13C nuclear magnetic resonance spectroscopy

¹³C NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY

Key Notes

Introduction

¹³C nmr spectroscopy is a valuable tool in structural analysis. The ¹³C nucleus is only 1.1% naturally abundant and so signals are weaker than those present in ¹H nmr spectra.

Coupling between ¹³C nuclei

No coupling is seen between ¹³C nuclei since the chances of neighboring carbons both being ¹³C are negligible.

Coupling between ¹³C and ¹H nuclei

Coupling is possible between ¹³C and ¹H nuclei. However, proton decou-pling is usually carried out by continually resonating all the protons while the ¹³C spectrum is run. This leads to one singlet for each non-equivalent carbon. Integration of signals is not possible since this process distorts signal intensities.

Advantages of ¹³C NMR

¹³C nmr spectra contain singlets over a wider range of chemical shifts than ¹H nmr meaning that there is less chance of signals overlapping. Direct information is obtained about the carbon skeleton of a molecule including quaternary carbons. The number of protons attached to each carbon atom can be determined by off resonance decoupling or by running DEPT spectra.

Interpreting ¹³C spectra

The number of signals indicates the number of non-equivalent carbons. The number of protons attached to carbon is determined by methods such as DEPT. The chemical shifts are compared to theoretical chemical shifts determined from nmr tables or software packages.

Introduction

¹H NMR spectroscopy is not the only useful form of nmr spectroscopy. There are a large variety of other isotopes which can be used (e.g. ³²P, ¹⁹F, ²D). However, the most frequently studied isotope apart from ¹H is the ¹³C nucleus. Like protons, ¹³C nuclei have a spin quantum number of 1/2. Thus, the same principles whichapply to proton nmr also apply to ¹³C nmr. However, whereas the ¹H nucleus is the naturally abundant isotope of hydrogen, the ¹³C nucleus is only 1.1% naturallyabundant. This means that the signals for a 13C nmr spectrum are much weaker than those for a 1H spectrum. In the past, this was a problem since early nmr spectrometers measured the absorption of energy as each nucleus in turn came into resonance. This was a lengthy process and although it was acceptable for 1H nuclei, it meant that it was an extremely lengthy process for ¹³C nuclei since several thousand scans were necessary in order to detect the signals above the background noise. Fortunately, that problem has now been overcome. Modern nmr spectrometers are much faster since all the nuclei are excited simultaneously with a pulse of energy. The nuclei are then allowed to relax back to their ground state, emitting energy as they do so. This energy can be measured and a spectrum produced. Consequently, ¹³C nmr spectra are now run routinely. At this point, you may ask whether a ¹³C nmr spectrum also contains signals for ¹H nuclei? The answer is that totally different energies are required to resonate the nuclei of different atoms. Therefore, there is no chance of seeing the resonance of an1H nucleus and a 13C nucleus within the limited range covered in a typical ¹H or ¹³C spectrum.

Coupling between 13Cnuclei

Unlike ¹H nmr where spin spin coupling is observed between different protons, couplingbetween different carbon nuclei is not observed in ¹³C NMR. This is due to the low natural abundance of ¹³C nuclei. There is only 1.1% chance of any specific carbon in a molecule being present as the ¹³C isotope. For most medium-sized molecules encountered by organic chemists, this effectively means that there will only be one ¹³C isotope present in a molecule. The chances of having two ¹³C isotopes in the same molecule are extremely small, and the chances of two ¹³C isotopes being on neighboring carbons are even smaller, so much so that they are negligible.

Coupling between 13Cand 1H nuclei

Although ¹³C–¹³C coupling is not seen, it is possible to see coupling between ¹³C nucleiand 1H nuclei. This might appear strange since we have already stated that ¹H signals arenot observed in the ¹³C spectrum. However, it is perfectly logical tosee coupling between¹³C and ¹H nuclei even if we don’t see the ¹H signals. This is because the ¹H nuclei willstill take up two possible orientations in the applied magnetic field, each of which produce their own secondary magnetic field. In practice, such coupling makes the interpretation of ¹³C nmr spectra difficult and so ¹³C spectra are usually run with ¹³C–1H coupling eliminated. This is done by continuously resonating all the proton nuclei while the ¹³C spectrum is being run such that the signal for each non-equivalent carbon atom appears as a singlet. This results in a very simple spectrum that immediately allows you to identify the number of non-equivalent carbon atoms in the molecule from the number of signals present. This process is known as proton decoupling. One disadvantage of this technique is that it distorts the intensity of signals and so integration cannot be used to determine the number of carbon atoms responsible for each signal. This distortion is particularly marked for signals due to quaternary carbons, which are much weaker than signals for other types of carbons.

Advantages of13C NMR

¹³C nmr gives a signal for each non equivalent carbon atom in a molecule and thisgivesdirect information about the carbon skeleton. In contrast, ¹H nmr provides information about the carbon skeleton indirectly and gives no information about quaternary carbon atoms such as carbonyl carbons. Another advantage of ¹³CNMR is the wide range of chemical shifts. The signals are spread over 200 ppm compared to 10 ppm for protons. This means that signals are less likely to overlap. Moreover, each signal is a singlet. This can also be a disadvantage since information about neighboring groups is lost. However, there are techniques that can be used to address this problem. For example, information about the number of protons attached to each carbon atom can be obtained by off resonancedecoupling. In this technique, the¹³C spectrum is run such that all the protons aredecoupled except those directly attached to the carbon nuclei. Hence, the methyl carbons (CH₃) appear as a quartet, the methylene carbons (CH₂) appear as a triplet, the methine carbons (CH) appear as a doublet and the quaternary carbons (C) still appear as a singlet.

In practice, off resonance decoupling is rarely used nowadays, since a technique known as DEPT is more convenient and easier to analyze. Unfortunately, it is not possible to cover the theory behind this technique here. However, a knowledge of the theory is not necessary in order to interpret DEPT spectra. Such spectra can be run so that only one type of carbon is detected. In other words, a DEPT spectrum can be run so that only the methyl signals are detected or the methylene signals, etc. This allows us to distinguish all four types of carbon, but it means that we have to run four different spectra. There is a quicker way of getting the same infor-mation by only running two spectra. A DEPT spectrum can be run such that it picks up the methyl and methine carbons as positive signals and the methylene carbons as negative signals (i.e. the signals go down from the baseline instead of up). The quaternary carbons are not seen. This one spectrum therefore allows you to identify the quaternary signals by their absence and the methylene signals, which are negative. We still have to distinguish between the methyl signals and the methine signals, but this can be done by running one more DEPT spectrum such that it only picks up the methine carbons.

Interpreting¹³C spectra

In general, ¹³C spectra of known structures can be interpreted in the followingstages. First, the number of non-equivalent carbon atoms is counted by counting the number of signals present in the spectrum, excluding those signals due to the internal reference (TMS at 0 ppm) or the solvent. The signal for CDCl₃ is a triplet (1 : 1 : 1) at 77 ppm (caused by coupling to the deuterium atom where I=1).

Second, each signal is identified as CH₃, CH₂, CH or quaternary using off resonance decoupling or DEPT spectra.

Third, the chemical shifts of the signals are measured, then compared with the theoretical chemical shifts for the carbons present in the molecule. There are a variety of tables and equations which can help in this analysis but it should be noted that the use of such tables is not as straightforward as for ¹H nmr analysis. However, software packages are now available which can calculate the theoretical chemical shifts for organic structures. These are often incorporated into chemical drawing packages such as ChemDraw.