A great advantage of CMR is that the NMR experiment can be run in two different modes.
a) The "off-resonance decoupling" experiment results in the splitting of each signal for each carbon
atom in the molecule by the hydrogen atoms attached to it.
i) This means that the number of hydrogen atoms attached to the carbon atom can be quickly
deduced simply by counting the peaks of the multiplet and subtracting 1.
 |
b) In the proton-decoupled mode each carbon nucleus appears as a singlet.
i) The protons are kept from coupling with the C-13 nuclei by irradiating the sample with
radio waves which flip the protons: they do not spend enough time in either spin state to
couple with the C-13 nuclei.
i) The protons are kept from coupling with the C-13 nuclei by irradiating the sample with
radio waves which flip the protons: they do not spend enough time in either spin state to
couple with the C-13 nuclei.
 |
ii) Adjacent carbon atoms do not split the signal in C-13 NMR. Why not?
Answer: Since there are so few C-13 atoms (~1%), the chances of having two C-13 atoms next to each other in a molecule are small.
Answer: Since there are so few C-13 atoms (~1%), the chances of having two C-13 atoms next to each other in a molecule are small.
6. In CMR as in NMR, electronegative atoms attached to the carbon atom results in a downfield shift.
