Explain how to obtain an NMR spectrum?

Question

Sarah Angotti · Accepted Answer

Two common types are #""^1 "H"# and #""^13 "C"# NMR, but I will explain #""^1 "H"# NMR.

#""^1 "H"# NMR Spectroscopy is an analytical technique that tells us, based on molecular behavior in response to an induced magnetic field at a particular frequency, what each proton environment looks like (what molecular connections there are near particular protons) in the molecular sample we are examining.

A typical NMR spectrometer would be the #"Bruker 300 MHz"#; the one at my university costs about #$200000#.

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 #"0 ppm"# reference. Depending on the positions of each substituent on the molecule and the presence and positions of electron-withdrawing groups on the molecule, peaks show up shifted downfield (higher #"ppm"#) from #"0 ppm"#.

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 #"7 ppm"#.

Here is an example of an NMR spectrum of #"1,3-cyclohexadiene"#:

The peaks at #"2.13 ppm"# correspond to the protons marked #"B"#, and the peaks at #"5.84 ppm"# correspond to the protons marked #"A"#.

Then you can construct possible structures of the molecule based on this, and some other spectroscopy technique like Infrared Spectroscopy.

Ellie Adams · Answer

undefined To obtain an NMR spectrum, prepare a sample in a suitable solvent, place it in the NMR instrument, set the parameters (frequency, pulse sequence), and acquire the spectrum. Analyze the resulting data for chemical shifts, coupling patterns, and peak intensities.