Coupling Revisited: Coupling constants, Decoupled Carbon Spectra, and Coupled Carbon Spectra.

Normally carbon spectra are run as decoupled spectra. As the data is collected at the carbon frequency (15 MHz on the EFT 60) the hydrogen frequency is also on so that the splitting of the C-13 peaks by H is NOT observed. This is done for two reasons: 1. The spectrum is simplified and so gathering the data is easier. 2. The carbon peaks of carbons attached to hydrogen are amplified by a factor of up to three when the hydrogens are decoupled. This is known as the Nuclear Overhauser Effect. The combination of these two factors makes the decoupled carbon spectra much easier to do.

In this experiment, do three spectra:

1. A normal H-1 spectrum. In addition to the normal summary table also calculate the three bond H-H coupling constant, J³_HH ,by measuring the distance between two adjacent peaks in a doublet,triplet or quartet and then converting the ppm to Hz. This calculation can be done by setting the ratio of ppm to 10⁶ = the coupling in Hz to the total Hz of the frequency of the instrument. (60x10⁶ for EFT H-1 spectra). Your value should be greater than 2 Hz and less than 50 Hz if calculated correctly.

2. A “normal” C-13 spectrum using the usual zg to initiate the collection of data after the C-13 parameters have been chosen.

3. Do a gated decoupled carbon spectrum. After you have run your normal carbon spectrum, use the command zggd (zero go gated decoupling) instead of zg to get a gated decoupled carbon spectrum. When it asks you for the number of scans put in 16 even though it will ask for a value greater than 60. During the acquisition period the decoupler is turned off and you should see coupling between the hydrogens attached to the carbon 13 atoms. This will result in a methyl peak (CH₃-) giving a quartet, a methylene peak (-CH₂-) giving a triplet, etc. Make a summary table of this including a column for the coupled information. Be sure to indicate on your coupled carbon spectrum the group giving the peak and explain the multiplicity obtained. Comment on the signal to noise you see in your spectra. Also calculate the single bond C-H coupling constant, J¹_CH , from one of your coupled peaks. Your value should be greater than 50 Hz and less than 400 Hz if calculated correctly.

1-5.  Consider the NMR of ethyl alcohol HOCH₂CH₃

1.  The C-13 coupled spectrum will show a _______ for the CH₂ or methylene group.

a) singlet   b) doublet  c) triplet  d) quartet   e) multiplet

2. In the proton spectra, from the highest chemical shift to the lowest, integration of the peaks will be:

a) 1,2,3 b) 1, 1, 1 c) 3, 2, 1

d) 0, 4, 3   e) 2, 3, 1

3. In the proton spectrm, the peaks from highest to lowest chemical shift will be:

a) singlet, doublet, triplet

b) singlet, quartet, triplet

c) singlet, triplet, quartet

d) triplet, quartet doublet

e) quartet, triplet singlet

4. In the coupled carbon spectrum, the peaks from highest to lowest chemical shift will be:

a) singlet, doublet, triplet

b) singlet, singlet

c) triplet, quartet

d) triplet, quartet, doublet

e) quartet, triplet, singlet

5.   The normal (decoupled) carbon 13 spectrum will consist of _______ different peaks.

a) 1 b) 2 c) 3 d) 6 e) more than 6

6.  On a 60 MHz instrument, 0.50 ppm in the proton spectrum corresponds to _______ Hz. On the same instrument C-13 is run at a frequency of 15 MHz. On its spectrum, 0.50 ppm corresponds to __________ Hz.

a) 60, 15 b) 15, 15 c)7.5, 7.5 d) 7.5, 30

e) 30, 7.5