Structure & Reactivity

Nuclear Magnetic Resonance Spectroscopy

NMR8.  Chemical Shift in Proton Spectra

The trends here are exactly the same as in carbon spectra. Wherever the carbon goes, it takes the proton with it. By analogy with carbon spectra,

Figure NMR11.1H NMR spectrum of hexane. 

Source: Simulated spectrum.

Figure NMR12.1H NMR spectrum of 1-hexene. 

Source: Simulated spectrum.

Figure NMR13.1H NMR spectrum of butanal. 

Source: Simulated spectrum.

 

As before, there are also hydrogens on linear carbons, although they are much less common than tetrahedral or trigonal carbons.

Remember, these are general rules that you should know. There will occasionally be exceptions; the proton in a carboxylic acid may be seen at 12 ppm, and the proton in chloroform shows up at 7 ppm although it is attached to a tetrahedral carbon. (World-record shifts occur for hydrogens attached to transition metals: "late" metals like ruthenium or rhodium can move hydrogen peaks all the way up to -20 ppm, but "early" metals like tantalum can move them down as far as 25 ppm.)

Problem:

NMR 5. Looking at the 1H NMR spectra of the following compounds, indicate which peak belongs to which proton.

a) dibutyl ether

b) methyl phenyl ether

c) benzaldehyde

d) acetophenone

Notice that a major difference from 13C NMR is that a carbon spectrum is spread out over 200 ppm, while a typical proton spectrum is compressed into about 10 ppm. There is a consequence of that difference, and it can be frustrating. It is usually easy to distinguish two different 13C peaks, whereas two peaks in the 1H spectrum could easily be so close together that they overlap. For example, the aliphatic hydrocarbons hexane and nonane display only two distinct peaks in the 1H spectrum, one for the methyl hydrogens and one for the methylene hydrogens, because the latter are all too similar to tell apart given the limited amount of resolution in the spectrum.

There is another complication in the chemical shifts seen in 1H spectroscopy, and that is the behaviour of protons attached directly to heteroatoms such as oxygen and nitrogen. Oxygen is very electronegative, and hydrogen is not, so it stands to reason that an OH proton would absorb at very low field, say 10 ppm. That's completely wrong. OH protons in aliphatic alcohols show up between 2 and 6 ppm, phenolic OH protons between 5 and 9 ppm, and carboxylic acid OH protons between 11 and 12 ppm. Water shows up around 1.6 ppm when dissolved in chloroform, but if the water is present in high concentrations it can show up further downfield. Thus:

Figure NMR14.1H NMR spectrum of benzoic acid. 

Source: Simulated spectrum.

Figure NMR15.1H NMR spectrum of ethanol. 

Source: Simulated spectrum.

 

 

Although oxygen is very electronegative, it also has two lone pairs of electrons; those lone pairs on oxygen are often crucial in understanding chemistry. The oxygen atom does pull electron density away from the hydrogen through the sigma bond between them, but the lone pairs also bathe the nearby hydrogen in the shielding effects of their electron density. Depending on how these two factors balance out, OH and NH protons don't absorb nearly as far downfield as expected.

 

 

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These materials are protected by copyright 2008, Chris P. Schaller

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