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The 13Carbon NMR spectrum of an organic compound provides information concerning:
The major differences that you will notice in 13C-NMR in comparison to 1H-NMR spectra include:
Before you log on -- Open the login book and enter your name along with the date and time. Check to make certain no one is currently using the instrument, enter you password one the log-on computer. If the insturment is currently being used, check with the person to see if you can log them off and log yourself on. Normally if a user forgets to logoff the instrument is locked.
Logging in -- Move the mouse to see the login window. Type your login name and when prompted, your password. Select the grant for the project you are working on. In the login process, a series of programs start. Wait until you see the Front Panel:
The most important applications are on the front panel: vnmr (an icon with a blue spectrum on a black background), and the File Manager (an icon with an open filing cabinet). To open an application click on it once.
Click on the vnmr icon on the Front Panel once. Wait until the Acqi button shows up at the right of the first row of buttons, and the Acquisition Status-window opens.
Menu-driven operation. The most important
commands can be called by clicking on the buttons in various menues.
However, it does not give access to the full range of commands.
The Main Menu button is on the first row of buttons at any time.
Click on it to retrieve the second row of buttons on the Main
Menu. These buttons are arranged in the normal sequence for obtaining
a plot of a spectrum. In any menu, the buttons on the second row
are defined by clicking the Help button.
Command mode. Type commands in the Input Window.
Selecting the Workspace -- NMR spectra are obtained in experiments. An experiment is actually a directory containing files of the text associate with the sample, the file with the parameters for acquisition and data processing, the FID file, and the spectrum file after you transformed the FID. Each user has his own experiments, which means data in an experiment will be the same, no matter what the other users have done. The data in an experiment is lost if the set up command is given, if another FID is retrieve, by starting an acquisition, or by delete the experiment. When you log in for the first time, you have just one experiment. To create more, click Workspace - Create New. Do not create more experiments that you need at a time. They occupy disk space. Use the Delete button to erase the unnecessary ones. To see the text associated with each experiment click Library. To move to (join) experiment 3, click the Exp 3 button, or type jexp3 in the input window.
Setting the Parameters -- From the Main Menu click Setup. The setup copies standard parameters file into the parameters file for the current experiment. First, specify the nucleus and the solvent. For the five solvents listed as buttons, appropriate lock parameters can be retrieved. There are buttons defined for proton or carbon in d-chloroform. All the frequencies are set based on the lock frequency. Select the solvent correctly or the spectral window will be improperly set and some region of the spectrum will show up folded. Or select the parameters by typing in the input window, e.g. setup(‘C13',’CDCl3'). To display the parameters at any time, type in the input window dg (display group). Not all the parameters are listed by dg. To inquire about the value of a parameter, type the parameter name followed by question mark, e.g. gain?. To modify the value of a parameter, type parameter_name=new_value. Note that text values should be in quotes, e.g. gain=’n’.
1. Remove standard and Insert sample into the magnet
5. Manual lock
6. Manual shim
7. Now change parameter nt, then type ga to collect your data
9. To Save Click Data, then type svf and file name
10. Plot Data
11. Remove Sample and put Lock Standard Back
1. Remove the sample from the top of the magnet by holding onto the top of the spinner. Remove the previous sample from the spinner with a firm pull on the top end and store it in a safe place. Insert your sample by typing commands e and i.
To Insert a Sample into the Magnet --
Push the sample tube through the spinner using a twisting motion
to avoid breaking the sample tube. The bottom of the sample should
be inserted into the wide end of the
spinner first. Adjust the spinner's position using the gauge.
Wipe the outside of the tube and the spinner with a soft tissue.
When moving the sample/spinner assembly, always hold the top of
the spinner to avoid repositioning of the spinner. Never insert
a sample unless you know the bore of the probe and have a sample
in the spinner.
• Place the sample tube and spinner into the transfer tube opening in the center of the magnet. Make sure that there is enough air to support the sample before letting go of the sample tube and spinner. Type 'i' so that the sample will be lowered into the probe. Listed for two click sounds (or a click and then a cluck) which indicate if the sample tube sits in the right position.
• If you do not hear the "click" of the sample resting in the probe or the spin indicator light does no stay on (it will blink until the sample is spinning at the desired rate), try ejecting and inserting again. Cover the transfer tube opening in the center of the magnet briefly with one hand and wait for the spinner light to turn on.
Note -- Always leave a sample in the magnet with the lock on at a level of 30% to 70%. The purpose of using a lock solvent is to minimize magnetic field drift.
2. Set up parameters by clicking setup, then select your nucleus and solvent, or by typing rtp, then answer carbon 13: C13. If the solvent is correct then type su.
3. Retrieve shim set by typing rts, plus the shim file name, such as cdcl3, then su.
• Type rts, at the prompt, enter cdcl3, and then su. This will load a standard shim settings of the using probe. This is a good start to shim your sample.
