Long-Range HETCOR

This experiment gives the correlations between protons and other nuclei such as ¹³C (standard) or ¹⁵N that are two or three bonds away from each other. This experiment is very similar to the absolute value HETCOR experiment. The regular HETCOR experiment, however, is optimized for correlations of directly bonded heteronuclei.

See Acquiring and Processing 2D Experiments for the basic features of two-dimensional experiments. Also, see for example S. Braun, H.-O. Kalinowski and S. Berger, "100 and More Basic NMR Experiments" for a detailed discussion of the experiment.

1.) Set up and execution:

The set up is very similar to the regular HETCOR experiment. You just have to set one more parameter (jnxh):

1.1. (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.)

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 typing

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

1.2. 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.

 

Here 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.

1.3. You are ready to set up the long range HETCOR experiment:

 

mp(2,3)	move the parameters from your carbon experiment (2) to an unused experiment (3).
lrhetcor	calls the long range HETCOR experiment. You will be asked to supply the experiment number that contains your proton experiment.

 

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 8, better a multiple of 16
ni=…	insert the value you found sufficient in 1.1.
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=…	average CH-one bond coupling constant (130 for aliphatic CHn- groups; this is a parameter you may need to optimize. Missetting will give you spurious or missing cross-peaks.)
jnxh=…	average CH-multi bond coupling constant(10-30 Hz may be good start; this is a parameter you may need to optimize. Missetting will give you spurious or missing cross-peaks.)
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 below)
au	you are ready to collect your HETCOR spectrum.

 

1.4 Considerations about the time the experiment will take and optimization:

The time for the duration of the experiment is approximately given by

  nt * ni * ( d1+ at ).

 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)

• Decrease nt which costs you signal to noise; however, nt has to be a multiple of 8.
• Decrease ni, which costs you resolution in the carbon (nitrogen) dimension. ni can be any number. Your resolution in Hz is sw1/ni. Zero filling will only visually increase the resolution, not give you more information.
• 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.
• Decrease at, that means decrease np or increase sw. This is not recommended, because you set your sweep width for your parameters. 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
8	128	2	0.155	0:38
8	480	2	0.155	2:28
8	128	4	0.155	1:11
8	480	4	0.155	4:36
16	128	2	0.155	1:15
16	128	4	0.155	2:36

1.5. Work-up of your experiment:

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