “Eigenbreite” is relatively large compared to the chemical shift. The efficiency of the method is demonstrated for some examples, i.e. ring compounds, for which the proton resonance signals of two pairs of geminal ligands, on different places in the ring system, can be analysed separately.