Tuesday, September 16, 2008
Fast Molecular Motions and Solid State Wide Line 2H NMR
On occasion, I have been asked why people make such a big deal about solid state wide line 2H NMR. After all, 2H has a low natural abundance and isotopic labelling is necessary to collect the data. The answer is that the 2H line shapes depend on the interaction between the 2H quadrupole moment and the electric field gradient tensor surrounding the 2H nucleus. The electric field gradient tensor is dramatically affected by the averaging of different types of molecular motions and this effect is readily observed in the NMR line shapes. The line shapes are sensitive to the rates, order and axes defining the motion. For molecular motions occurring at rates fast with respect to the width of the static 2H NMR spectrum, one can determine the type of molecular motion by doing reasonably straight forward calculations or often simply by inspecting the motionally averaged spectrum. The figure below shows the effects that some common fast molecular motions have on 2H NMR line shapes. The same static quadrupolar coupling constant of 160 kHz was used in the calculation of all of the spectra in the figure.
The line shapes observed for molecules undergoing motions at rates comparable to the width of the static 2H NMR spectrum can also be calculated however, the calculations are a bit more involved.