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Current time:0:00Total duration:5:51

Video transcript

in the previous video we looked at the protons on benzene and we said all six protons were in the same environment therefore all six protons are chemically equivalent and should give us only one signal on an NMR spectrum and so here's the one signal on a spectrum due to the protons on benzene if we compare benzene to this compound this is tetramethylsilane or TMS and the protons on TMS are all in the same environment and so therefore we would expect one signal for TMS and here's a signal for TMS right here in an earlier video we said that as you go to the right on an NMR spectrum you're talking about a lower frequency signal so a lower a lower frequency signal as you move to the right on an NMR spectrum and so if you move to the left on NMR spectrum you're talking about a higher frequency signal so a higher a higher frequency signal and so therefore the protons on benzene have a higher frequency signal than the protons on TMS TMS is actually our standard because the protons on TMS are more shielded than almost all organic compounds and so therefore it's our reference and so instead of talking about frequency we could talk about we can talk about chemical shift values here and the chemical shift would be would be a similar idea to the frequency so as you go to the right you're talking about you're talking about a lower chemical shift lower chemical shift and as you move to the left on an NMR spectrum you're talking about a higher chemical shift so a higher shift as you move to the left so the protons on benzene have a higher chemical shift than the protons on TMS actually we set this equal to zero so this is our standard so how do we how do we figure out what the chemical shift is for example for the protons on benzene it looks like it looks like the signal appears a little bit past seven here so how do we get this number for a chemical shift well again everything is compared to TMS and so let's look at let's look at the formula for calculating chemical shift so if I move down here we can see the formula for chemical shift chemical shift is equal to the observed shift from TMS and Hertz times ten to the six divided by the spectrometer frequency in Hertz for example let's say that we are using an NMR spectrometer operating at 300 megahertz so we're using a 300 megahertz spectrometer here if you're using a 300 megahertz spectrometer the protons on benzene absorb a frequency 2181 hurts more than the protons on TMS and so once again TMS is our standard our reference so this difference if you're thinking about frequency right this difference between our two signals is 2181 hertz if we are using if we are using a 300 megahertz spectrometer and so let's go ahead and figure out the chemical shift for the protons on benzene so let's get some more room down here and so here's a here's a symbol for a chemical shift so chemical shift is equal to the observed shift from TMS that was 2181 so that's 2181 hertz and we need to multiply that by 10 to the sixth and the reason we multiply that by 10 to the 6 is because the spectrometer is in megahertz here so 300 megahertz is 300 times 10 to the sixth Hertz and so we can cancel out we can cancel out the hertz we can cancel out 10 to the sixth and so we have a simple calculation here to figure out the chemical shift and so let's go ahead and do that so we turn the calculator on 2100 81 divided by 300 gives us seven point two seven so this is equal to seven point two seven notice how the Hertz will cancel out and we have right here at ppm or parts per million because these signals are reported as a fraction of the operating frequency of the spectrometer and so there's a reason why we do that so we got this number seven point two seven here let's do this calculation again let's say let's say we did this we ran the spectrum on a different spectrometer let's say we're using a 60 megahertz spectrometer so let's let's change it up a little bit so a 60 megahertz spectrometer if you use a 60 megahertz spectrometer the protons on benzene absorb a frequency 436 hurts more than the protons on TMS so to calculate the chemical shift now the difference would be 436 Hertz times 10 to the sixth divided by now we're using a 60 megahertz spectrometer so 60 times 10 to the sixth Hertz once again the Hertz cancels the 10 to the 6 cancels and we can do that calculation so we take a 436 we divide that by 60 and we get 7.27 again so we get 7.27 notice we got the same value we did up here alright so 7.27 is a constant no matter what kind of spectrometer you're using so you could be using a 300 megahertz spectrometer or a 60 megahertz spectrometer you're going to get the same value for the chemical shift and then so that's why we go through this calculation here so we get a constant value for the chemical shift so the protons on on benzene have a chemical shift of 7.27 and we'll talk much more about about chemical shifts in the next in the next few videos