Summary Decoupling is one of the most used NMR techniques. The aim of the method is to remove unwanted spin-spin interactions and it can be used either for structure elucidation purpose or for improving resolution and sensitivity due to simplified multiplet structure. Depending on the implementation decoupling methods can be divided into homonuclear and heteronuclear ones. In the former case both continuous wave irradiation(double resonance) and bandselective homodecoupling may be used, more detailed description of the homonuclear methods is available elsewhere. Heteronuclear decoupling also makes use of continuous wave irradiation (selective and off-resonance decoupling), however, it has been shown that for broadband applications pulsed technique gives superior results. Recently many decoupling sequences were developed covering almost all practical needs. A typical decoupling pulse sequence consists on wideband composite or shaped inversion pulse united into a cycle or supercyle to compensate residual imperfections. A short description of the most popular modern decoupling methods for liquid state NMR is available on the Internet. The problems discussed in the literature: Books and Reviews Shaka A.J., in "Encyclopedia of NMR", 1996, v.3, pp. 1558-1564 (19 References). Decoupling Methods. Levitt M.H., Freeman R., Frenkiel T., in "Advances in Magnetic Resonance", ed. Waugh J.S., Academic Press, N.Y. and London, 1983, v. 11, p. 47-110. Shaka A.J., Keeler J., Progress NMR Spectroscopy, 1987, v. 19, p. 47-129. Derome E., Pergamon Press, Oxford, 1987-280. Modern NMR Techniques for Chemistry Research. Ernst R.R., Bodenhausen G., Wokaun A., Clarendon Press, Oxford, 1987-610. Principles of NMR in One and Two Dimensions.
Homo-decoupling is a double-resonance technique because it uses two RF fields to affect magnetically active nuclei. Homo-decoupling involves applying a second 1H RF field to cause selective saturation of a nucleus A while observing all other nuclei in the molecule; B, C, D, etc. If nucleus A is spin-coupled to nucleus B and if the second RF field is strong enough, the result is that A is effectively prevented from spin-spin interacting with B. The observed B nucleus spectrum will appear as if it is not coupled to A. The A resonance commonly appears as a glitch as a result of this experiment.
1H - 1H DOUBLE RESONANCE(1H双共振实验): PRESATURATION, HOMODECOUPLING(同核去偶), & NOE DIFFERENCE
(ARX spectrometers)
OVERVIEW
1. The f1 channel is used for the observe channel and the f2 channel is used for the additional irradiation (e.g. a low power continuous wave during the relaxation delay for presaturation and NOE difference experiments, or time-shared irradiation during the data acquisition period for homodecoupling). The two channels must first be routed correctly by specifying what nucleus will be excited by each channel. Then the precise values of the frequencies must be set.
2. The zg command will start the current pulse program (i.e. the PULPROG parameter). The current pulse program always determines which channels are actually used and the timing of the pulses on each channel that has been activated through the eda menu.
DETAILED SET-UP
1. Obtain a properly phased 1H spectrum of the sample.
2. The dflt1h.400 and dflt1h.500 parameter files contain the pulse program zg30 which use 30 degree pulses. For the NOE difference experiment, you should use the pulse program zg which uses a 90 degree pulse. The only reason to make this change for the normal 1H spectrum is to set the gain properly before proceeding to the NOE difference experiment.
3. Turn on the f2 channel with the following steps. Type eda. Select "NUCLEI edit." In the submenu, select DEC. This will bring up a menu of nuclei. Select 1H. Select SAVE to return to the eda menu, then select SAVE again.
4. Use this spectrum to set the O2 frequencies to be used by the f2 channel for decoupling.
a. For cases where you only need to set a single value for O2:
1) Enter the utilities subroutine. Select the "button" labeled O2. This will put the display into the mode where the arrow is "stuck to the spectrum".
2) Position the arrow on the resonance that you want to irradiate. Press the middle button to "define" the frequency. This will set the value of the O2 parameter.
3) Select return to exit the utilities subroutine.
b. For cases where you need to create a list of O2 frequencies for the f2 channel (e.g. a series of homodecoupled spectra or an NOE difference experiment):
1) Enter the utilities subroutine. Select the button labeled frqlist. The following dialog will occur:
Please enter type of list: f2
Please enter name of f2 list: input desired list name
Write name of f2 list to acqu parameters? y
(If the list already exists, you will get the following:)
Freq. list exists, append (a), overwrite (o), or quit(q): select appropriate choice
2) At this point, you will use the mouse to select the positions of the various irradiations. The message window will tell you the function of the three mouse buttons, i.e.
