1H Homonuclear Decoupling

The traditional homonuclear decoupling experiment is done with weak, selective decoupling (homonuclear decoupling) of one or a group of protons. Observation of the collapse of 1H fine splitting enables identification of through-bond coupled protons. This is an easy and quick experiment if only a few coupled partners are studied. Each coupling is studied separately with an on-resonance decoupling of one partner or separately both partners for the sake of symmetry, reducing its efficiency if many such couplings are examined. A majority of such studies can be done nowdays with 2D COSY-type experiments (such as gcosy) which allow simultaneous observation of all through-bond couplings in less time. However, a unique adavantage of this experiemnt is to offer clarity to a crowed 1H spectrum with complicated J-coupling splittings at a high resolution, or to quickly confirm a few well-guessed couplings.

To perform the homonuclear decoupling experiment (homodec sequence on the Varian systems), the user should pay attention to the following:

• Decoupling power must be weak so that it is selective in order to minimize excitation of adjacent peaks which may lead to misinterpretation.
• The 1H decoupling power level (dpwr), however has to be strong enough to cause clear visible effect to coupled partners. This means the power level may have to be re-adjusted during the experiment in order to get a clean-cut effect on the coupled partner peak(s).
• Be very cautious when raising the power level for a long acquisition period with decoupling; the probe may be burned if the power is too high (see procedure below). A limitation is set to 20dB on nmr500 for this experiment. The recycle delay must be long enough (>1 sec) for the probe to cool in between scans.
• Coupled peaks that are close to each other may be difficult to study with this experiment.
• Homonuclear decoupling introduces a glitch, often as an out-of-phase peak, at the position of selective decoupling.

The 2D COSY experiment (gcosy) does not have any of the issues mentioned above. Linewidth, however, in gcosy is usually broader because of the use of short acquisition time and the absolute-value mode, but it is mostly sufficient and the peaks can be sharper if needed with longer at.

Homodec

Pulse sequence and example reference (1st) and 3 selective decoupling offset frequencies (dof).

Procedure (on nmr500)

Step 1: Preparation

• Optional: Temperature should be regulated for best results (done by FTS on NMR500)
• Optional: Spin should be turned off for best stability, unless better linewidth with spinning is desired.
• Join exp1 (jexp1, or another exp #). Type init1h to load standard 1H parameters and shims
• Adjust solvent setting
• Lock and shim Z1, Z2. Adjust X1 and Y1 a bit if spin is turned off, and re-shim Z1 and Z2.
• Lock sample at ~ 70-80%.
• Check 1H channel tuning

Step 2: Collect 1H spectrum

• Collect a standard 1H spectrum with nt=1 (or nt=4 for cleaner spectrum)
• Put the box cursor to enclose the signal region and to include ~10% flat baseline area at both ends of the signal region. Type movesw . This sets a new spectral width sw and spectrum center tof .
• Collect another 1H spectrum with nt=1 with the new setting. Phase and reference the spectrum carefully.

Step 3: Set up array for decoupling frequency offset (dof)

• Join exp2 (or another experiment #) with jexp2. Type mf(1,2) to move FID from exp1 to exp2. Type wft f full to display spectrum.
• Type setexp('homodec')
• Type ds to display spectrum. Place cursor in a clean baseline area away from peaks. Keep cursor away from spectrum edges (>0.5ppm). Type sd. This sets the reference decoupling position (1st element in array).
• Place cursor centered at a peak or a group of split peaks to be studied and type sda. Repeat this step through all peaks to be studied.
• Type da to display the dof array in Process->Textoutput window. Make sure you have the right number of array elements (reference + peaks to be decoupled).
• If a mistake is made during setting dof array, start from setting reference spectrum with sd command.

Step 4: Set nt and submit experiment

• Based on the signal strength of the 1H spectrum, set proper, equivalent nt.
• decoupling power dpwr is set to 5 by default. If the peak to be decoupled is narrow, set it lower (ie. dpwr=0, minimum is -16) if higher selectivity is needed. For broader peaks, set dpwr up to 20. Do not use more than 20 for dpwr.
• Type go to start experiment.

Step 5: Processing

• As in most experiments, click Process->Autoprocess to use vnmrJ's autoprocessing feature during or after experiment.

For manual processing:

• After the 1st array element (reference) is done, type wft aph f full. Manually adjust phase if necessary.
• Type dssa dscale to show arrayed spectra vertically. Type vs=vs*0.5 dssa dscale to shrink peaks by half.
• To zoom in a region, type ds(1) f full and expand region of the 1st spectrum. Type dssa to display array
• To display any one of the arrayed spectra, type ds(#) f full where # is the index number of the element (1,2, 3 .. etc.)

Default Parameters

• at=2 (2 sec acquisition time)
• d1=3 (3 sec recycle delay, minimum set to 1 sec)
• dpwr=5 (5 dB decoupling power. Max is set to 20. Reduce to 0 or down to minimum -16 if higher selectivity is needed)
• nt=4 (4 scans)

Example

Sample: Strychnine at ~ 25mg/mL (~ 100mM) in cdcl3 Data collected November 2010 (~ 4 mins with nt=8)

H. Zhou updated Nov 2010