Assignment 2
NMR Guided Reading
January 28, 2000
There are quite a few parameters which are crucial to acquiring and processing a NMR spectrum. We will first deal with the acquisition parameters and then a few of the processing parameters.
You will want to have Derome with you. Hopefully you have taken a look at section 2.3 through 2.5. These sections will be particularly helpful for these exercises.
The acquisition parameters in which we are most interested are the following:
I believe I have the correct abbreviations for the MacNMR.
Start out by placing the ODCB (ortho-dichlorobenzene, Bruker sample) sample in the magnet. Lock on the d6-acetone sample and shim. With d6-acetone you will find it much easier to saturate the lock signal than with D2O. Play with it a bit and make sure that you get the lock power low enough (my guess is somewhere in the mid-20's). Get as good of a shim as you possibly can. It probably won't be good enough -- this one is really hard. You will refine it even more, once you get your proton spectrum.
Receiver Gain: Load in standard acetone acquisition parameters, do an automatic receiver gain setting and acquire a spectrum with 8 scans in it. Save that FID somewhere safe so you can look at it later. Let's first explore what happens when you set the receiver gain too high or too low. Note the receiver gain and then crank it up quite a bit. Acquire a spectrum. Notice that the FID will be chopped at the top and bottom for the first portion of the FID. When you fourier transform the FID you will notice a substantially rolling baseline. Reduce the receiver gain substantially and observe how the signal to noise ratio is decreased substantially. Set the gain back to what it should be.
Offset Frequency and Dwell Time: The offset frequency is fairly easy to understand. It is simply where the center of the spectrum lies. It is expressed in KHz in the MacNMR software as O1. This is an offset frequency which is added to a base frequency expressed in MHz. The two combine to give the spectral frequency (also in MHz) which corresponds to the center of the spectral range in a given experiment. One normally centers that in the middle of the proton range of 0 to 10 ppm. We now want to set the spectral frequency to be in between the two proton resonance envelopes in the aromatic region of ODCB (each of those signal envelopes corresponds to a pair of magnetically inequivalent, but chemically equivalent, protons in the AA'XX' system of ODCB). To do that set your cursor in between the two and then click on Set Spectrometer Frequency under one of the menu items.
Acquire a spectrum. After transform, the spectrum should now be centered between the two proton envelopes. Expand around one of the envelopes and examine the signals. Compare the spectrum to those shown on page 21 of Derome. When we are done we should have one that looks like the sharp spectrum showing the details of the coupling and one which looks lumpy like the bottom one. This one will look pretty lumpy. The problem is that you have not taken enough data points to fully characterize the signals (and possibly your shim is still not good enough).
We want to concentrate on solely this area of the spectrum, so let's change the sweep width (SW). First, look at the values for Acquisition Time, SW and Dwell. As described in Derome there is a reciprocal relationship between these two parameters. Dwell stands for the amount of time in between each data point in the FID. The Sweep Width is the frequency range in Hz covered by the experiment. If we change either parameter the program is smart enough to change the other one. So, change the SW to +/- 200 Hz. This gives us a total window width of 400 Hz. Notice that the Dwell has changed with it. The Dwell has become much larger than it was. This is because we have only lower frequency signals to characterize (those that are under 200 Hz from the carrier frequency) and thus need to sample our data less often.
Look at the acquisition time now. It is something like 20 seconds. Also a big change. This is because our Dwell is longer but we are acquiring the same number of data points (DW * 16K = ~20 seconds). To make our experiment reasonable we need to change our No. of Points to something smaller; let's try 1024. Now if you look at the Acquisition time it will be much more reasonable. Finally change the number of scans to 1 so we don't have to fiddle around so long during the experiment.
Take a spectrum, transform it and phase it. You can't use APPLE-J; you will have to rephase it (using both the zero-order and first-order phase corrections) because we have such a difference in sweep width from before.
Look at the spectrum, do you have a lumpy or a sharp spectrum.
Switch the view between interpolated points and acquired points. How well are the peaks digitized? Are there several points for each peak or are the points spread out? If you don't have several points for each peak, then you cannot tell small differences in frequency.
We need to talk about the Nyquist frequency now. The Nyquist frequency defines the smallest frequency difference that can be differentiated. It is based on acquisition time.
Acq. time = 1 / 2 N
where N stands for the Nyquist frequency. Let's say we want to define frequencies which have a difference of 0.1 Hz. Therefore we need to sample for 1 / 2 * 0.1 Hz or for 5 seconds. Fix the appropriate parameter so that we are doing so. Remember that we also want to characterize the full 400 Hz sweep width.
Acquire a new spectrum, transform, and phase. If you are still getting a lumpy spectrum, it is now up to you to shim better.