Carbon-13 NMR

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Chapter: Organic Chemistry : Structure Determination of Organic Compounds

While proton magnetic resonance (PMR) is the most common type of NMR, it is also possible to observe other nuclei which have spin quantum numbers not equal to zero.


CARBON-13 NMR

While proton magnetic resonance (PMR) is the most common type of NMR, it is also possible to observe other nuclei which have spin quantum numbers not equal to zero. Of greatest interest to organic chemists is 13C NMR spectroscopy. Carbon-13 has a spin quantum number I = 1/2 , the same as a proton, so that when placed in a magnetic field, two possible orientations with respect to the field are possible — one of lower energy and one of higher energy. Transitions between these two spin states occur at discrete frequencies in the radio frequency region. Absorption of energy at the resonance frequency causes nuclei in the lower energy level (aligned) to undergo a transition to the higher energy level (opposed). This process is the same as discussed previously for protons, and the equations which govern the absorption are the same and will not be repeated.

There are significant differences between a 13C nucleus and a proton which must be dealt with:

1. Low (1%) natural abundance of 13C.

2. Lower magnetogyric ratio of 13C, making the signal for 13C much lower than that of a proton.

3. Strong coupling to protons, although first order, gives complex multiplets which often overlap, making peak assignments difficult.

These limitations made the development of 13C NMR spectroscopy lag sig-nificantly behind the development of 1H NMR. In the earliest work the relatively weak sensitivity of 13C was a major stumbling block and compounds specifically labeled with 13C had to be prepared in order to obtain usable spectra. Today it is possible to obtain excellent 13C spectra on natural abundance samples of <25 mg in less than 30 min. The hardware and software advances which have enabled such progress to be made lie in three areas:

1. Improved signal detection

2. Fourier transform techniques

3. Digital signal averaging

Suffice it to say that modern NMR spectrometers are capable of obtaining 13C spectra quickly and easily so that 13C NMR is now a routine tool for structure identification.

These instrumental improvements do not solve the problems of coupling between protons and carbon that complicate 13C spectra, but other techniques have. The proton-coupled 13C spectrum of 3-octanone demonstrates that pro-ton – carbon coupling significantly complicates the spectrum due to the large number of lines produced. From this spectrum, it is impossible to tell how many carbons are present or what are their chemical shifts because of overlapping mul-tiplets in the spectrum. To solve this problem, broad-band proton decoupling is used to remove all proton – carbon couplings, and one is left with proton decou-pled or fully decoupled spectra which have only singlet absorptions for each carbon present. For example, 3-octanone has eight lines in the fully decoupled 13C spectrum, as predicted by the fact that each of the carbons is in a unique chemical environment and thus has a unique chemical shift. A distinct advantage of 13C NMR is that 13C absorbs over a range of 250 ppm (compared to 10 ppm for 1H). This means that each carbon can be distinguished by a unique chem-ical shift. Thus it is possible to tell exactly how many nonequivalent carbons are present in a molecule merely by counting the lines in the fully decoupled spectrum (Figure 11.30). (The small three-line signal at 77.3δ is from the solvent CDCl3. It appears in all 13C spectra run in CDCl3 and is normally ignored.)


In addition to the number of nonequivalent carbons present, the chemical shifts of the carbons can reveal a great deal about the types of bonding patterns and substituents which are present. Because of the great range of chemical shifts observed for 13C (250 ppm), even small changes in the environment around carbon can result in a significant change in chemical shift. Figure 11.31 is a brief compilation of 13C chemical shifts for representative classes of organic compounds. By assigning the chemical shifts in many series of compounds, it has been possible to develop correlation equations for calculating 13C chemical shifts based on structural features present in the molecule. These correlation equations  generally provide excellent agreement between calculated and observed 13C chemical shift values. 


Thus it is now routine to test possible structures by calculating 13C chemical shifts and comparing them with the observed spectra. For example, the calculated and observed 13C chemical shift values for cocaine are seen to be in remarkable agreement for most of the carbons in this reasonably complex compound.


