Applications of 13C NMR.

To illustrate some of the concepts developed above, we will first concentrate on Carbon. The most abundant isotope 12C has no overall nuclear spin, having an equal number of protons and neutrons. The 13C isotope however does has spin 1/2, but is only 1% abundant. Carbon NMR spectra are characterised by the following;

* A chemical shift range of about 220 ppm, normally expressed relative to the 13C resonance of TMS.

* A natural linewidth of ca 1Hz, related to the values of the relaxation times T1 and T2.

* A Larmor frequency in the range of 20-100 MHz, for typical spectrometers.

* Typically about 5-20 mg of sample dissolved in 0.4 - 2 ml of solvent (normally CDCl3) are required, and a good spectrum would be obtained in 64 - 6400 scans.

Lets start by looking at a 13C spectrum of diethyl phthalate obtained by the FT technique;

First we note the wide chemical shift range of the signals. Note also that the signal we attribute to the methyl group is approximately a 1:3:3:1 quartet, the methylene approximately a 1:2:1 triplet, and the aromatic CHs approximately 1:1 doublets. The intensity ratios suggest this is due to coupling and the multiplicities that it is due specifically to coupling of the spin 1/2 13C with the spin 1/2 protons and nothing else (ie the 2nI+1 rule, I being the spin number). Before we move on to discuss how this coupling may be useful to us, let us remind ourselves of how the coupling arises. Remember the energy level diagram of one spin 1/2 nucleus in a magnetic field;

The precessing magnetisation vector either reinforces or opposes Bo, so that locally at least, other nuclei will perceive two slightly different values of Bo. Since the populations of each energy level are practically identical (equation 5), another nucleus close-by will resonate with equal probability at two slightly different Larmor frequencies (equation 1). The difference between these two frequencies is what we know at the coupling constant J. Its value depends on how the perturbation in Bo is transmitted between the two nuclei, and this is normally achieved via the intervening electrons (hence the term through bond coupling). When two identical nuclei are involved, three slightly different and equally spaced values of Bo, with the middle one being twice as probable as the highest or lowest, hence the 1:2:1 triplet coupling pattern we are familiar with. In spin terms, we say that four configurations are possible; +1/2,+1/2; +1/2, -1/2; -1/2, +1/2; -1/2, -1/2. As the middle two are of equal energy, this manifests as a double height peak, ie a 1:2:1 triplet. If we remember that J(coupling) =  x o/106, these visible in the carbon spectrum above look in the range JC-H ~ 6 x 22 ~ 130 Hz. Notice that carbon appears not to couple with other carbons, only with protons in the same molecule. This is because the probability that two 13C-13C nuclei will be close enough to couple is 100 times less than the probability of finding 13C-12C as adjacent nuclei.

Now look at the next spectrum;

There are seven singlets; the spectrum is certainly less cluttered because the coupling has gone! It has been removed by a technique called broad band 1H decoupling. During measurement of the 13C FID, the entire proton resonance region of the compound (ie from about -2 to + 10 ppm) was irradiated with "white noise" radio frequency. This has the effect of increasing the rate of transition between the.proton low and high energy spin states such that only one average magnetic field is experienced by any individual carbon nucleus, and the manifestation of coupling vanishes. As far as the carbon is concerned all the protons cease to exist (well, almost. There is an effect called the nuclear Overhauser effect or nOe which does very odd things to the intensity of the carbon line, normally increasing it by a factor of two or more and hence is one of the factors making integration of 13C spectra unreliable. More of which in other lecture courses!). Normally for 13C both decoupled and coupled spectra are recorded, but the latter with a small modification called "off-resonance proton decoupling";

Here the proton decoupler is actually switched on during 13C measurement but its frequency is centred at about -10 ppm. It has the effect of removing any coupling between 13C and 1H that occurs through more than one bond (H-C-C and longer range couplings of < 10Hz) but leaving coupling due to directly bonded atoms (ie H-C of >125 Hz) still visible (above). This reduces the complexity of the spectrum and offers a significant gain in the intensity of the signals (~ four fold reduction in measurement time) from the nOe enhancement referred to above.

Together, these two spectra give the following information;

The typical chemical shift ranges of carbon nuclei are as follows;

C (alkane) ~ 0 - 30 ppm
C (alkene) ~ 110 - 150 ppm
C-N ~ 50
C-O ~ 60
C-F ~ 70 ppm.
Aromatic ~ 110 - 160 ppm
Ester, amide, acid, ~ 160-170 ppm
Ketone, aldehyde ~ 200-220 ppm.
You should be able to use this information to deduce typical functional groups present in molecules. One or more examples of this will be presented in the lecture course.

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Copyright (c) H. S. Rzepa and ICSTM Chemistry Department, 1994, 1995.