![]() ![]() And this goes to parts per million and multiplied by 10 to 6th because the shifts are very tiny. And then by dividing by the reference frequency, you come independent of the actual field, and it's universal for all NMR machines. And it records the absorption peak as a shift from the reference. So as we say here, the delta scale uses a reference compound. All right, so this is the way you monitor an NMR spectrum. So when you multiply by 10 to the 6 you get ppm and they usually range from about 0 up to 12. And you multiply it by 10 to the 6 because you're going to get a very small number here if you don't multiply it by 10 to the 6. If you divide it by the reference, then it'll be the same for all spectrometers. So if you didn't divide it by the reference, then you'd get different values for each spectrometer and that would be confusing. So the reason you divide it is because sometimes you can use, and NMR you have spectrometers with different frequencies. Now the reference for protons in carbon 13 is almost always tetramethylsilane and you divide it then. And as I defined it on the last overhead, it's nu minus nu reference. Okay, all right, so this is just a, so the chemical shift is measured in what they call a delta scale. But you're using a scale, the delta, the delta scale. So generally when you see anymore spectra, you're not comparing that the absolute frequency is new. And that's because you multiply by 10 to 6 it's called parts, PPM or parts per million. And the exact definition is the frequency of, say, a sample minus the frequency of tetramethylsilane divided by the tetramethylsilane multiplied by 10 to the 6. So what this is called when you do it like this it's called the delta scale. And then you measure the position of these other peaks relative to TMS. So in effect what you do is you say this is zero. And then you measure these relative to that. You'd also get a peak down here for the TMS. Say this is our molecule with three different, say, proton positions. So your typical spectrum then will contain something. So what you do is you measure relative shifts. You get the spectrometer to measure the shifts relative. And you measure the shifts in your molecule, that you're looking at, you put that along in with your sample, in the sample tube. We talk about this in the notes in a minute, it's called tetramethylsilane. What you usually do is you put in a, what's known as a standard And the usual standard we talk about. But in practice you don't actually do that. And these are absolute values of the frequency. We said we had these, Different shielding positions and we had a spectrum we plotted against new, to frequency and we get three different values. Right, okay, so, When you measure we talked about let's go back again to the, So our demonstration here. You add to it and the effective field at the proton and nucleus is larger. Right, so again you represent that this way you have your outside external field. Or they make the applied field increase in magnitude at the actual position. Sometimes in systems like this, benzene and ethylene they actually deshield the nucleus. So that means that this will field an actual field that's larger than the applied. And you can see the currents generated here at the protons are in the same direction as the magnetic field. Here you have applied magnetic field in this direction. And the magnetic field can be shown to be in this direction. And the idea here is that these aromatic ring system here, and the pi bonds here, generate a magnetic field. And the most common ones that you come across are benzene and ethylene. And that means that they'll add to the magnetic field, so that the field that you feel at the nucleus position will be enhanced, if you like, compared with the external field you're using. And some will actually deshield the nucleus. I'm just going to mention that, that's always not the case. Right, so just we talked about, the idea that all electrons tend to shield the nucleus. And again, you'll always come across these terms and in a more upfield and downfield. Okay, so we've developed that point a little bit about deshielded, high frequency, shielded, low frequency. ![]()
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