The value of fully decoupled 13C NMR spectra is primarily tied to deter-mining how many nonequivalent carbons are present and their chemical shifts. Unfortunately, the integrated areas of 13C signals are not directly proportional to the numbers of carbons responsible for those signals under most circumstances. Thus both n-heptane and 4-(1-propyl) heptane have four signals in their 13C NMR spectra, but it is not possible to determine if the ratio of different carbon types is 1 : 2 : 2 : 2 as expected for n-heptane or the 1 : 3 : 3 : 3 expected for the branched compound.


It is possible to distinguish them based on the chemical shift of C-4, which is calculated to be 36.5 ppm in n-heptane but 53.1 ppm in the branched compound.

A second difficulty of fully decoupled 13C NMR spectra is that the connectiv-ity in the molecule is difficult to establish (except by chemical shift correlation) because coupling patterns are absent. This dilemma is partially resolved by the use of a technique called off-resonance decoupling. In off-resonance decoupled 13C spectra, the carbons are coupled only to those protons directly attached to them and the coupling is first order. Thus quaternary carbons are singlets, methine carbons are doublets, methylene carbons are triplets, and methyl carbons are quar-tets. It is possible to use this information to establish proton – carbon connectivity, which can be used to add protons to partial structures determined by 13C chemical shift data.

The carbons of 1,2-epoxy-5-hexene can be assigned from the off-resonance decoupled spectrum (Figure 11.32). In the fully decoupled spectrum it is clear that the olefinic carbons (115 and 138δ) are distinct from the epoxide carbons (47 and 52δ) and from the methylene carbons (30 and 32δ), but it is not possible to assign which is which. In the off-resonance decoupled spectrum, both the olefinic and epoxide carbons are distinguished by their splitting patterns from the numbers of directly attached protons. The methylene carbons, however, are both triplets and cannot be distinguished.


A final feature of importance in 13C NMR spectra is the notion of equiva-lency. Because some type of decoupling is normally done, either broad band or off resonance, magnetic equivalency is not an issue in 13C NMR, but chemi-cal equivalence remains an issue. If two carbon atoms share the same chemical environment, then of course they will have the same chemical shift. Thus it is important to recognize local or molecular symmetry elements. In a previous example n-heptane is seen to have four signals. 

The internal plane of symme-try results in three equivalent pairs of carbons in addition to the unique central carbon. Toluene (or any monosubstituted benzene) has four signals for the aro-matic protons in addition to the methyl carbon signal. The xylenes offer another example of equivalency.


o-Xylene has four signals, m-xylene has five signals, and p-xylene has only 3. In general, the more symmetric is a molecule, the fewer 13C signals it will have. For example adamantane has only 2 absorptions and buckminsterfulluene (C60) has only a single line in its 13C spectrum.


Thus when the number of 13C signals is less than the number of carbon atoms present in the molecule, there must be symmetry elements present that make some carbon atoms equivalent. The pyrolysis of 2-acetoxy-2.3-dimethylbutane in a hot tube at 200 C gives two products which are both found to have the formula C6H12. The major product has only two 13C absorptions while the minor product has five 13C signals (Figure 11.33). Thus the major product is likely to be the symmetric olefin while the minor product is the less symmetric olefin. 


Note that even the minor product has a pair of equivalent carbons giving rise to five rather than six lines, but the symmetry is still significantly less than that of the major olefin.


The foregoing has been a brief introductory discussion of NMR which has concentrated on some basic principles that are very useful in understanding the technique. The actual practice of NMR today is much more advanced. The incor-poration of Fourier transform techniques has revolutionized NMR spectroscopy. All types of pulse sequences and two-dimensional (2D) techniques have been developed to provide even greater structural detail than has been discussed above. A discussion of such techniques belongs in a more specialized text, but it must be remembered that while these techniques are faster, more sensitive, and much more sophisticated, they are still largely based on the principles presented here, as is the interpretation of the results.

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