NMR gives details on the structure and functionality of a chemical. It is an immensely powerful technique that can fully identify many chemicals, especially ones previously unheard of, and it is the most common identification technique for organic chemists because both hydrogen and carbon are easily analysed with it. For this standard, ¹³C NMR is the technique assessed. Watch: Carbon-13 NMR spectroscopy Opens a YouTube search: pick a video that clicks for you
How NMR works
The technique uses the magnetic spin property of some nuclei, which depends on the exact isotope being studied (¹³C, ¹H, ²H, ¹⁵N…). Some nuclei are ideal for analysis while others show very little absorption.
The orientation of the spin of an NMR-active nucleus can be influenced by an external magnetic field, but this effect is relatively weak, so powerful superconducting electromagnets are used to produce a significant effect.
Under these intense fields, nuclei either align with the field (lowered energy, increased stability) or against it (raised energy, decreased stability), and there is a difference in energy between the two arrangements.
Photons in radio waves carry a very small energy, just the right amount to flip the nuclei from the lower to the higher energy state. The machine detects which frequencies the nuclei absorb. Over time they return to the lower state.
Think of the nuclei as tiny bar magnets, with the north end turning to face the south of the external field, and radio waves being just right for knocking them around to the opposite orientation.
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Why ¹³C specifically
¹³C NMR analyses the ¹³C nuclei in carbon-containing (organic) molecules. Statistically, ¹³C nuclei make up 1.1% of carbon nuclei, which results in a small signal. While that low abundance historically made ¹³C NMR difficult, modern machines with powerful superconducting electromagnets, advanced noise reduction equipment and sophisticated computer software can analyse even the most complex examples.
Shielding
The shielding effect
Nuclei are not present in isolation. They are surrounded by spinning electrons, each with their own magnetic properties. When an external magnetic field is applied, these electrons reduce the intensity of the field that reaches the nucleus. This is the shielding effect.
The result is that the energy of the radio-wave photons absorbed depends on both the type of nucleus and the density of electrons surrounding it.
In covalently bonded molecules the sharing of electrons depends on the atoms present: more electronegative atoms draw electrons towards them (making themselves shielded) and away from other atoms (making those deshielded). Each bond has a shielding or deshielding effect and these effects add together.
This in turn changes the effect an atom has on adjacent atoms: the more deshielded an atom, the more it deshields the atoms it is covalently bonded to.
In addition, extra magnetic fields are created by delocalised electrons circulating about a section of the molecule. These are present in double and triple bonds and are especially active in conjugated systems (bonds alternating single and double along a chain or around a ring). These fields reinforce the external field and so deshield the surrounding atoms, particularly those directly involved in the bonds.
Chemical environments
The definition and the test
The chemical environment of an atom is the sum total of all the shielding effects acting on that atom.
Atoms in a molecule will have a different chemical environment unless they have the same sequence of bonds as each other.
The number of chemical environments is very important, it directly translates to the number of peaks in an NMR spectrum. When counting environments, carbons that are chemically equivalent are treated as being in one environment.
Worked example Counting environments: propane vs 1-chloropropane How many chemical environments does each of propane and 1-chloropropane have? Propane. Carbon 1 is bonded to 3 H’s and a CH₂CH₃; carbon 2 is bonded to 2 H’s and 2 CH₃’s; carbon 3 is bonded to 3 H’s and a CH₂CH₃. Carbons 1 and 3 have exactly the same groups bonded to them, so they have the same chemical environment. Carbon 2 is different. 1-chloropropane. Carbon 1 is bonded to 3 H’s and a CH₂CH₂Cl; carbon 2 is bonded to 2 H’s, 1 CH₃ and 1 CH₂Cl; carbon 3 is bonded to 2 H’s, 1 Cl and 1 CH₂CH₃. None of these carbons have exactly the same groups attached, so all three have different chemical environments. ✓ Propane has TWO chemical environments (so two peaks). 1-chloropropane has THREE. Worked example The symmetry shortcut: 3-ethylpentan-2-ol Commonly we short cut the process and look for symmetry. How many environments does 3-ethylpentan-2-ol have? When an atom (typically a carbon or nitrogen) has identical groups attached to it, those groups will have identical chemical environments. If a group has multiple environments, each environment is mirrored by the other symmetric group. Carbon 3 has two identical groups off it. This lets us identify carbons 4 and 6 as chemically equivalent, as well as carbons 5 and 7. ✓ FIVE chemical environments: even though the molecule has seven carbons.
