Coupling, Decoupling and NOE in NMR Spectroscopy Experiment

This experiment was developed by S.E. Van Bramer for Chemistry 465 at Widener University.

Introduction:

Coupling between nuclei is an important part of NMR. In proton NMR this coupling provides valuable information about how a molecule is connected. Unfortunately, it also complicates the NMR spectrum. By selectively irradiating the coupled nuclei, it is possible to collapse the splitting patterns that are observed. Broadband decoupling is usually used for collecting carbon spectra. Here all the protons in a molecule are irradiated to collapse the splitting patterns in the carbon signal.

Leaving the decoupler transmitter on has the additional effect of producing Nuclear Overhauser Enhancement (NOE). In this experiment you will use special pulse sequences to observe the NOE and decoupling seperately. You will obtain the following carbon spectra:

coupled without NOE
coupled with NOE
decoupled without NOE
decoupled with NOE

This demonstrates the effect of the decoupler on the NMR spectrum.

In addition homonuclear selectively decoupled spectra will be obtained. The experimental details outlined here are in addition to the basic steps outlined in "NMR Operations for QE-300" and this should be treated as a supplement to that document.

Experimental:

Typographic Conventions

Special keys are written out, press the key shown, ie: <return> means to press the key that says "return".
Most commands are entered as 2 letter commands. These commands are represented here as bold caps in brackets ie [EJ]. They are entered without a <return>.
Some sequences require holding the control key and pressing an additional key. [^W] means hold the <control> key and press W.
Additional comments are made in italics.
A sequence of keystrokes or commands is separated by commas.

Safety

DO NOT ENTER KB 421 if you have a pacemaker or other metal implant.
DO NOT place any magnetic material near the magnet.
DO NOT drop small metal material in the room.
DO NOT bring large metal objects into KB 421.
Credit Cards, Computer Disketts, and ATM cards can be erased inside the yellow warning rope.
DO NOT work unsupervised
Follow Directions and Ask questions. The NMR costs about $150,000, be careful
Sign-in the logbook

Sample Preparation

Sample Loading

Setup the Magnet For The New Sample

Obtain a Proton Spectrum

Select the Current Nucleus for proton. [CN]
Set experiment to 1 Pulse. [EX], 1PULSE,
Set Pulse Width to the Ernst angle for your sample, (Set angle for your sample.) [P2], 20D, <return>
Set the Receiver Gain. [SG]
Set Number of Acquisitions (Suggest 8, increase to improve S/N). [NA]
Set the Block Size (suggest 16k). [CB]
Set recycle delay to 0 sec. (For optimum S/N) [D5], 0,
Collect Data. [ZG]
Process the FID
1. Save FID. [SA], filename, <return>
2. Baseline Correct FID. [BC]
3. Exponential Multiplication (optional to improve S/N) [EM]
4. Fourier Transform FID. [FT]
Process the Spectrum
1. Autophase. [AP]
2. Plot the spectrum. [YS], [OX]

Obtain a Decoupled 13-C spectrum with NOE.

Select the Current Nucleus for carbon. [CN]
Set experiment to 1 Pulse. [EX], 1PULSE, <return>
Set Pulse Width to the Ernst angle for your sample, (Set angle for your sample.) [P2], 20D, <return>
Set the Receiver Gain. [SG]
Set Number of Acquisitions (Suggest 64, increase to improve S/N). [NA]
Set the Block Size (suggest 32k). [CB]
Set recycle delay to 0 sec. (For optimum S/N) [D5], 0, <return>
Select Decoupling Frequency (ppm). [F2], 5, <return>
Select Decoupling Power (attenuation). [L1], 2800, <return>
Turn Decoupling on and select MLEV-64 modulation. [DN], 2, <return>
Collect Data. [ZG]
Process the FID
1. Save FID. [SA], filename, <return>
2. Baseline Correct FID. [BC]
3. Exponential Multiplication (optional to improve S/N) [EM]
4. Fourier Transform FID. [FT]
Process the Spectrum
1. Autophase. [AP]
2. Plot the spectrum. [YS], [OX]

Obtain a Coupled 13-C spectrum with no NOE.

Select the Current Nucleus for carbon. [CN]
Set experiment to 1 Pulse. [EX], 1PULSE, <return>
Set Pulse Width to the Ernst angle for your sample, (Set angle for your sample.) [P2], 20D, <return>
Set the Receiver Gain. [SG]
Set Number of Acquisitions (Suggest 64, increase to improve S/N). [NA]
Set the Block Size (suggest 32k). [CB]
Set recycle delay to 0 sec. (For optimum S/N) [D5], 0, <return>
Turn Decoupling off. [DF], <return>
Collect Data. [ZG]
Process the FID
1. Save FID. [SA], filename, <return>
2. Baseline Correct FID. [BC]
3. Exponential Multiplication (optional to improve S/N) [EM]
4. Fourier Transform FID. [FT]
Process the Spectrum
1. Autophase. [AP]
2. Plot the spectrum. [YS], [OX]

Obtain a Coupled 13-C spectrum with NOE.

