Friday, August 29, 2008
The NMR Time Scale
Many people use statements like, "It is fast on the NMR time scale" without really having an understanding of what the "NMR time scale" means. Assume we have a compound with two inter-convertible conformations, A and B, each of which gives a different NMR spectrum. If the interconversion between A and B is very slow then we expect to get a spectrum with both conformations A and B resolved. If the interconversion is very fast then we would get an unresolved spectrum which represents the average conformation of A and B. The interconversion between A and B is fast on the NMR time scale if it occurs at a rate much greater than the difference in frequency between A and B. Since the difference in frequency between A and B depends on the Larmor frequency of the nucleus being observed and the strength of the magnetic field, the "NMR time scale" depends on the particular experiment and the field strength. Therefore, when one uses the expression, "NMR time scale" one must qualify it with details of the measurement. An example of different NMR time scales is shown in the figure below. Here the 1H (300 MHz) and 13C (75 MHz) NMR spectra of the N-methyl groups of N,N-dimethylacetamide are shown at various temperatures. The spectra were plotted on the same scale in Hz. In this molecule there is a hindered rotation about the central C-N bond. One can see that the rotation is fast on the 1H NMR time scale before it is fast on the 13C NMR time scale because the difference in frequency between the methyl groups is smaller in the 1H spectrum compared to the 13C spectrum.