HETCOR- Heteronuclear Correlation Experiment

 

Absolute value 2D HETCOR- Heteronuclear Chemical Shift Correlation. See Acquiring and Processing 2D Experiments for the basic features of two-dimensional experiments. Also, the Varian Guide to NMR Experiments has a detailed description of this experiment. The setup is similar to that of the phase sensitive HETCOR experiment.

 

1.      Set up and execution:

a.       On the I500, please make sure that the system is set up to acquire protons.  If you are not sure, ask the staff. You can damage the spectrometer, if it is not wired correctly. Other spectrometers do not require hardware changes.

 

b.      Acquire a standard proton spectrum set up for your solvent. Optimize the sweep width to those resonances that you want to study by setting the cursors around the resonances and type movesw.  (You may want to repeat the experiment with these new parameters. Be aware of resonances that may have folded in. While folded lines are not necessarily bad, you want to watch out that they do not overlap with your lines of interest.  Since this is the indirectly detected dimension, different 13C-chemical shifts may help you to disentangle the resonances in the final 2-dimensional spectrum.  At this point you can also examine the resolution you want to get for the proton dimension by setting np to the number you will later use as ni, for example np=sw/20).

 

 

c.       Next you want to take a carbon spectrum. On I500 ask the staff to recable the spectrometer for carbon detection with proton decoupling. The other spectrometers do not require hardware changes.  jexp2, join another experiment, for example experiment 2, if not used yet. This allows you to look back at your parameters later.  Also optimize the sweep width by typing movesw.  You may want to repeat the experiment with the new parameters. Be aware of resonances that may have folded in. If you narrowed the proton sweep width around your resonances of interest, you may need the full carbon sweep width to avoid folding in both dimensions.

 

d.      You are now ready to set up the phase sensitive HETCOR experiment.

Type:

·        mp(2,3)                  move the parameters from your carbon experiment (2) to an unused

                        experiment (3).  

·        hetcor                calls the absolute valute HETCOR experiment. You will be asked to

supply the experiment number that contains your proton experiment.

2.Set the following parameters:

 

·        temp=30                (set a temperature to keep it stable)

·        spin=0                   (also turn off the spinning in the acquisition window)

·        su

·        np=2048

·        d1=                        1 to 2 times the proton(!) relaxation time

·        nt=                         a multiple of 4

·        ni=                         insert the value you found sufficient in 1.b.

·        phase=1,2             (this makes it phase sensitive)

·        tpwr=                     transmitter power (listed on monitor)

·        pw=                       (90o pulse on carbon (listed on monitor)

·        dpwr=40

·        pplvl=                    proton pulse power level (listed on monitor)

·        pp=                        proton 90o pulse (listed on monitor)

·        dmm=’ccg’

·        j1xh=140               (average CH-one bond coupling constant)

·        chonly=‘y’             gives CH only spectrum, otherwise chonly=’n’

·        oddeven=’y’         gives CH and CH3 positive and CH2 negative, otherwise oddeven=’n’            (irrelevant if chonly=’y’).

·        hmult=’n’              removes non-geminal proton-proton couplings in F1, ‘y’ preserves

non-geminal proton-proton couplings in F1     

·        fn=2*np

·        time                       approximate time your experiment will take. (see table below)

 

 

Considerations about the time the experiment will take and optimization:

a.       The time for the duration of the experiment is approximately given by:

 nt * ni * 2 * ( d1+ at ).

b.      If your experiment is taking too long you can change one of the following parameters: (Of course for extending your experimental time, you can increase those values)

c.       Decrease nt which costs you signal to noise; however, nt has to be a multiple of 4.

d.      Decrease ni, which costs you resolution in the carbon (nitrogen) dimension. Your resolution in Hz is sw1/ni. Zero filling will only visually increase the resolution, not give you more information.

e.       Decrease d1, which lets your signal relax not quite as much. There is an optimal value for the relaxation compared to the T1 time. However, this is the parameter where you may be able to save time the best.

f.        Decrease at, that means decrease np or increase sw. This is not recommended, because you set your sweep width for your parameters.

g.       Decreasing np decreases your resolution in the direct dimension. Anyway, usually at will be small compared to d1 and you do not gain that much.

 

The following table gives you an idea about the duration for some

HETCOR-experiments:

 

Nt

Ni

d1/s

at/s (np=2048)

Time (hours)

4

128

2

0.155

0.38

4

480

2

0.155

2:31

4

128

4

0.155

1:12

4

480

4

0.155

4:39

16

128

2

0.155

2:32

16

128

4

0.155

4:48

 

3.  You are now ready to collect your HETCOR spectrum, type au.

4. Work-up of your experiment:

See Acquiring and Processing 2D Experiments for details of the work-up of 2-dimensional experiments.

 

In my experience the phasing is the trickiest part. To get the full benefit of the phase sensitive method, you want absorptive peaks in both dimensions, (that is real-real spectra). In the color display you want almost all yellow contour lines around a peak (or all violet for the negative peaks). Concentrate on those peaks you are most interested in. You can examine, whether you are well phased and correct the phasing by looking at the F1 and F2 slices through your peaks of interest.

Find the slice numbers of your peaks by placing the cursor on that peak (a vertical projection and expanding the scale may help you to locate its maximum). The slice index numbers (say n) will be displayed on the top line.  Typing ds(n) you can look at that one dimensional slice. You can now phase that peak in the usual way. If your slice contains only one peak, you want to do the phasing once on that slice to get the 0-order phase correction. Then load a slice with another peak with ds(m). Now phase by clicking once in the area where the first peak was, but do not press the left or right buttons again. That keeps the 0-order phase correction. Afterwards phase the present peak in the usual way. That sets the 1st-order phase correction. You can then scan through all other slices of interest to see if they are phased well. You may need to compromise on the overall phasing. Peaks folded in from resonances outside your window (see 1.b.) and peaks with very different j-couplings may not be phased well, when you are done. To phase the other dimension, you need to switch the axis. Do that by clicking on Main Menu -> Display -> More -> F2 Mode (or F1 Mode). Repeat the same phasing procedure for this dimension.