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Interpreting NMR 2 spectra.docx - NMR 2 Interpreting NMR...

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NMR 2Interpreting NMR spectraNMR spectrometry is the most widely used analytical technique fordetermining the structure of small to medium sized organic molecules.This document will not describe the physics and engineering conceptsleading to the concept of the nuclear magnetic resonancephenomenon.The following simple NMR spectrum illustrates the appearance andsome of the information that can be extracted from an NMR spectrum.The X axis of the spectrum is scaled to a derived set of frequency unitscalled parts per million, PPM.The derivation of these frequency unitscan be found in the lecture notes.In the above spectrum there is onesignal at 0.23 PPM.The frequency, PPM, at which a signal appears iscalled its ‘chemical shift’.This one signal indicates that there is onlyone type of hydrogen in the molecule and the four of them inhabit thesame magnetic environment.Typically an NMR spectrum will have asignal for each group of hydrogens that occupy different magneticenvironments.In NMR spectra the area under each signal will beproportional to the number of hydrogens that generate the signal andeach signal will be labeled with this number.The signal in thespectrum above is labeled ‘4H’; this is referred to as the signal’sintegration.As we will see in later examples, a signal may consist of several peaksthe number owhich is indicated by the ‘multiplicity’ of the signal.Thespectrum shown above, the one signal consists o a single peak and istherefor called a singlet.The frequency at which a particulargroup of protons (hydrogens)resonate, the chemical shift, depends on the magnetic environment of
those protons.The magnetic environment is influenced (modulated)by the surrounding electron density.The circulation of electronsgenerates a magnetic field, which opposes the magnetic field.Protons(hydrogens) in an electron rich environment are said to be ‘shielded’and resonate at lower frequencies.In spectra such protons wouldexhibit signals at PPM’s at low end of the spectrum i.e. closer to 0 PPM.Protons in an environment of low electron density are said to be‘deshielded’ and resonate at higher frequencies i.e. closer to 10 PPM.The above spectrum is that of chloromethane in which one of thehydrogen of methane has been replaced by chlorine.Due to its highelectronegativity, the electrons are drawn to the chlorine leaving thehydrogens’ surroundings low in electron density.As a result, theprotons are deshielded and resonate at a higher frequency than thoseof methane.The following NMR spectrum is that of ethane.In this compound wehave two groups of protons that are in identical environments and thusgenerate only one signal.Another important observation is that thetwo groups of protons do not couple with each other.
Now, in the spectrum of propane on the following page, we have two

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Term
Summer
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Tags
Nuclear magnetic resonance, Proton NMR

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