The limitations of 19F GARP Decoupling

In a previous post, it was shown that distorted line shapes are obtained for resonances in broadband decoupled NMR spectra when the resonances of the decoupled nuclide are outside of the effective decoupling bandwidth.  This can be a particularly difficult problem when observing ¹H NMR spectra with ¹⁹F decoupling.  ¹⁹F has a large chemicals shift range so, if there are multiple widely spaced ¹⁹F resonances, it will be difficult or impossible to decouple all ¹⁹F sites at once, particularly at higher magnetic field strengths.  If one is not aware of this problem, data misinterpretation may be an issue as distorted line shapes will  lead incorrect splittings used to measure coupling constants.  The figure below illustrates this problem.  The top three panels of the figure show the 300 MHz ¹H[¹⁹F] NMR spectra for the three ¹H resonances of 1,2-difluoropyridine as a function of the ¹⁹F decoupler offset.  The GARP decoupling scheme was used with 90° pulses of 80 µsec.  The decoupler offsets, depicted in the bottom panel of the figure, were varied in 5 ppm increments.

Of the 11 decoupler offsets used, only offset 6 (at -116 ppm) effectively decoupled both ¹⁹F sites.  Varying the decoupler offset by only ± 5 ppm leads to distorted line shapes, which are particularly pronounced for the H₃ resonance.  These distorted line shapes could easily lead to data misinterpretation and erroneous coupling constants.  In this case, the ¹⁹F decoupling bandwidth is 55 ppm.  Since the chemical shift difference between the two  ¹⁹F resonances is 52 ppm, one is able to obtain a fully ¹⁹F decoupled ¹H spectrum with the careful choice of the decoupler offset frequency however, there will be cases where the decoupling bandwidth would not be sufficient to decouple all ¹⁹F resonances in some molecules.  How then can one generally evaluate all of the coupling constants in fluorine containing molecules?  The ¹⁹F-¹⁹F couplings can be evaluated in a ¹⁹F[¹H] spectrum (not shown).  Specific ¹H-¹⁹F coupling constants can be determined by measuring a ¹H PSYCHE spectrum or be collecting ¹H spectra with selective ¹⁹F continuous wave (CW) decoupling  for each of the¹⁹F resonances.  The latter is shown in the figure below.  The bottom panel shows a standard ¹H spectrum.  The middle two panels show the ¹H spectra for each of the ¹⁹F sites decoupled separately using CW decoupling.  The top panel shows the fully ¹⁹F decoupled spectrum.

Using these data, all of the coupling constants can be evaluated and are shown in the figure below.

In conclusion, one must be careful in interpreting ¹H[¹⁹F] spectra and understand the limits of the ¹⁹F decoupling scheme used.

Decoupling Bandwidth and Distorted Line Shapes

Broadband X nucleus decoupling (X = ¹³C, ¹⁵N, ³¹P, ¹¹B, ¹⁹F etc.....) is frequently used in ¹H detected 2D HSQC/HMQC data collection or in standard 1D ¹H spectra to aid in structure assignment.  When broadband decoupling schemes are used, one must keep in mind that they are not infinitely broadbanded.   They have finite bandwidths over which they are effective thus limiting the chemical shift range for the decoupled nuclide.  The effective bandwidth depends on the particular decoupling scheme and the decoupling power used.  If multiple peaks are to be decoupled, one must insure that all peaks are within the decoupler bandwidth.  One can determine experimentally the effective decoupling bandwidth by running a series of ¹H spectra varying the decoupler offset frequency.  Such a measurement is shown in the figure below for the P-CH₃ methyl resonance of dimethyl methylphosphonate. 300 MHz ¹H [³¹P] NMR spectra were collected in a pseudo-2D fashion, incrementing the decoupler offset frequency from 200 ppm to -200 ppm from the ³¹P resonance frequency in 1 ppm steps.  Broadband GARP decoupling was employed with a power of 3125 Hz  (80 µsec 90° pulses).  The pseudo-2D contour plot is shown in the left-hand panel and a stacked plot is shown in the right-hand panel.

One can see that the effective decoupling bandwidth is 15.56 kHz or 128 ppm on a 300 MHz instrument.  When the decoupler offset exceeds ±64 ppm from the ³¹P resonance frequency, one obtains distorted line shapes.  Representative distorted line shapes are shown in the figure below.  The bottom spectrum was collected with no ³¹P decoupling.  The top, fully decoupled spectrum was collected with on-resonance ³¹P GARP decoupling.  The middle spectra, highlighted in pink, are representative distorted spectra outside of the effective decoupling bandwidth.

If one is not aware of the decoupling offset and available bandwidth, one may obtain misleading line shapes subject to misinterpretation.

Positive NOE's and the Decision on Which Decoupling Mode to Use

Nuclei with positive gyromagnetic ratios, such as ¹³C, exhibit positive NOE's with nearby protons. When observing ¹³C directly with proton decoupling, the intensity of the resonances will be increased to an extent dependant on the magnitude of the NOE's and the amount of time over which they are allowed to build up. When quantitative results are not sought after, it is always best to collect the NMR data with decoupling during both the acquisition time and the relaxation delay so that the effect of the NOE's on the intensity of the lines is maximized. The figure below compares the ¹³C NMR spectra for formamide collected with inverse gated decoupling (left) and full decoupling (right).
The above spectra were collected at 11.7 tesla with 4 scans, using a recycle delay of 30 seconds and an acquisition time of 2.3 seconds. This effect of positive NOE's on the resonance intensity should be compared to that for negative NOE's.

