CHE 450G: Practical Inorganic Chemistry

U of KY Dept of Chemistry

Varian Gemini NMR Instruction Sheet

Step by Step ¹H NMR Instructions

1. Log onto the machine. This requires a username and password. You may use this instrument ONLY with the assistance of a trained user.

2. Eject the sample that is in the machine and put your sample in. Your TA will explain how to do this.

3. Your TA or John Layton can log you in. If you don't see an open window with a big black empty space click on VNMR in the long window at the bottom of the screen to open up the VNMR program. If you don't even have that, try double clicking an icon or two to see if you can open one of these two windows.

4. Click on the Setup button and choose an appropriate nucleus and solvent. For example, for a proton spectrum in chloroform click on the H1,CDCl3 button, or for a proton spectrum in benzene click on Nucleus,Solvent followed by H1 and Benzene.

5. Click on the Acqi button which will open up a smaller new window titled 'gembb.chem.uky.edu ACQUISITION'.

6. Click on the Lock button. You will be presented with the deuterium lock signal. If the spectrometer has locked onto the signal the trace should look like a flat step function and the lock level will be fairly constant and above 30.
   If it is not locked, try raising the lockpower and or lockgain. The controls for this work just like the shim controls (see below). If that doesn't work, close the window. Type rts(solvent) su (where solvent = your solvent such as cdcl3, c6d6, toluene etc.) and start again from the previous step . If you still don't get a lock, ask for assistance.

7. "Shimming" means that we are simply adjusting the magnetic field inside the instrument to be as homogeneous as possible. To shim the field, click on the Shim button. You'll see two horizontal bar graphs that show the lock level. The bar on the top is a tens unit and the one on the bottom is a ones unit. The same information is also displayed more simply as a number.

   Below this display next to the word 'Shim' there is a small triangle that switches between different lists of shimming parameters when you click on it with the left mouse button. You do not need to adjust all the parameters, just those listed below.

   To adjust a value, simply click on the buttons labeled -1+, -4+, -16+ and -64+. Think of these as fine through coarse controls; clicking on -16+ changes the value 16 times as much as clicking on -1+. If you click with the left mouse button you decrement the value of that parameter and if you click with the right mouse button you increase the value of the parameter.

   Shimming is accomplished by adjusting each shim parameter to maximize lock level. It is not the absolute number we care about (these can always be changed using the power and gain), rather we want to get the highest level we can while having the bars (lock level) read on scale.

8. To shim the instrument simply:
   1. Make sure your lock level reads on scale as described above.

   2. Adjust the Z1C (z1 coarse) and Z2C (z2 coarse) buttons to shim. You shouldn't need to use more than -1+ or -4+ to do this. Maximize the reading on the scale using Z1C and then do the same using Z2C. Iterate this procedure back and forth until you can not get the bars/numbers to go any higher. If the lock level pegs at the top of the scale, reduce the lockpower and/or lockgain until the meter again reads mid-scale (you'll need to toggle through the parameter list to see these parameters).

   3. Use the Z1 and Z2 controls to finish shimming (these are about 5x less sensitive than the coarse controls). Normally all you need to use is the -16+ button.

   4. Now maximize the lockphase (you'll need to toggle through the paramter list to see this parameter).

   5. When you are finished, adjust the lock power and lock gain so the meter level is somewhere between 40 and 80.

   6. Click on the Close button to exit the Acquisition window. Do not attempt to adjust the other shims such as Z0, Z3, Z4 etc.

9. Click on the top window, type nt=4 and hit return. This sets the number of scans to 4. We're going to make sure that our peaks look OK before collecting a full set of data.

10. Type su ga. The spectrometer will collect 4 scans and then display your spectrum (see below on how to manipulate things). Adjust the phase by typing aph and make sure that your peaks look good -- they should be Gaussian in shape and not have humps to either side. Typical problems with shims will give humps or even extra peaks on the side of your real peaks. Fortunately, this problem will show up in all of the peaks of your spectrum, so they are easy to spot.
    If you suspect your peaks are misshapen due to a bad shim, go back and try shimming again. If this still does not help type try using the rts command described above. The rts command ("return to shims") loads in a set of values that should be close to the values that you need for a particular solvent.

    If you are still not having any luck, ask your TA or John Layton for help. There is a manual phase adjustment, but its use is too complicated to explain here.

11. If everything looks good, decide how many scans (transients) you want to collect. For example, to collect 16 scans and begin acquisition type nt=16 ga.

12. While your scans are collecting, it is a convenient time to enter text about your sample. Type text('your sample information here'). Don't forget the single quotes and parentheses. Use a double backslash to begin a new line. For example:
    text('RTI-155A, Crude product from\\ether/pentane, orange oil\\yellow C6D6 solution\\8:55P 09-10-98') will produce:
    RTI-155A, Crude product from
    ether/pentane, orange oil
    yellow C6D6 solution
    8:55P 09-10-98

13. When your scan is done type aph f to automatically phase your spectrum and display the full spectrum.

14. There is a button that toggles between Cursor and Box. Click the left and right mouse buttons to get two cursors. Place these on either side of your residual protio solvent peak and then click on the Expand button. Place one cursor on the center of this peak and type nl rl(x.xxp) where x.xx is a number you need to supply. A list of residual solvent shifts can be found be the spectrometer. Typical values are 7.24p for CDCl₃ and 7.15p for C₆D₆. Do not forget the lowercase "p".

15. Click on the Full button to display the full plot. Use the two cursors to expand the area you wish to plot. Alternatively, you can type in something such as sp=0p wp=8p to display, for example, 0 to 8 ppm automatically. Typing in the values ensures you can plot all your spectra on the same scale and overlay them for easy comparison later. To change the vertical scale click and drag on the spectrum with the middle mouse button. Make sure all your peaks are as high as they can be without being cut off at the top. You can also set the value of the vertical scale, for example vs=200.

16. To integrate your spectrum find the button that toggles between No Integral, Part Integral and Full Integral. Select Part Integral to automatically integrate and type cz dc to automatically clear any integral resets and level your baseline.

17. Click the Resets button and then cut (zero) your integrals as needed. Working from left to right, move the mouse and click the left mouse button once where you want the integral to be cut. If you make a mistake the right mouse button undoes a cut (or you can use the cz command to wipe all the cuts). Adjust the height of the integrals by clicking and dragging with the middle mouse button when you are done. Make sure the integrals are as tall as possible without being cut off at the top of the page.

18. To plot your spectrum type pl pscale hpa which plots the spectrum, scale and parameters, respectively. If you are not going to do inset plots, type page to dump the spooled file to the plotter.

19. To generate a peak list first toggle through the Integrals button to get rid of your integrals and then press the Th (horizontal threshold) button. Click and drag with the left mouse button to set the height of the peak list threshold. Pick a level that intersects all your peaks of interest, but don't put it so low that it picks off every single point in the baseline. Then type printon dll printoff to send the peak list to the printer.
    dll will list only those peaks that are displayed on the screen and that are above the threshold. For complex spectra it may help to expand several regions and dll them individually.

20. When you are finished, remove your sample and replace the one that was in the machine when you got there. NEVER PLACE THE SPINNER BACK INTO THE INSTRUMENT WITHOUT A SAMPLE!

21. Log off the instrument by typing exit. A confirmation message will come up. Click OK or hit return to finish logging out.

    As always, be sure to report any problems to your TA immediately.

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This page was last updated and is copyright 1996-2000 by Rob Toreki. All rights reserved.