Explain how to obtain an NMR spectrum?
1 Answer
Two common types are
A typical NMR spectrometer would be the
The general preparations are fairly simple:
- Acquire sample (roughly
#"30 mg"# ) - Put it into a designated NMR tube (they tend to be about
#8.5 "in."# long or so, and maybe half a#cm# in diameter), fill it to the specified depth with deuterated solvent, like#"CDCl"_3# (usually your professor will specify how deep) - Put the sample tube into a spinner for placement into the NMR spectrometer
The acquisition process of this is essentially the following (most of this is accomplished by pressing "go" or "start" somewhere, and you should have a manual somewhere to follow):
Manual: (should take maybe 15~30 minutes)
- Follow directions from a manual to set everything up with the shimmer (essentially like calibrations)
- Press "go" or similar to start the spectrometer's acquisitions
Automatic: (tends to take about an hour at most)
- Apply a magnetic field at a certain frequency
- Adjust that frequency until it lines up with the resonant frequency of the functional group
- Trace the intensity of the signal (y-axis) as that frequency is adjusted
- Convert the frequency to
#"ppm"# (it's the conventional x-axis unit for "chemical shift") - Plot the spectrum
Manual:
- Process the spectrum (zero the reference point, mark peaks with integrations to determine # of protons per peak, print out on-scale close-zooms)
- Analyze the spectrum by hand (this is the hard part!)
You tend to plot relative to a sample called Trimethylsilane, or TMS, which is used as a
There are literature values that (I would think) are provided in a textbook that should come with your lab course, such as aromatic regions being near
Here is an example of an NMR spectrum of
The peaks at
Then you can construct possible structures of the molecule based on this, and some other spectroscopy technique like Infrared Spectroscopy.