Negative NOE's and the Decision on Which Decoupling Mode to Use

Nuclei with negative gyromagnetic ratios, such as ¹⁵N and ²⁹Si, exhibit negative NOE's with nearby protons. When observing these nuclei directly with proton decoupling, the intensity of the resonances will be decreased to an extent dependant on the magnitude of the NOE's. Usually it is best to collect the NMR data for these nuclei with inverse gated decoupling so that the effect of the NOE's on the intensity of the lines is minimized. There are cases however where the magnitude of the NOE is so large that the intensity of the resonance collected with full decoupling becomes negative and even stronger than that observed with inverse gated decoupling. In such cases it is advantageous to leave the decoupler on 100 % of the time. As the figure below shows, this is indeed the situation with the ¹⁵N resonance of formamide where the magnitude of the signal is smaller when inverse gated decoupling decoupling is used.
The above spectra were collected at 11.7 tesla with 4 scans, using a recycle delay of 120 seconds and an acquisition time of 2.3 seconds.

Effect of 1H Tuning on the Signal-to-Noise Ratio in 13C NMR Spectra

Most liquid state ¹³C NMR data is collected with ¹H decoupling so that all of the ¹³C resonances (not coupled to nuclei other than protons) will appear as singlets. Since ¹³C is only 1.1% naturally abundant and often the chemist is limited by the quantity of sample, acceptable signal-to-noise ratios in ¹³C NMR spectra are often difficult to obtain in a reasonable period of time. Any hints to improve the signal-to-noise ratio are welcome. One way to ensure that the signal-to-noise ratio is what it should be, is to make sure that the proton channel of the probe is well tuned. A poorly tuned proton channel will lead to incomplete decoupling resulting in broad lines or residual splittings and hence a low signal-to-noise ratio. The reason for this is that when the proton channel is poorly tuned the sample receives less power from the proton channel of the spectrometer and the decoupler pulses in the decoupling scheme (usually WALTZ 16) are no longer calibrated for proper decoupling. This is illustrated in the figure below.For the spectrum in the left-hand panel, the ¹H channel was well tuned. In the right-hand panel, the ¹H channel was poorly tuned. One can see that the spectrum acquired with the poorly tuned ¹H channel has a lower signal-to-noise ratio. There are even more problems with pulse sequences (such as DEPT experiments) which require hard pulses from the ¹H channel. This sort of problem can also be seen in proton detected experiments with poorly tuned ¹³C channels when ¹³C decoupling is required (eg. HMQC/HSQC experiments)

Modes of Heteronuclear Broadband Decoupling

In talking to many novice NMR users over the years, I have often been discouraged to learn that some do not even realize that they are using heteronuclear broadband decoupling when they run (for example) a ¹³C spectrum. They do not question why their signals are singlets or even realize why they may not be singlets if ¹H decoupling is not used. It is for this reason I have added this BLOG post. Heteronuclear broadband decoupling is the irradiation of one nuclide while observing another nuclide. It is normally applied using multiple pulse decoupling schemes such as WALTZ 16 or GARP. These schemes effectively allow the decoupling to be efficient over a range of frequencies (sometimes even the entire chemical shift range!) with a minimum of power. The figure below shows some common modes of heteronuclear decoupling.In the top trace, broadband ¹H decoupling is applied with a 100% duty cycle. This is most common for the collection of ¹³C NMR data. It is applied during the acquisition time to decouple protons and during the recycle delay to take advantage of any nuclear Overhauser enhancement. In the second trace, decoupling is applied only during the acquisition time to decouple protons but not during the recycle delay. This is called inverse gated decoupling and is often used when collecting NMR data for nuclei like ¹⁵N or ²⁹Si which have negative gyromagnetic ratios and hence negative nuclear Overhauser effects. In the third trace the decoupling is applied only during the recycle delay but not during the acquisition time. This is called gated decoupling and is used when a fully coupled spectrum is desired while still taking advantage of any nuclear Overhauser enhancement. The bottom trace shows an example of inverse gated decoupling applied to an X nucleus while observing protons. For examples of this, click here and here.

The Effect of Proton Decoupling on the Signal-to-Noise Ratio for 31P NMR

I have seen some students collect 31P NMR data without proton decoupling. Their reason was that there were no directly bound protons to the phosphorus. What they have not taken into account are the remote protons which have small couplings to the phosphorus. There are usually many of these remote protons and the cumulative effect of the couplings is to broaden out the 31P resonance. This broadening causes a severe loss in the signal-to-noise ratio. The upper trace of the figure below is a single scan 31P spectrum of triphenylphosphate where the closest protons are 4 bonds away from the phosphorus atom. The lower trace was run under identical conditions with the proton decoupler turned on. In this case, the gain in signal-to-noise using proton decoupling is 17.