Wednesday, October 31, 2007
Tuesday, October 30, 2007
These spectra were acquired while the magnetic field was sweeping. This will happen on a Bruker spectrometer if you do not bother locking the field (or fixing the field when running unlocked). On rare occasions in automation, if there is difficulty in locking your sample, I have seem the spectrometer run a spectrum while sweeping the field. This however is very infrequent.
Monday, October 29, 2007
Friday, October 26, 2007
Thursday, October 25, 2007
Wednesday, October 24, 2007
Tuesday, October 23, 2007
Monday, October 22, 2007
One should use this method with care as artificial cross peaks will appear for uncoupled signals with excessive t1 noise. Before symmetrization, one should look for the smallest real off-diagonal signal. Make a mental note of the signal. Symmetrize the spectrum and then scale it such that the smallest real off-diagonal signal noted above is the smallest signal in the symmetrized spectrum.
Sunday, October 21, 2007
Friday, October 19, 2007
1. Use a standard setup and get more sample or collect more scans. Yeah yeah..... I know ..... if you could do this you wouldn't have a problem.
2. Reduce the volume of the solution keeping the same mass and lift the NMR tube such that all of the sample is in the receiver coil. This is not a very desirable option as the magnetic susceptibilities of the solution inside the coil and the air outside the coil are very different and will result in a severe distortion of the magnetic field. It is very difficult to shim the magnet to compensate for this effect and the resolution and signal-to-noise ratio in your spectrum will suffer greatly.
3. Reduce the volume of the solution keeping the same mass and use a "Shigemi" tube. These tubes have plastic parts which go above and below the sample which is centered in the receiver coil. The plastic parts are chosen to have the same magnetic susceptibility as the solvent used to prepare your solution.
4. Reduce the volume of the solution keeping the same mass and use an NMR tube of a reduced diameter with a probe designed for such a tube. This is an option we have at Ottawa U. on the Varian INOVA 500 for which we have a 3 mm probe.
5. Use a Cryogenically cooled probe. These probes have cryogenically cooled coils and electronics which dramatically reduce the amount of noise thereby increasing the signal-to-noise in the spectrum. They are very expensive to both purchase and maintain and not currently an option at Ottawa U. Maybe we can ask Santa.....
Thursday, October 18, 2007
Wednesday, October 17, 2007
Tuesday, October 16, 2007
Ryan's NMR Blog is a very valuable resource for those interested in ACD's NMR prediction software as well as other issues related to NMR.
Thank you Ryan!
Monday, October 15, 2007
Friday, October 12, 2007
If you inadvertently collect a spectrum with a Nyquist fold-back you can still calculate the correct chemical shift, as the signal will be the same number of ppm away from the wrong end of the axis as it is outside of the correct end of the axis.
Thursday, October 11, 2007
Wednesday, October 10, 2007
To avoid the problem, simply stop spinning the sample.
Tuesday, October 9, 2007
Some people find it surprising that the trifluorinated methyl group comes in the aromatic region of the spectrum. Fluorinated carbons are often difficult to find in 13C spectra with low signal-to-noise ratios as the signal is spread over multiple lines and can be buried in the noise.
Friday, October 5, 2007
Thursday, October 4, 2007
Wednesday, October 3, 2007
Tuesday, October 2, 2007
The next time you are fighting a resolution problem, try using benzene-d6 as your solvent!