Select the Current Nucleus for carbon. [CN]
Set experiment: One Pulse, Decouple Off During Acquisition. [EX], 1PDFA,
Set Pulse Width to the Ernst angle for your sample, (Set angle for your sample.) [P2], 20D, <return>
Set the Receiver Gain. [SG]
Set Number of Acquisitions (Suggest 64, increase to improve S/N). [NA]
Set the Block Size (suggest 32k). [CB]
Set recycle delay to 10 sec. (Decoupler is on during this delay) [D5], 10, <return>
Select Decoupling Frequency (ppm). [F2], 5, <return>
Select Decoupling Power (attenuation). [L1], 2800, <return>
Turn Decoupling on and select MLEV-64 modulation. [DN], 2, <return>
Collect Data. [ZG]
Process the FID
1. Save FID. [SA], filename, <return>
2. Baseline Correct FID. [BC]
3. Exponential Multiplication (optional to improve S/N) [EM]
4. Fourier Transform FID. [FT]
Process the Spectrum
1. Autophase. [AP]
2. Plot the spectrum. [YS], [OX]

Obtain a Decoupled 13-C spectrum without NOE.

Select the Current Nucleus for carbon. [CN]
Set experiment; 1 Pulse, Decoupler On During Acquisition. [EX], 1PDNA, <return>
Set Pulse Width to the Ernst angle for your sample, (Set angle for your sample.) [P2], 20D, <return>
Set the Receiver Gain. [SG]
Set Number of Acquisitions (Suggest 64, increase to improve S/N). [NA]
Set the Block Size (suggest 32k). [CB]
Set recycle delay to 5-10 times T₁ sec. (So that NOE is completely relaxed after acquisition) [D5], 60, <return>
Select Decoupling Frequency (ppm). [F2], 5, <return>
Select Decoupling Power (attenuation). [L1], 2800, <return>
Turn Decoupling on and select MLEV-64 modulation. [DN], 2, <return>
Collect Data. [ZG]
Process the FID
1. Save FID. [SA], filename,
2. Baseline Correct FID. [BC]
3. Exponential Multiplication (optional to improve S/N) [EM]
4. Fourier Transform FID. [FT]
Process the Spectrum
1. Autophase. [AP]
2. Plot the spectrum. [YS], [OX]

Homonuclear Selective Decoupling.

Retrieve the Proton Spectrum (Or acquire a new proton spectrum). Use the zoom and peak picking functions to determine the shift of the proton to decouple from the carbon spectrum.
Select the Current Nucleus for proton. [CN]
Set experiment to 1 Pulse. [EX], 1PULSE, <return>
Set Pulse Width to the Ernst angle for your sample, (Set angle for your sample.) [P2], 20D, <return>
Set the Receiver Gain. [SG]
Set Number of Acquisitions (Suggest 8, increase to improve S/N). [NA]
Set the Block Size (suggest 16k). [CB]
Set recycle delay to 0 sec. (For optimum S/N) [D5], 0, <return>
Select Decoupling Frequency (ppm). [F2], selected shift of proton peak, <return>
Select Decoupling Power (attenuation). [L1], 2800, (You will probably have to adjust this to effectively decouple, do not go below 1800.)
Turn Decoupling on. [DN]
Collect Data. [ZG]
Process the FID
1. Save FID. [SA], filename, <return>
2. Baseline Correct FID. [BC]
3. Exponential Multiplication (optional to improve S/N) [EM]
4. Fourier Transform FID. [FT]
Process the Spectrum
1. Autophase. [AP]
2. Plot the spectrum. [YS], [OX]

Laboratory Write-up:

Your Laboratory write-up should include the following information

An Abstract of your experiment.
The Carbon pages from your laboratory notebook.
Plot's of all spectra obtained. Clearly labeled with all relevant information about the data acquisition. Scale to show spectral features relevant for this experiment.
A brief discussion of each experiment. What was going on in the instrument? Draw a diagram of the pulse sequence.
How does decoupling work?
What effect does decoupling the spectrum have on it's appearance?
What effect does NOE have on the spectrum?
What happens when the spectrum is decoupled and has NOE?
Determine the S/N for each of the spectra collected.
Calculate the NOE observed?

This page is maintained by
Scott Van Bramer
Department of Chemistry
Widener University
Chester, PA 19013

Please send any comments, corrections, or suggestions to svanbram@science.widener.edu.

This page has been accessed times since 1/5 /96 .
Last Updated Sunday, August 18, 1996 8:22:15 PM