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Current time:0:00Total duration:6:10

- [Voiceover] If you look
at the red and blue protons, they're both attached to this carbon, and if we see this double bond here, we have 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 into 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 on 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
proton's 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 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 the signal for the red proton. I went into 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 you 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 Hz, and if it's 1.4 Hz for this one, it must be 1.4 Hz for this one, because those protons
are coupled 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 spectrums, 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 the
spectrum with interaction, the spectrum with coupling
between the protons, we just assumed that the heights of these
two peaks were the same, but if I look at the actual NMR spectrum, they're not quite the same. So this one right here
is a little bit higher, and if you draw an arrow
pointing towards the higher peak, that arrow points towards the signal of the proton that's
causing the splitting. 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 doublet points towards the proton with which it is coupled, and the same thing for this signal. So this peak's 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. So if you could imagine this
roof over them like that. So, sometimes you'll see
this on an NMR spectrum, and 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, so 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. So how many neighboring
protons do we have? Well, those protons in blue
are attached to this carbon. The next-door carbon is this one. So how many neighbors? One, two, three, so three
neighboring protons, so n is equal to three. I'm 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 would expect a quartet. So let me go ahead and
draw that down here, so we would expect a
quartet 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 all attached to this carbon, 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'd 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 is the same. So the distance between the
peaks should be the same, so it turns out to be 7 Hz, so this distance should be 7 Hz, the same with this distance, so we just have to pretend like
they're all equivalent here, and the same for this one, so a coupling constant of 7 Hz. Same for this signal. So this distance should be 7 Hz, 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 complex splitting, which we'll talk about in the next video.