One gets sharp lines in the NMR spectra of liquids (or solutions) because the rapid isotropic motion of the molecules averages out dipolar interactions, chemical shielding anisotropy and quadrupolar coupling, all of which can broaden NMR resonances. In rigid solids, there is no isotropic motion and the interactions mentioned above will broaden out the NMR resonances to the point where they will lost in the baseline of typical spectral widths employed for solution state NMR. So, for rigid solids, you are likely to see nothing. Despite what many people believe, all solids are not rigid. Many exhibit some type of molecular motion (eg. methyl group rotation, phenyl ring flips, rotation about eta bonds etc...). Some compounds called "plastic crystals" even exhibit isotropic motion. An example of such a compound is adamantane. The bottom trace of the figure below shows a conventional solution state 13C NMR spectrum of adamantane with proton decoupling. The top trace shows the spectrum of the solid acquired in exactly the same way. Although the resolution for the solid is not as good as that for the the solution, one can still resolve the two types of carbon.
2 comments:
Glenn, your readers might be interested to know that one of the first experiments I did on our Nicolet 360 was to measure the Pb-207 shielding powder pattern for a lead salt (which salt escapes me - no protons around obviously). The important point for your post is that it was measured with a 10 mm high-resolution BB probe...and gave a beautiful result! The principal components of the shielding tensor were easily obtained.
Mike
nmrfreak,
I had a similar experience with the same 10 mm BB probe on that Nicolet 360 back in 1983. I ran the solid state 77Se and 2H NMR spectra od H2Se, HDSe and D2Se at low temperature to observe the phase transitions. Collecting that data really made an impression on me. From T1 experiments, we found that the molecules in the plastically crystalline solid phase move faster than those in the liquid state. I think that is what really go me interested in NMR.
Post a Comment