NMR spectroscopy

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Organic Chemistry
Interpreting NMR spectra (C13 and H1)
12/6/12
Interpreting NMR spectra
NMR spectroscopy, also known as Nuclear Magnetic Resonance, is a form of spectroscopy that deals with the magnetic properties of organic compounds. This type of spectroscopy applies to the atoms of a molecule magnetic moments due to their nuclear spin properties. The process of NMR spectroscopy reveals the type, number and positions of the atoms with magnetic presence in the molecule to be determined if done properly. The technique that will be described later is of C13 and the hydrogens (H10) that surround it. This is also known as C13 spectroscopy and proton spectroscopy. This type of spectroscopy is less common than IR spectroscopy, but still an effective way of determining structure of certain organic compounds. NMR spectroscopy works by when the magnetic pulse of the machine interacts with the molecule, the nucleus will either be with or against the field. This is true to both C13 and proton spectroscopy. Because the molecules have charged poles, positive and negative, this allows the molecule to be with or against the magnetic field of the machine depending on the compound being tested. Hydrogen has two nuclear spin states (+1/2 and -1/2), and they can either be aligned with the magnetic field (+1/2) or against it (-1/2). Most of them align with the field in lower energy spin states. The frequency of radiation needed to cause nuclear spin in directly in correlation with the strength of the magnetic field being applied. Also, within this technique the radio waves applied to the molecule are what get the atoms moving and can cause them to flip to be observed. This how the atoms are able to be seen on your data, because they have magnetic fields of their own once they come into contact with the machines magnetic field this will tell you how many radio waves needed to get them to flip so that you can actually observe them. All the radio waves really do in the machine is measure the time it takes for the atom to move and then move back to its original position known as shift, as well as the spin of the atom whether it be C13 or a proton. The machine is really less complicated to use than it seems, and practice like all things, makes perfect. All of the information stated above is the basics you need to grasp in order to be able to move to the next step in the process, which is analyzing the graph the machine puts out.

Now that you have reached this step in the process, you are ready to start interpreting your graph. The machine prints a graph very similar to that of the equipment used to measure seismic activity. The needle will lay basically at flat line until an atom with magnetic pull moves up and down or begins to spin, then the needle will jump and come back down. This type of graph will help in determining the chemical shift of the molecule as shown in the graph below. On this graph, the As seen on the graph below the Y axis shows the intensity or field strength of the magnetic field, while on the X axis, chemical shift is shown.

There is also a reference solution used to provide a base to measure against. This solution is tetreamethylsilane also known as TMS. This shows up on the graph at higher field strengths, and always occurs on the same strength on every graph. It is used in both types of NMR spectra, but is most effective in proton NMR as it allows for the showing of chemical shifts of the atoms. Thus the number of chemical shifts helps to determine the structure of the compound. The chemical shift are measure in part per million in the solution. Another thing to consider when looking at the graph is if the TMS absorption is upstream or downstream. If it is on the very right it is considered to be upstream, thus if it’s at the left its considered downstream. This tells the person looking at the graph where the C13 or proton is at on the on the graph by the chemical shift of the atoms present. Because the...
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