Chemical shifts
What δ means and what changes it
Chemical shifts, symbol δ, are a measure of how shielded or deshielded an atom is, measured in ppm. The greater the chemical shift, the greater the magnetic field experienced by the nuclei and therefore the more deshielded they are. It is a relative measure compared to a reference molecule.
The chemical shift of a carbon is influenced by the atoms bonded to the carbon. How each bonded atom modifies the shift depends on the element (in particular its electronegativity), the chemical environment of that atom, and the type of bonding present (single, double, triple or conjugated).
The basic idea: anything that draws electrons away from a carbon is deshielding and consequently raises the chemical shift. Bonding to highly electronegative atoms, and carbons that are highly deshielded, produce high chemical shifts; bonding to low electronegativity atoms, and carbons that are relatively shielded, produce low chemical shifts. Double and triple bonds have additional deshielding properties.
In alkanes, carbon is more electronegative than hydrogen, so the more carbon atoms bonded to a given carbon, the more deshielded it will be. As each carbon is influenced by the others, longer chains are more deshielded and have higher chemical shifts.
It is not possible to predict chemical shifts without significant computation, but the general rules apply, and most importantly, carbons in the same chemical environment always have the same chemical shift.
¹³C shifts for a selection of 4-carbon molecules (ppm)
Molecule
Shifts along the chain
Butane
13.8 · 25.1 · 25.1 · 13.8
Butan-1-ol
14.4 · 19.7 · 35.5 · 62.2 (the C–OH)
Butanal
13.5 · 15.8 · 45.8 · 202.0 (the CHO)
1-chlorobutane
13.2 · 20.0 · 34.8 · 44.2 (the C–Cl)
1-aminobutane
14.1 · 20.4 · 36.6 · 42.2 (the C–NH₂)
But-2-ene
14.3 · 125.0 · 125.0 · 14.3
✓
Read that table sideways
All six have four carbons, and the first two or three shifts barely move. What changes is the carbon carrying the functional group: 62.2 for the alcohol, 44.2 for the chloride, 42.2 for the amine, and 202.0 once it becomes a carbonyl. Electronegativity plus a double bond gives you by far the biggest jump, which is exactly why the carbonyl region sits alone at the far left of every spectrum.
AS91388 data sheet: approximate ¹³C chemical shifts
Carbon environment
δ (ppm)
C=O (in ketones)
205–220
C=O (in aldehydes)
190–200
C=O (carboxylic acids, acid chlorides, amides and esters)
160–185
C=C (in alkenes)
115–140
RCO₂CH₂R′ (esters)
60–80
RCH₂OH (C–OH between 50–90)
50–70
RCH₂Cl
40–45
RCH₂Br
30–40
RCH₂NH₂
35–45
RCH(NH₂)R
30–60
R₃CH
25–60
CH₃CO
20–30
R–CH₂–R
15–50
R–CH₃
8–35
The three major divisions. These apply WITHOUT EXCEPTION to any molecule in this course
δ (ppm)
Carbons associated with this region
0–100
Carbons with single bonds only, or carbons with a triple bond
100–150
Carbons with double bonds to other carbons (i.e. C=C)
150–250
Carbonyl carbons (C=O)
The most distinguished signals are in the low region (0–20 ppm), corresponding to CH₃, and in particular the very low region (0–15 ppm) is almost always a CH₃ bonded to a CH₂. For the other signals, the information gained is supportive when combined with other data such as IR and MS. ℹ
Why carbonyls split into two ranges
Carbonyls have especially deshielded carbons, due to the very high electronegativity of oxygen and the presence of a double bond. Secondly, when another atom with lone pairs is attached to the carbonyl carbon, the carbonyl draws electrons to it, making the carbonyl carbon more shielded and therefore giving it a reduced chemical shift.
