13.11: Characteristics of ¹³C NMR Spectroscopy
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Objectives
After completing this section, you should be able to
- Determine the number of distinct C atoms in a molecule.
- Use the chemical shifts table to determine functional groups present in a molecule.
- Assign a chemical shift to each carbon in a given molecule.
The Basics of 13 C-NMR spectroscopy
Unlike 1 H-NMR signals, the area under a 13 C-NMR signal cannot be used to determine the number of carbons to which it corresponds. This is because the signals for some types of carbons are inherently weaker than for other types – peaks corresponding to carbonyl carbons, for example, are much smaller than those for methyl or methylene (CH 2 ) peaks. Peak integration is generally not useful in 13 C-NMR spectroscopy, except when investigating molecules that have been enriched with 13 C isotope.
The resonance frequencies of 13 C nuclei are lower than those of protons in the same applied field - in a 7.05 Tesla instrument, protons resonate at about 300 MHz, while carbons resonate at about 75 MHz. This is fortunate, as it allows us to look at 13 C signals using a completely separate 'window' of radio frequencies. This means you will only see the 13 C nuclei in a 13 C NMR experiment like in the 1 H NMR experiments we just looked at, we only saw hydrogens. Just like in 1 H-NMR, the standard used in 13 C-NMR experiments to define the 0 ppm point is tetramethylsilane (TMS), although of course in 13 C-NMR it is the signal from the four equivalent carbons in TMS that serves as the standard. Chemical shifts for 13 C nuclei in organic molecules are spread out over a much wider range than for protons – up to 200 ppm for 13 C compared to 12 ppm for protons (see Table 3 for a list of typical 13 C-NMR chemical shifts). This is also fortunate, because it means that the signal from each carbon in a compound can almost always be seen as a distinct peak, without the overlapping that often plagues 1 H-NMR spectra. The chemical shift of a 13 C nucleus is influenced by essentially the same factors that influence a proton's chemical shift: bonds to electronegative atoms and diamagnetic anisotropy effects tend to shift signals downfield (higher resonance frequency). In addition, sp 2 hybridization results in a large downfield shift. The 13 C-NMR signals for carbonyl carbons are generally the furthest downfield (170-220 ppm), due to both sp 2 hybridization and to the double bond to oxygen.
Example \(\PageIndex{1}\)
- How many sets of non-equivalent carbons are there in each of the molecules shown in exercise 5.1?
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How many sets of non-equivalent carbons are there in:
- toluene
- 2-pentanone
- para-xylene
- triclosan
(all structures are shown earlier in this chapter)
Solution
a
a) 8 signals (each carbon is different)
b) 11 signals (the two enantiotopic CH 2 CH 3 groups are NMR-equivalent)
c) 6 signals (each carbon is different)
d) 16 signals (the fluorobenzene group only contributes 4 signals due to symmetry)
b
a) 5 signals
b) 5 signals
c) 3 signals
d) 6 signals
Because of the low natural abundance of 13 C nuclei, it is very unlikely to find two 13 C atoms near each other in the same molecule, and thus we do not see spin-spin coupling between neighboring carbons in a 13 C-NMR spectrum . There is, however, heteronuclear coupling between 13 C carbons and the hydrogens to which they are bound. Carbon-proton coupling constants are very large, on the order of 100 – 250 Hz. For clarity, chemists generally use a technique called broadband decoupling , which essentially 'turns off' C-H coupling, resulting in a spectrum in which all carbon signals are singlets . Below is the proton-decoupled 13 C-NMR spectrum of ethyl acetate, showing the expected four signals, one for each of the carbons.
One of the greatest advantages of 13 C-NMR compared to 1 H-NMR is the breadth of the spectrum - recall that carbons resonate from 0-220 ppm relative to the TMS standard, as opposed to only 0-12 ppm for protons. Because of this, 13 C signals rarely overlap, and we can almost always distinguish separate peaks for each carbon, even in a relatively large compound containing carbons in very similar environments. In the proton spectrum of 1-heptanol , for example, only the signals for the alcohol proton (H a ) and the two protons on the adjacent carbon ( H b ) are easily analyzed. The other proton signals overlap, making analysis difficult.
In the 13 C spectrum of the same molecule, however, we can easily distinguish each carbon signal, and we know from this data that our sample has seven non-equivalent carbons. (Notice also that, as we would expect, the chemical shifts of the carbons get progressively smaller as they get farther away from the deshielding oxygen.)
This property of 13 C-NMR makes it very helpful in the elucidation of larger, more complex structures.
13 C NMR Chemical Shifts
The Carbon NMR is used for determining functional groups using characteristic shift values. 13 C chemical shifts are greatly affected by electronegative effects. If a H atom in an alkane is replaced by substituent X, electronegative atoms (O, N, halogen), 13 C signals for nearby carbons shift downfield (left; increase in ppm) with the effect diminishing with distance from the electron withdrawing group . Figure 13.11.1 shows typical 13 C chemical shift regions of the major chemical class.
Spin-Spin Splitting
Comparing the 1 H NMR, there is a big difference thing in the 13 C NMR. The 13 C- 13 C spin -spin splitting rarely exit between adjacent carbons because 13 C is naturally lower abundant (1.1%)
- 13 C- 1 H Spin coupling : 13 C- 1 H Spin coupling provides useful information about the number of protons attached a carbon atom. In case of one bond coupling ( 1 J CH ), -CH, -CH 2 , and CH 3 have respectively doublet, triplet, quartets for the 13 C resonances in the spectrum. However, 13 C- 1 H Spin coupling has an disadvantage for 13 C spectrum interpretation. 13 C- 1 H Spin coupling is hard to analyze and reveal structure due to a forest of overlapping peaks that result from 100% abundance of 1 H.
- Decoupling : Decoupling is the process of removing 13 C- 1 H coupling interaction to simplify a spectrum and identify which pair of nuclei is involved in the J coupling. The decoupling 13 C spectra shows only one peak(singlet) for each unique carbon in the molecule (Fig 13.11.2). Decoupling is performed by irradiating at the frequency of one proton with continuous low-power RF.
Summary
¹³C NMR (Carbon-13 Nuclear Magnetic Resonance) Spectroscopy is a powerful analytical technique used to study the structure and connectivity of organic molecules. Unlike proton NMR, which detects hydrogen nuclei, ¹³C NMR specifically targets the carbon nuclei within a molecule. In ¹³C NMR spectroscopy, carbon atoms resonate at characteristic frequencies based on their local chemical environment, which is influenced by neighboring atoms and functional groups. These resonances appear as peaks in the NMR spectrum, providing valuable information about the types of carbon atoms present and their surroundings. By analyzing the chemical shifts, intensities, and coupling patterns of these peaks, chemists can deduce crucial structural details such as the number and types of carbon atoms, their connectivity, and the presence of certain functional groups. This information is essential for determining the molecular structure and understanding chemical properties and reactivities.