200 MHz NMR Operating Procedures

NMR Preparation

Sample Preparation

1. Obtain clean and dry NMR tube.
2. Put in enough sample to cover the bottom of the tube
3. Add about an inch of deuterated solvent
   • do not add too much, it is expensive
   • do not leave the bottle open to avoid exposure to atmospheric water and evaporation.
   • do not use anything but a scrupulously clean and dry pipette and NEVER return solvent.
4. Turn off spin and lock (on the NMR console).
5. Remove the metal NMR cap on the magnet.
6. Eject sample (Press the button labeled 2^nd then the one labeled lift).
7. Replace lock standard with your NMR tube and use the depth gauge to determine proper height (Make sure the depth gauge is set for a 5mm probe).
8. Wipe the NMR tube with an alcohol or acetone-moistened lint-free tissue and dry. Make sure not to get solvent on the collar.
9. Make sure the air is flowing and place NMR tube w/ collar in the magnet
10. Turn the lift off using the console, replace the cap, and activate the spinning.

Lock Procedure A "lock" keeps the field from drifting by locking on the deuterium signal from your solvent. All lock commands use the console buttons. The lock signal is seen on the computers "fly video" monitor.

1. Adjust the field to center the peak (approximate values are on the console).
2. Temporarily increase the lock gain and lock power if necessary.
3. Auto-lock the sample (the sample is locked when the lock light is steady).

   If auto-lock fails you must do a manual lock by adjusting the field, lock gain, and lock power till a lock is achieved.

4. Decrease the lock power to the low 20's (or below if you don't lose lock in so doing) and adjust the lock gain to the point where you can see a line across the console screen. (Reduce lock power if the line is drifting up and down.)
5. Switch the console from auto-lock to lock by pressing the lock button.

Software Preparation

1. The application being used is called NTNMR. If not already open on the computer, it can be opened using the icon shortcut on the desktop of the PC.
2. After opening the application, the main window will be displayed. This is the document window. All data will be displayed in this window. Only one spectrum can be displayed per window, however, several windows can be opened at once.
3. Start by opening an NMR method. ACH1-refers to proton NMR. For C-13 NMR the appropriate initial file can be loaded from the Chem 450 folder.
4. Immediately use the "Save As ..." command to save under your file-name. Save the file frequently so that your changes are not lost.

Shimming

Since NMR is based on exposing the nuclei of a sample to magnetic fields, it is important to create highly homogenized field strengths in the solenoid prior to taking a spectrum. Homogeneity provides for the greatest resolution, sensitivity, and reproducibility. The process of homegenizing the field strength is called "shimming."

The goal in shimming is to get the lock signal as high as possible. The lock sweep is seen on the fly video monitor. (Lock gain can be used to get the signal on the monitor if it goes too high or too low.) Any shim adjustment that raises the signal is good but each shim variable is affected by the others. It is important not to select shim values that would alter the sweep so much that the magnet will be so far out of shim that the lock is lost.

To shim the magnetic field, the following procedure is followed.

1. Select "console toolbar". This can be done by either clicking on the icon on the toolbar at the top of the screen, or by selecting "console toolbar" from the "view" menu. The toolbar icon looks like a computer monitor and can be toggled on and off. Select only this button and no others. Choosing more than one button can cause difficulties in bringing up the console toolbar.
2. From the console toolbar select the tab labeled "shims".
3. Read the shims from the console or Load the most recent shim file from a similar experiment.
4. Then under spinning shim values, select boxes next to Z₁ and Z₂. This selects the two shims being adjusted. (Don't adjust Z₃ and Z₄ unless you know what you're doing.)
5. Select as step value of 20-100 in the "step value" box on the console toolbar.
6. Click the start button to start autoshimming. Autoshim is done when the stop button becomes grayed-out and the start button returns from gray to black.

Collecting a Spectrum

1. Adjust the number of scans using the dashboard button and acquisition tab (multiples of 4 are required for quadrature detection).
2. To adjust the receiver gain automatically use the Scripts>Misc. Scripts>AutoGain script or do set it manually. To manually set the receiver gain hit the RS button to rescan one scan at a time (each one writing over the last). Adjust the Reciever Gain under the "hardware" tab to position the FID so that the top and bottom of the FID are between 8 and 10.5 bits (you may need to stop the scanning to do this).
3. From the toolbar at the top of the document window, select the button labeled "ZG"-zero go (ctrl+Z). This will cause the NMR to zero out what has been collected already and start collecting scans.
   • The original output is called the Free Induction Decay (FID). It will not be Fourier transformed. To obtain Fourier transformed output while the spectrum is being collected, use the button on the tool bar that looks like it has a spectrum on it. The button that looks like an FID will toggle back to the FID.
4. As the spectrum is being acquired you can see the number of scans that have been collected in the lower right. When all of the scans are complete you should save the FID using "Save As".
5. Fourier transform and auto-process the spectrum using the ND-FT button or cntrl-F.

Analyzing the samples

• Placing the mouse pointer on a data point that you wish to look at, and then click the left mouse button- this will display the position (as ppm or frequency) and amplitude.
• To zoom in on a portion of the spectrum, click and drag over the desired portion. In the top right hand corner of the display screen is another small version of the spectrum in a box.
• Setting a reference can be done by choosing "Set Reference" from the View/Reference menu. For protn NMR, set TMS to 0 ppm or CHCl₃ is 7.27 ppm in CDCl₃. Other solvents and impurities can be found here.

Y-axis Scaling: Amplitude

• Use the up and down arrows on the keyboard for y-axis scaling.

Phase Adjustment The phase adjustment allows you to correct for leaning of the peaks.

• Select Options>Phase Adjustment. This will bring up a pop-up window.
• From this window the data can be phase corrected manually or the "auto" function will allow this to be carried out automatically. Start by selecting "auto". This will automatically correct the zero and first order phases.
• If this does not give the right adjustment, phase adjustments can be adjusted manually by using the slider bars under zero and first order phase data boxes. The zero order corrects near the pivot point and the first order corrects farther away.
• For phase corrections to be applied. Select apply at the bottom of the screen.

Integration

1. Select Options>Integrals from the menu.
2. The easiest method to use is to select auto-integrate from this menu. The selected portion of the spectrum will be integrated. The limits can be adjusted after the fact.
3. Right-clicking the integral brings up a dialog box. Selecting "Scale" will choose to make the smallest integral = 1.
4. Save your integration.

Printing

Print preview allows toggling on/off several additional parameters (including a Title or other comment, collection parameters, integration values, date, etc.). The output is set to print to the Sci 274 printer.

Wrap-up

The instrument should be relocked (see lock procedure) with the standard sample in it.