In the previous post, we talked about the principles behind the chemical shift addressing questions like how the ppm values are calculated, why they are independent of the magnetic field strength, and what is the benefit of using a more powerful instrument.

Today, the focus will be on specific regions of chemical shift characteristic for the most common functional groups in organic chemistry.

Below are the main regions in the ¹H NMR spectrum and the ppm values for protons in specific functional groups:

 

 

The energy axis is called a δ (delta) axis and the units are given in part per million (ppm). Most often the signal area for organic compounds ranges from 0-12 ppm.

The right side of the spectrum is the low-energy region (upfield) and the left side is the high-energy region (downfield). This might be a confusing terminology and we talked about its origin earlier, so read that post if you need to know more but you definitely need to remember that:

Downfield means higher energy – left side of the spectrum (higher ppm)

Upfield means lower energy – right side of the spectrum (lower ppm)

 

Let’s start with the chemical shift of protons of alkyl C-H groups.

 

The Chemical Shift of Connected to sp³ Hybridized Carbons

We can see in the table that sp3 hybridized C – H bonds in alkanes and cycloalkanes give signal in the upfield region (shielded, low resonance frequency) at the range of 1–2 ppm.

The only peak that comes before saturated C-H protons is the signal of the protons of tetramethylsilane, (CH3)₄Si, also called TMS. This is a standard reference point with the signal set exactly at 0 ppm and you can ignore it when analyzing an NMR spectrum. There are a lot of compounds especially organometallics that give signals at negative ppm, but you will probably not need those in undergraduate courses.

One trend to remember here is that protons bonded to more substituted carbon atoms resonate at higher ppm:

 

 

 

The Chemical Shift of Protons Connected to Heteroatoms

The second group of protons giving signal in this region is the ones bonded to heteroatoms such as oxygen and nitrogen. And even though the signal can be in the range from 1-6 ppm, it is usually in the downfield end of this spectrum.

This is due to the higher electronegativity of those atoms pulling the electron density and deshielding the protons. As a result, they are more exposed to the magnetic field and require higher energy radiation for resonance absorption.

The image below can visualize the effect of electron-withdrawing groups on the chemical shift. Suppose the light is the magnetic field, and the triangular object is the electron cloud/density around the given nucleus. The larger the object (the electron cloud) in front of the nucleus, the less it is exposed to the light (magnetic field), thus the smaller the chemical shift. On the contrary, towards the left, we have nuclei that are surrounded by a smaller electron density, and thus the stronger field exposure causes their signal to appear in a higher energy region (downfield):

 

 

Once again, recall that the electron density around a nucleus is affected by the electronegativity of the neighboring nucleus.

The stronger the electron-withdrawing group, the more deshielded the adjacent protons and the higher their ppm value.

Now, 1-6 ppm for protons on heteroatoms is a broad range and to recognize these peaks easier, keep in mind that they also appear broader as a result of hydrogen bonding.

The O-H and N-H protons are exchangeable, and this is a handy feature because when in doubt, you can add a drop of deuterated water (D₂O) and make the signal disappear since deuterium does not resonate in the region where protons do:

 

 

Other groups that give broad, and sometimes, deuterium-exchangeable signals are the amines, amides, and thiols.

And one more thing, which we will discuss in the signal splitting, is that the OH signal is not split by adjacent protons unless the sample is very well-dried.

 

 

The Chemical Shift of Protons on sp² Hybridized Carbons

The protons of alkenes are deshielded and their signals appear downfield from the saturated C-H protons in the 4-6 ppm range.

There are two reasons for this. First, sp2 hybridized carobs are more electronegative than sp3 carbons since they have more s character (33% vs 25% s). So, sp² orbitals hold electrons closer to the nucleus than the sp³ orbitals do which means less shielding, therefore a stronger “feel” of the magnetic field and a higher resonance frequency.

The second reason is a phenomenon called magnetic anisotropy.  When protons on carbon-carbon double bond are placed in a magnetic field, the circulating π electrons create a local magnetic field that adds to the applied field which causes them to experience a stronger net field and therefore resonate at a higher frequency:

 

 

This effect is more pronounced in aromatic compounds which have resonance in the range from 7 to 8 ppm. The circulation of the p electrons in benzene is called a ring current and the protons experience an additional magnetic field that is induced by this ring current.

Interestingly, aromatic compounds with inner hydrogens such as, for example, porphyrins, [18]-annulene and the ones with hydrogens over the ring are shielded by the induced magnetic field and appear scientifically upfield:

 

 

Interestingly, antiaromatic compounds generate a different ring current which in turn generates an induced magnetic field with opposite directions than in aromatic compounds. Thus, antiaromatic systems show the opposite trend: the inner protons appear in a higher ppm area than the outer protons. For example, the protons outside the ring of [12]annulene appear at 5.91 ppm whereas the inner protons are characterized by a chemical shift of 7.86 ppm:

 

 

 

The Chemical Shift of Alkynes

The p electrons of a triple bond generate a local magnetic field just as we discussed for alkenes and one would expect to see their signal more downfield since the sp carbon is more electronegative than sp² carbons.

However, hydrogens of external alkynes resonate at a lower frequency than vinylic hydrogens that appear in the 2-3 ppm range.

The reason is that, unlike alkenes, the induced magnetic field of the p electrons in the triple bond is opposite to the applied magnetic field. This puts the proton in a shielded environment and thus it feels a weaker magnetic field:

 

 

The conflicting effects of magnetic anisotropy and the higher electronegativity of sp hybridized carbons put the signal of acetylenic hydrogens in between alkanes (1-1.8 ppm) and alkenes (4-6 ppm).

 

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