Homo-Nuclear Decoupling Experiments
AM and AMX Spectrometers

Homo-Nuclear Decoupling: AMX-300, AMX-400

Homonuclear decoupling on the AMX spectrometers can be done in two different ways. Depending on whether you need a quick and simple experiment, or good signal-to-noise, select one or the other.

Two possible modes are described:
Method A) results in some loss (~30 %) in S/N but no cable changes.
Method B) requires changing cables. It uses a directional coupler to reduce loss to ~6 %.

Method A)

Set the following parameters: (use macro hhdec) 
PULPROG      hdamx            (Pulse program)
P1          Set to 30 (same as P0)to 90 deg
Hl2         27-32 (VBAMX-400)  (Dec power)
            52  (AMX-300)
DS          1                  (Dummy scans)
DECSTAT     HD                 (Decoupler Mode)
DECNUC      1H                 (Decoupler Nucleus)

In Calib, use {SFO1 2 3} (middle button) to select O2

Method B)

AMX-3/400 Homodecoupling (Values measured on the AMX-300):

Use the pulse program zg0hdh2, level dl0 = 35, put the "directional coupler" between the pre-amp (1H) and the probe.
Insert it at the "*" marks: 

  PRE-AMP Housing               ___________________
  --------|      _____         |                   |
       ___|_1H  |     |       _| Directional       |_
       ___|_|---|     |---*--|_|                   |_|-*--> Probe
          |     |_____|        | Coupler           |
          |                    |                   |_
  --------|     300 MHz        |                   |_|--< From
                Filter         |___________________|    Console
The 90 deg pulse increases from 5.1 to 5.4 usec (and S/N loss is thus about 6 %). Therefore, it seems best to remove the coupler when not actually needed.

Values for the VBAMX-400 should be similar (3/1/93: not yet measured), the 90 Deg pulse should increase by about the same percentage.

Homo-Nuclear Decoupling: AM-400, AM-500

One Single Frequency Decoupling Experiment

1) Obtain and process a normal spectrum.

2)Enter EP, expand around the area where you want to set the decoupler. Place the cursor at the position to be irradiated. Type 'O2' followed by 'M'. After EP is exited, type O2 to check the value. Selecting 1 to 4 dummy scans (DS) might be useful for acquiring the decoupled FID. Set the decoupler power with 'DP', use about '15 L'. The optimal level depends on the probe used as well as the desired decoupling bandwidth.

DO NOT, EVER, USE 'H' in place of 'L'!!!

3) Type 'HD' to activate homo-decoupling mode. Acquire the FID with 'ZG'.

4) Type 'PO' to turn the decoupler off after the acquisition has completed.

5) Process and plot (EFP, EP etc.) as usual.

Multiple Single Frequency Decoupling Experiments

(A 2D-COSY might be easier, provide more information and probably will be just as quick.)

1) Acquire and process a regular spectrum.

2) Enter EP. Place the cursor at the position to be irradiated. Type 'O2' followed by 'L' to enter cursor frequency in a "Frequency List", FL. Place the cursor on the next region, type 'O2', 'L', repeat for more experiments. exits EP and closes (writes) the frequency list.

3) Use 'AS HOMODEC.AU=D1' to set up the parameters. The 'FL' command shows frequency list, scrolls the list. The list should contain as many values as experiments desired. 4) Set NE= of number of values in the FL list.

5)Set D1=1 sec for loading the frequencies. Set RD to a short value (or 0) to keep the decoupler on most (or all of) the time.

6) Set NS=8 or a multiple of 8.

7) DS=1 to 4 dummy scans.

8) S3=15L for starters.

9) 'AU' is used to execute the .AU (or micro-) program. The name of the frequency list file will requested as well as the name to be used for the FID files. The program creates NE numbered files starting with .001, e.g. JUNK.001 to JUNK.004.

10) Type 'QUIT' when finished.

11) 'RE ' to read the FID file(s). Process and plot etc. Repeat for NE files.

12) Look up how to plot stacked plots (STACK.AU) as an automated alternative.

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Questions, comments to: Rudi Nunlist rnunlist@bloch.cchem.berkeley.edu

Last Update: 12/22/94.