Decoupling modes on the Benchtop NMR

One-dimensional NMR (Nuclear Magnetic Resonance) experiments are one of the most helpful tools a scientist has for quantification and structural elucidation of chemical compounds. 1D experiments can be broken down into two categories: regular 1D experiments, and 1D decoupled experiments. The simplest 1D experiment consists of a 90° radio frequency (rf) pulse, applied at the Larmor frequency of the observed nucleus, followed by acquisition; this pulse knocks the bulk magnetization from the Z-axis into the XY-plane. During the acquisition, the detected signal is the FID (Free Induction Decay) which is the decay of the transversal magnetization (M_xy) over time. A Fourier transformation is applied to the FID (time domain) in order to convert it into the NMR spectrum (frequency domain). A 1D decoupled experiment is similar to a regular 1D experiment but incorporates a secondary rf pulse in the second channel. This secondary pulse is applied differently depending on the type of 1D decoupled experiment being run. The three modes of 1D decoupled experiments are decoupled, gated, and inverse gated.^[1,2]

Figure 1. The figure above represents the different pulse sequences for a (a) regular 1D decoupled with NOE, and (b) inverse gated and (c) gated experiment.

Decoupled (¹H decoupling is applied with a 100% duty cycle)

The general idea behind a decoupled experiment is to transmit a continuous rf pulse (during both acquisition and relaxation) with a broad bandwidth at the Larmor frequency of the coupling nuclei. This rf pulse is used to annul the spins of the coupling nuclei and results in a lack of observed coupling when the nuclei of interest are pulsed upon. In addition to the decoupling effect, the Nuclear Overhauser Effect (NOE) is also observed during this type of decoupling amplifying the signal intensity significantly in carbon spectroscopy (this will be covered in another blog post). For this reason, this is the most common mode for the collection of carbon NMR data.

Inverse Gated

In this decoupling mode, opposite to the mode described in “Gated”, the decoupler is only ON during the acquisition period. By having the decoupler ON during this period, the influence of NOE is minimized, and no coupling is observed. This mode is used if quantitative results are desired.

Gated

The gated decoupling mode is used when the user would like to preserve the coupling information between nuclei and get the benefits of NOE (note that NOE can sometimes decrease signal intensity, like in the case of ¹⁵N and ²⁹Si). This is done by applying the decoupling pulse during the relaxation period but not during acquisition.

In the following example you can see the difference between decoupled and inverse gated. Figure 2 shows the carbon spectra of tert-butanol. The spectrum on the left was acquired using decoupled mode and you can see how the integral of each peak is affected differently due to NOE enhancement. The spectrum on the right was acquired using inverse gated decoupling. You can see that the integral ratio is now 1:3. You can also see that the signal to noise ratio on the left spectrum is higher due to NOE. (Please note that in order to make the spectra quantitative we used an interscan delay of at least 5·T₁).

Figure 2. A 13C NMR spectrum of tert-butanol is shown above. Left) This spectrum was obtained using decoupled mode and shows how NOE affects the intensity of the signals present in the spectrum. Right) This spectrum was obtained by using inverse gated decoupling mode. Both spectra are obtained using the NMReady-60PRO.

Please don’t hesitate to contact us if you have any questions about the decoupling experiments or if you want to see how our instrument can be incorporated into your workflow!

References

^[1] http://chem.ch.huji.ac.il/nmr/techniques/1d/row2/c.html (accessed December 20, 2019)
^[2]https://u-of-o-nmr-facility.blogspot.com/2008/05/modes-of-heteronuclear-broadband.html (accessed December 20, 2019)