"return / define frequency / unused". Position the arrow on the resonance that you want to irradiate. Press the middle button to "define" the frequency. Repeat for each irradiation point. (If this list is to be used for a NOE difference experiment, be sure to include a position not on a resonance for the subtraction. Processing will be easier if you make the off-resonance value either the first or the last in the list.) When all irradiation points have been defined, press the left mouse button to terminate the selection process. At this point you will have a frequency list whose name has been entered as the parameter f2list.
3) Select return to exit the utilities subroutine.
5. If you do not wish to overwrite your original spectrum, create a new data set. (The original spectrum is often useful later, e.g. for a dual display.)
a. On the top bar, select File, then New. to change the Name or Expno to something that doesn't already exist.
b. This creates a new data set that contains parameters identical to those of the data set displayed at the time the new data set was created. In this case, the parameters will be identical to those used to obtain the original spectrum except that the f2 channel is turned on to 1H with the O2 value or the name of the f2 frequency list set properly.
6. Select a pulse program that will control the timing of the 1H irradiation for the desired experiment.
a. Type eda, select the PULPROG parameter (top left). Type in the name of the pulse program followed by Enter. Select SAVE to exit the menu.
1) For presaturation, use presat30.js (uses a 30 pulse) or alternatively presat.js (90 pulse).
2) For homodecoupling, use homodec30.js (30 pulse) or alternatively homodec.js (90 pulse).
3) For NOE difference, use noediff.js (composite 90 pulse).
b. It is always a good idea to have a hard copy of the pulse program you are using. To obtain one, type edpul <pulse program name>. The pulse program will be displayed. Select list to obtain a hard copy, then OK to exit from the display.
c. From the pulse program listing, you will be able to tell which output power register is used for the power level of the f2 channel. For all three of these pulse programs, it is dl2. (These are the letters d then l, followed by the number 2.) The value depends on the kind of experiment and the probe. Look in the spectrometer notebook for a suggested value.
7. There are a few other parameters that you will have to set.
a. For presaturation - The presaturation occurs during the d1 relaxation delay time. If the presaturation removes the largest signal, you should re-do the rga using this pulse program.
b. For homodecoupling - Probably no additional parameters need to be changed. If the largest peak is decoupled, you may need to re-do the rga using this pulse program.
c. For NOE difference - It is recommended that you use at least 4 "dummy scans". The irradiation occurs during the d1 relaxation delay time. For steady state NOE measurements you should use approximately 5×T1.
8. Type ii and wait for the ii:finished message
9. To acquire a single data set (e.g. for presaturation or homodecoupling):
a. Start the data acquisition with zg. Transform and phase the data. The phasing will probably be somewhat different from the regular spectrum because of additional delays due to switching.
b. To obtain a dual display of this data set along with the regular spectrum, you must first input the name of the other data set to be displayed. Type dual. If you have not previously defined the name of the "2nd data set" you will be asked for that information before the dual display starts. Input the name of the regular spectrum in the spaces provided for the "2nd data set", then SAVE the menu.
10. To acquire a sequential set of spectra with a different values of O2 for each (e.g. a set of homodecoupled spectra or data for NOE difference):
a. There is an "au program" that will create a sequential set of increasing experiment numbers starting at the current experiment number. To start this program, type multio2. You will first be asked to input the name of the pulse program to be used. You then must specify the number of irradiation points (which will be the number of data sets to be acquired). Make sure that any of the experiment numbers that will be created by this process do not already exist if you don't want them to be overwritten.
b. The question about "# of time average cycles" refers to cases where you need a fairly large number of scans to obtain an adequate signal to noise ratio. You will have fewer systematic errors if you acquire a moderate number of scans at each irradiation point, then cycle through the entire set several times. If you only need a modest number of scans to obtain a reasonable signal to noise ratio, input 1 in response to this question.
c. To process these data sets, read in the first one, transform and phase it. Then type multiefp and answer the questions to process all the rest exactly the same.
d. For NOE difference there is an "au program" to do the subtractions provided that the off-resonance data set is the first or last in a sequential set of increasing experiment numbers. Read in the data set that contains the off-resonance data. Then type diffe and answer the questions.
e. There is an "au program" to produce a stacked plot of a sequential set of spectra. Set the plot parameters with the edg menu such that integrals and peak labels are turned off. Then type stack1d and answer the questions. An example is given in the "Measuring T1 Relaxation Times" handout.
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