• Once a good shim setting has been established for a sample type, it is generally possible to swithc between different neucleus without further shimming.
4. Tune. Only if necessary, details are given at the end of these instructions.
5. Manual lock . Or try autolock (type lock) if manual lock is difficult.
• Enter 'acqi' to open the aqcisition window. Turn the lock control off. The lock signal appears as a sine wave if the lock signal is off-resonance, and as a dc signal if the lock signal is on-resonance. Adjust Z0 until the lock signal is on resonance.
• Increase the lock power to 20 and the lock gain to 30 or higher until you see sine wave signal. Adjust Z0 to get a dc signal.
• Adjust the lock power and lock gain until the dc signal is from 30% to 70% of the full screen.
• Turn the lock on. Adjust the phase so that the dc signal is highest at the above setting of the lock power and lock gain.
6. Manual shim. Or try autoshim (type shim) if manual shim is difficult.
• The screen will show a digital lock amplitude and two horizontal lock signal bar graphs. The top one shows the total lock signal, the bottom one shows either 0-20%, 20-40%, 40-60%, 60-80%, or 80-100% of the full scale, using different colors for each range.
(A) Adjust each of the following to maximize lock signal- Z1C, Z2C, Z1, Z2.
(B) Increase Z1 by 20 units, maximize lock signal with Z2.
(C) If level is higher or the same, repeat step (B). If it is lower go to step (D).
(D) Decrease Z1 by 20 units, maximize lock signal with Z2.
(E) If level is higher
or the same, repeat step (D). If it is lower, increase Z1 by 20
units
and
maximize lock signal with Z2 and the sample is shimmed.
Continue the steps until you can no longer maximize the signal.
7. Now change parameter nt, then type ga to collect your data.
8. Process and print out data.
Type command wft if you have used go instead of ga. The following are a few possible
commands to further process the spectrum if needed: to display
the scale click Dscale or type dscale if
the scale is in hertz but you need ppm, you can type axis=’p’
to view the full spectrum in the full screen f full, to
adjust the vertical scale use the middle mouse button directly
on the spectrum or type vs=value wanted, to zoom in on the region
within the cursors left-click on the left and right-click on the
right of a region in the spectrum then click Expand to change
the limits of the spectrum to specific values, for example 1.0
ppm and 6.0 ppm: sp=1.0p wp=5.0p. For baseline correction
type dc, or bc to reference a peak, for example,
set acetone to 2.05 ppm (in organic
solvent) or 2.225 ppm (in D2O) click as near as possible
to the centre of the acetone peak, nl rl(2.225p).
For peak-picking
dpf
displays the peak frequencies
th allows
you to adjust the threshold with the left mouse button.
9. Data are not normally saved but it possible to save fid in a data directory. Click Data, then type svf and file name.
10. Plot Data -- Click Main Menu/ Display/Plot/Plot/Scale/other options/Page
Page is the command that sends the information to the plotter. Every option you click before page will be included in the plot.
• Take out your sample and insert the lock sample back into the magnet.
• Reset lock for the lock standard.
• If somebody is waiting for you to finish, insert his/or her sample instead of the lock sample. Make sure the sample is spinning. In return, the person will log you off the computer.
First close down VNMR and then exit the software. Reverse the logon procedure.
Tuning -- NOTE: If you are tuning for the first time on this machine, ask for help of someone who has done it before! You may not tune the probe unless you have been qualified.
1. Insert the sample.
2. Setup 1D proton parameters (make sure tn=’H1’, dn=’C13’ and correct solvent). Type su. Now machine knows CHAN1 for H1 and CHAN2 for C13.
3. Disconnect the cable from the 1H preamplifier and connect it to the PROBE J5321 port on the TUNE INTERFACE panel.
4. Press the CHAN + button so that the readout is channel 1 for proton. Make sure the red indicator light is not flashing before you proceed. If it is flashing, check the connections you just made with the two cables.
5. Tune the probe using the appropriate stick (note that the
red stick always for proton). The
upper, smoother part is the tune and the lower, rougher part is
the match. As the probe gets closer to being tuned, the
number on the INTERFACE display will decrease.
6. Press the ATTEN buttons until the readout is 8. Continue tuning the probe until the displayed number is less than 10.
7. If the numbers do not change easily (or no response) when you turn the rod, there must be something wrong. Find someone who can help you. Do not force the tuning rod or turn it too far.
8. Disconnect the tuning function by pressing the CHAN buttons until the readout is 0 (otherwise you will not be able to acquire).
Reconnect the rf cables to their original positions. The red indicator light should turn off when you have done so.
Repeat the process for tuning C13, by choosing CHAN 2, and C13 cable connections.