Advanced Organic Chemistry. Structure and Mechanisms by Carey F., Sundberg R.

By Carey F., Sundberg R.

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The first zero-crossing point is at 1/(2× pulse width) – this would be at about 150 kHz for a 3 µs pulse. For a 400 MHz spectrometer, we need to cover a bandwidth of about 8 kHz for a proton spectrum. 4 shows, there is minimal power fall off for such a small pulse. Of course, it is quite easy to solve the bandwidth needs of proton spectra – they only have a spread over about 20 ppm (8 kHz at 400 MHz). Things get a bit more difficult with nuclei such as 13 C where we need to cover up to 250 ppm (25 kHz) spread of signals and we do notice some falloff of signal intensity at the edge of the spectrum.

5 Correct line shape and some typical distortions caused by poor shimming. Sometimes the (automatic) shimming process goes wrong and the instrument is unable to generate the field homogeneity that is needed. You will need to spot this otherwise you may make the wrong judgement about your compound. So how can you tell? Well, the key is to understand what physically happens if the field is not homogeneous. Your sample should experience the same strength field wherever it is in your sample tube. If it doesn’t, then molecules in different parts of the tube will resonate at slightly different frequencies.

Tuning and matching allow us to tweak the circuit to compensate for the sample load on the coils. In older systems (many of which are still in use), tuning and matching is carried out on the probe using tuning knobs. In more modern systems this is done under automation by the instrument. , salt solutions). 10 Frequency Lock Because the magnetic field of an NMR spectrometer can drift slowly over time, it is necessary to ‘lock’ the spectrometer frequency to something that drifts at the same rate.

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