Dear NMR colleagus,
as a novice in solid-state NMR I dare to aks whether the three methyl groups of a tert-butyl moiety in a organic molecule may show distinct NMR signals in a 13C CP-MAS spectrum? Or is the rotation (if present?) in the solid still fast enough to average the signals?
Thanks in advance from
Alfred
Dear Alfred!
Good question and also my knowledge in solid state NMR is limited, BUT - I think I know someone you could ask: Dr Robert Guy Griffin at the Francis Bitter Magnet Lab. at the MIT. His e-mail address is: [email protected]
Send Bob my best regards!
All the best
Sture F.
If the sample is a polymer then the tacticity(actactic/syndiotactic/isotactic) could introduce non-degeneracy in the methyl signals of t-butyl groups as a whole in both solution and solid-state NMR. However, in solid-state CP MAS NMR spectrum also rotation of C-CH3 and C-C(CH3)3 should be fast enough leading to no non-degeneracy. Again, if the spectrum is recorded at a lower temperature, then t-butyl methyl groups collectively could show non-degeneracy due to steric hindrance to rotation of C-C(CH3)3 bond.
Dear NMR colleagues,
thanks for your answers. In the case I am investigating currently (CP-MAS of a small organic molecule with a tert-butyl group) I actually see all carbons at (within limit of the experiment) identical chemical shift.
Alfred
To summarize, whether you see 1 shift or 2 to 3 shifts will depend on whether the C-CH3 is hopping/rotating quickly. (This assumes there is not high symmetry in the molecule, otherwise the three could be identical anyway). The hopping rate will depend on the structure and on the temperature. A quick google search turned up this paper which may be of some help: http://repository.brynmawr.edu/cgi/viewcontent.cgi?article=1031&context=physics_pubs.
- Badges
- Science topic
- Similar topics
- Engineering
- Materials Engineering
- Powders