Coupling constant

if you look at the red and blue protons are both attached to this carbon and if we see this double bond here with these different groups attached to this double bond and since there's no rotation around the double bond the red and the blue protons are locked in two different environments therefore they are not chemically equivalent and since those protons are not equivalent they can couple together and since this is occurring on the same carbon we call this geminal coupling so geminal coupling here so geminal referring to the fact that both protons are in the same carbon and coupling can occur so those protons are close enough where they can affect each other so let's think about first the NMR spectrum with no coupling so we would expect one signal for the blue proton and one signal for the red proton so here's the spectrum with no coupling but we know that the red protons magnetic moment can align either with the external magnetic field or against the external magnetic field and that causes the signal for the blue proton to be split into two so if I go down here so we actually see a doublet for the signal for the blue or the blue proton same thing for the blue proton the magnetic moment can be aligned either with the external magnetic field or against it and that splits the signal for the red proton into a doublet so two peaks for two Peaks for the signal for the red proton I went to much more detail about this in the spin-spin splitting spin-spin coupling video in this video we're more concerned with the idea of the coupling constant and the coupling constant refers to the distance between the peaks of a signal so if we think about the distance between the two peaks of this signal that is the coupling constant and the coupling constant is the same for both of these signals because these protons are splitting each other they are coupled together the coupling constant is measured in hertz so it turns out to be 1.4 or Hertz and if it's 1.4 Hertz for this one it must be 1.4 Hertz for this one because those photons are a couple together the reason why we use Hertz is because it's the same coupling constant no matter what NMR spectrometer you're using so it doesn't matter what the operating frequency is you're going to get the same coupling constant alright if we look at the actual NMR spectrum so over here is a zoom in of the actual NMR spectrum the signal for the red proton is right here and the signal for the blue proton is over here so when I looked at this when I looked at the spectrum with interaction right the spectrum with coupling between the protons we just assume that the the heights of these two peaks were the were the same but if look at the actual NMR spectrum they're not quite the same right so this one right here is a little bit higher and if you draw an arrow pointing towards towards the higher peak right that arrow points towards the signal of the proton that's causing the splitting all right so that arrow is pointing to the right and that's where we find the signal for the red proton which is causing the splitting of the blue proton so the the doublet points towards the proton with which it is coupled and the same thing for for the signal right so this Peaks a little bit higher so we draw an arrow pointing towards the higher peak and so the doublet points toward the proton with which it is coupled and so you get this situation where you get these doublets with like a roof over their head right so if you can imagine this roof over them like that so sometimes you'll see this on NMR spectrum and if you think about if you think about that they're pointing towards the proton with which it is coupled sometimes it can help you when you're trying to understand what's going on in your NMR spectrum alright let's look at another example for a coupling constant so let's look at this molecule and let's focus on the ethyl group so over here this carbon has two protons we expect one signal for those protons and then over here this carbon has three protons so we would expect another signal for these protons let's focus in on the protons in blue alright so how many neighboring protons do we have well those protons and blue are attached to this carbon the next door carbon is one so how many neighbors one two three so three neighboring protons so n is equal to three and using the n plus one rule we expect n plus one Peaks so three plus one is equal to four so we would expect a signal with four peaks so we'd expect a quartet so let me go ahead and draw that down here so we would expect a quartet four for that signal so this is supposed to represent what you would see on an NMR spectrum next let's do the protons in red here so how many neighboring protons do they have well they're both they're all attached to this car but and the next door carbon is here and we have two protons on the next door carbon so we have two neighbors so n is equal to two and so we expect two plus one Peaks so three peaks or a triplet so let me see if I can draw in a triplet here so this would be the signal for these protons since the red and the blue protons are splitting each other the coupling constant should be is the same right so the distance between the peaks should be the same so it turns out to be seven Hertz so this distance should be seven Hertz alright same with this distance so just have to pretend like they're all equivalent here and the same for this one so a coupling constant of seven Hertz alright same for same for the signal right so this distance should be seven Hertz and also for this one so hopefully this just gives you an insight into the idea of a coupling constant which you'll need to understand more more complex splitting which we'll talk about in the next video