Thursday, September 27, 2007
Wednesday, September 26, 2007
If you have used gradient shimming or another type of auto-shimming and you have peculiar line shapes, then you should check the following:
1. Was the depth set correctly on the tube?
2. Does the tube have a column of solution of approximately 5 cm? Tubes that are underfilled or overfilled will cause problems.
3. Is the sample well mixed? (see BLOG entry for September 4, 2007).
4. Is the solution cloudy?
5. Are there floaters or sinkers?
6. Is the sample paramagnetic?
7. Is there any slow chemical or dynamic exchange?
Monday, September 24, 2007
1. Lock and shim your sample as usual. If you are not using a deuterated solvent then you should lock and shim on a sample filled to the same level as your sample containing the deuterated version of the protonated solvent you are using in your sample.
2. Push the "lock" button on the BSMS hand controller such that the "lock" light is NOT illuminated.
3. Push the "sweep" button on the BSMS hand controller such that the "sweep" light is NOT illuminated. (Steps 2 and 3 can also be carried out by clicking on the "Run Unlocked" button on the command panel)
4. Run the spectrum of your sample.
5. In another experiment, run the standard sample. Do not try to lock or shim.
6. Calibrate the chemical shift in the spectrum of the reference compound. For example for 31P NMR, the accepted reference is 85% phosphoric acid at 0 ppm.
7. Type "sr" and write down the number the spectrometer reports.
8. Return to the experiment where your sample was run.
9. Type "sr" and enter the number you recorded in step 7. Your spectrum is now referenced.
Friday, September 21, 2007
Thursday, September 20, 2007
Wednesday, September 19, 2007
Tuesday, September 18, 2007
Monday, September 17, 2007
"lb -1" (The more negative lb - the more resolution enhancement)
"gb 0.4" (gb is a value between 0 and 1. The larger the number, the more resolution enhancement)
"gm" (This command calculates the Gaussian weighted FID)
"ft" (This command carries out the Fourier transform)
"apk" (This command calculates the proper phase correction. You may have to phase the spectrum manually)
Try it on your own data with different values of "lb" and "gb" to get a feel for what it can do for you.
To learn more about what you are doing, read the online TOPSPIN manual or take CHM 4380/8309B.
Friday, September 14, 2007
The proton signal for the 99.6% D benzene-d6 is due to C6D5H. The lone proton on the benzene ring is coupled to the deuterons on the ring. If you look closely at the resonance, you can see partially resolved coupling as bumps on the side of the signal and a distinctly non-Lorentzian shape. If the coupling of the proton to the meta and para deuterons can be neglected, we would expect the resonance to be a 1:2:3:2:1 quintet due to coupling to the ortho deuterons.
The signal from 99.8% D chloroform-d on the other hand is due to CHCl3 (ordinary chloroform). There is no H-D coupling and as a result the resonance is sharper.
Thursday, September 13, 2007
The small peak is due to a small amount of CHCl3 in the CDCl3 solvent. The CHCl3 signal is a singlet because proton decoupling was used to collect the data. The CDCl3 signal is a 1:1:1 triplet due to the J coupling to the deuteron which is a spin I=1 nucleus having three energy levels. The chemical shift difference between the CHCl3 and CDCl3 isotopomers is called an isotope shift.
Question 1: Since chlorine has two NMR active isotopes (35Cl and 37Cl) why don't we observe J coupling between the 13C and the 35/37Cl like we do for the 13C and 2H?
Answer 1: The relaxation between the energy levels of the chlorine isotopes is very fast and the 13C "sees" each Cl at an average energy. 2H on the other hand relaxes slowly between the energy levels and the 13C "sees" all three energy states of the deuteron.
Question 2: Why don't we see an isotope shift between CD 35Cl3 and CD 37Cl3?
Answer 2: The effect is there ...... just too small to observe.
Tuesday, September 11, 2007
Daniel D. Traficante, "Elementary Trouble-Shooting for Poor Sinal-to-Noise Ratios of NMR Spectrometers" Concepts in Magnetic Resonance, 2, 63 (1990).
I must admit that when students come to me with poor signal-to-noise ratio problems, the first thing I check is the sample. Most often this is the problem as many people really have no idea how much sample they have in their tube or the sample is cloudy or has precipiate or is not filled properly or is paramagnetic or is a complex mixture......... If the sample appears OK, I use the logic in the note above to help isolate the problem.
Sunday, September 9, 2007
Friday, September 7, 2007
Part 1 (1925-1955, 225 references)
Part 2 (1956-1959, 301 references)
Wednesday, September 5, 2007
Hmmmmmm....... I see that "PENIS" is not listed on the above website. It stands for Proton Enhanced Nuclear Induction Spectroscopy. For reasons of political correctness, it is now called "CP" (Cross Polarization).
Tuesday, September 4, 2007
Mix your samples well - get better data.