Chemistry

Nuclear Magnetic Resonance Spectroscopy (NMR)

Nuclear Magnetic Resonance Spectroscopy (NMR)


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1H broadband decoupling of 1313 C-NMR spectra

1H broadband decoupled 1313 C-NMR spectra are the most frequently recorded 13C spectra.

For all carbon atoms only singlets are observed in the spectrum (provided there are no other coupling nuclei such as 19F or 31P are included in the compound), since the otherwise dominant 1H-13C-couplings are switched off.

Example acetylsalicylic acid

In the picture are the decoupled for comparison 13C spectrum of acetylsalicylic acid (above) and the spectrum with all couplings (below - more detailed explanations in the "gated decoupling experiment") are shown.

In broadband decoupled 13C spectrum gives each of the nine carbon atoms of acetylsalicylic acid a signal.

Due to the Kern-Overhauser-Effect (see "Kern-Overhauser-Effect") the signals of the C-atoms, which are bound directly to protons (2,3,4,5,9), are more intense than the signals of the quaternary C-atoms (1,6,7,8).

Experimental:

At the 1H-broadband decoupling experiment, the protons are decoupled during the entire measurement process.

That means in addition to 13C resonance frequency, the proton resonance frequency is radiated in and the transitions of all protons are saturated, i.e. N1= N0at 1H! (See also chapter "Basics - Nuclei in a magnetic field, resonance condition").

The protons change their orientation very quickly, so that the 13C atoms do not "recognize" the different spin states and thus no C-H coupling and no signal splitting takes place.

Advantages and disadvantages of broadband decoupling:

Tab. 1
Advantages and disadvantages of broadband decoupling
advantagesdisadvantage
No signal splittingNo information on the coupling constant
Simplified evaluationNo evaluation of the integral intensities possible
Signal amplification through Kern-Overhauser-Effect
Shorter measurement time


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