Chemistry

Nuclear Magnetic Resonance Spectroscopy (NMR)

Nuclear Magnetic Resonance Spectroscopy (NMR)



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Gated decoupling process

Since the inclusion of paired 13C-NMR spectra (for the evaluation of the coupling constants) require a very large amount of time (often several hours), a method was developed with the gated decoupling experiment that allows 13C-NMR spectra with all 13C-1Record H couplings in a reasonable time (approx. 1 hour).

The protons are not decoupled during the entire measurement process, as in the broadband decoupling experiment, but only in the part where no data is recorded. During the decoupling the Kern-Overhauser-Effect (see chapter "Kern-Overhauser-Effect") can build up, which leads to a signal amplification. At the 13C-pulse and data acquisition, the decoupling is then switched off so that all couplings can be recorded.

It follows:
Increase in signal intensities, couplings visible, time savings

Example acetylsalicylic acid

The following picture shows the gated decoupling spectrum of acetylsalicylic acid.

The spectrum is dominated by the C-H couplings through a bond; 1JCHlies between 130.2 Hz (CH3Group) and 164.1 Hz (aromatic CH group).

Furthermore, C-H couplings via two and three bonds are visible (J <9 Hz).

It is very nice to see the dependence of the C-H coupling constants on the electronegativity of the substituents and the s-bond component (see also "Spin-spin couplings, coupling via a bond"). An increase leads to increasing values ​​of JCH.

The coupling patterns correspond to the rules for spin 1/2 nuclei (chapter: "Spin-spin couplings, splitting rules").

example

CH3-Group (coupling C-H via a bond), according to M = n + 1 = 3 + 1 = 4

That 13The C-Signal is split into a quartet by the three neighboring H atoms.

Advantages and disadvantages of the gated decoupling experiment:

Tab. 1
Advantages and disadvantages of the gated decoupling experiment:
advantagesdisadvantage
All couplings visibleSpectra often confusing due to the couplings
Determination of the coupling constants possibleSignal assignment difficult

Reversed gated decoupling experiment

This experiment is used to record decoupled spectra for quantitative evaluation.

The protons are only used during the 13C-impulse and the data acquisition are decoupled. In the course of time, no more Kern-Overhauser effect can build up, so that the "real" signal intensities are retained, but the spectrum no longer shows any couplings.

It follows:
no Kern-Overhauser-Effect, none 13C-1H couplings