The result is that ketones and aldehydes are typically distinguishable from carboxylic acids and their derivatives (esters, amides and acyl chlorides):
• 180–220 ppm, ketones and aldehydes • 160–185 ppm, carboxylic acids, acyl chlorides, amides and esters
This is a genuinely useful piece of reasoning to be able to state, not just a pair of numbers to memorise.
Student NMR table. This is the table you are given in the assessment
δ (ppm)
Carbons associated with this region
0–15
CH₃CH₂–
15–30
CH₃–
20–35
–CH₂–
30–60
C–N, C–Cl, C–Br, C–C=O
50–70
C–O
60–90
C≡C
100–150
C=C
160–185
C=O present in carboxylic acids, esters, acyl chlorides and amides
180–220
C=O present in aldehydes and ketones
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Why there is no “C–H” row in any of these tables
It is not an omission, there is no distinct C–H environment in ¹³C NMR. You are observing the carbon, and a carbon’s shift is set by its hybridisation and by how electronegative its neighbours are. Hydrogen barely pulls electron density away from carbon, so a C–H bond shifts the carbon hardly at all. A carbon carrying only C and H is the plain alkyl entry at the bottom of the table.
Use the electronegativity logic instead of memorising the order: C–N (30–60) sits below C–O (50–70) because nitrogen is less electronegative than oxygen, and both sit below anything with a π bond.
Reading a ¹³C spectrum
What the axes do
¹³C spectra appear as a series of lines, with intensity on the y-axis and chemical shift (δ, ppm) on the x-axis.
The x-axis values increase from right to left. Low chemical shifts on the right relate to carbons that are highly shielded; towards the left, carbons are increasingly deshielded.
The full range recorded is normally 0–250 ppm and is then trimmed of any “blank” space, so you must be careful to read the scale before making interpretations. The workbook’s hexane example is plotted over only 13–34 ppm.
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Peak height tells you nothing
The intensity (the height) of a peak does not necessarily give any usable information. If you are given a spectrum with varied peak heights, each peak can be treated as one environment regardless of height. There are many factors that contribute to differences in peak height and they are not easily interpreted. ✓
A deduction you can make from the number of signals
Double bonds always involve at least 2 carbons. Therefore if only one signal is present in the C=C region, the molecule is completely symmetric around the double bond, and the consequence is that every peak has at least 2 carbons in its environment.
Example: cis-hex-3-ene has six carbons but produces only three peaks. Worked example A spectrum with all three regions H₂C=CH–CH₂–CH₂–CO–CH₃ gives signals at δ 207.9, 137.2, 115.2, 42.8, 29.9 and 27.9. Assign the regions. The carbonyl carbon has δ = 207.9 ppm, in the 150–250 carbonyl region, and specifically in the 180–220 ketone/aldehyde band. The double bond carbons have δ = 137.2 and 115.2 ppm, the 100–150 C=C region. The remaining saturated carbons sit at δ = 42.8, 29.9 and 27.9 ppm, the 0–100 region. ✓ Six environments spread across all three major divisions: one carbonyl, two alkene carbons, three saturated carbons.
¹³C NMR spectra: what do you learn?
First and most important: the number of carbon environments. This tells you the minimum number of carbons present in the molecule.
Secondly, divide these environments into three groups: saturated or triple bonded carbons, double bonded carbons, and carbons in carbonyl groups.
Thirdly, further divide these carbon environments up based on the table, to indicate the likely functional groups responsible for those shifts.
Lastly, combine all the information from steps 1–3 and you may conclude additional things about the molecule as a whole.
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The CDCl₃ peak at 77.0 ppm
Many of the spectra you will encounter have been altered to not include solvent peaks. However, some spectra you may see have been run in CDCl₃ and have a three-peak signal at 77.0 ppm. If you encounter this peak, you should ignore it for your analysis. Counting it as a carbon environment will give you the wrong number of carbons.