Carbon NMR

Now I want to briefly touch on another form of nuclear magnetic resonance except this type of NMR is going to detect carbon 13 isotopes instead of protons and this is fittingly called carbon 13 NMR, carbon 13 NMR is a more limited type of nuclear magnetic resonance in contrasts of proton NMR, there's actually less information that we can get from carbon 13 than we can from proton NMR and this is largely in part to the low natural incidence of the carbon 13 isotope, I'm not sure, if you guys your call from Gen chem but if you guys remember, carbon 13 has a natural abundance of about one out of every 100 carbon atoms will be a carbon 13, okay? So, because there's so few carbon 13 in, you know, in molecules that means that splitting. Remember, one of the major forms of information that we get in proton NMR is not observed at all, okay? You do not get any splitting in carbon 13 NMR and if you do the math that makes sense because if you think about it the only way splitting could occur is if you have a carbon 13 that's next to another carbon 13 so they can interfere with each other, right? But, if the natural incidence of carbon 13 is 1 in 100 that means that the chances of getting two carbon 13 next to each other are 1 in 100 times 2, so that means that the chances that you would actually get these two carbons to split each other is actually 1 over 10 thousand, okay? So, the math just gets too crazy, it's such a small percentage of our carbon 13 that will split that we just basically say that is not absorbs at all, okay?

Other than that we're going to see all of the major themes that we learned in proton NMR carry over into carbon 13 NMR, the only differences being we're not detecting hydrogen's anymore, we're detecting carbons, we're not getting any splitting and we are gonna have to learn some new shift values in terms of our chemical shifts because the instrument is calibrated differently, okay? Now, what you're going to notice that the same general pattern applies, if you were just to not look at these numbers and just look at the order of these groups you see they're all in the same exact order, we have alkane, we have alkyne, we have our electronegatives, we have our alkene, we have benzene, we have carbonyls. So, really the order hasn't changed at all, it's just the absolute values that have changed because the spectrum of the carbon 13 NMR goes from about 0 to about 210. So, it's just a different set of values, okay? Now, to make things a little bit better for you, just the fact that like these values didn't really change a lot helps, but also it's extremely rare for professors to ask you to memorize these values because they tend to not care as much about carbon 13, because just not as not as helpful of a analytical method, so many times they'll tell you that you don't really need to know these shift values, you should just be familiar with them. So, in that sense if you're familiar proton NMR you're already familiar with carbon 13 NMR in terms of the ranges and in terms of the order of the different types of shifts, okay? So, we're going to go straight into some practice problems that kind of you know help us to solidify our knowledge of carbon 13 NMR, I want you to go ahead and answer this question and then I will go ahead and answer it for you. So, take a shot.

Alright, so how many different signals did we get? Well, this is definitely going to depend on whether you saw symmetry or not and actually turns out that there is symmetry in this molecule because we've got the benzene ring, that's you know exactly the same on both sides, and we actually have symmetry along this bond. Now, you might be wondering, maybe you're under the impression that there wasn't any symmetry because you see this CH3 that's kind of off tilted to one side. So, you're thinking that, you know, this is an asymmetrical molecule but again guys, remember, single bonds can rotate as much as they want. So, really even though it's drawn that way right now, remember that this CH3 could easily go to the top, could easily go to the side, it could easily rotate all around there. So, actually there's a perfect plane of access all the way through this molecule, just imagine that the CH3 to be rotated. So, it's, right underneath that dotted line, sorry, I know that was tricky, but it's stuff you have to be able to visualize. So now recall, we're not concerned about hydrogen's here, just different types of carbons so that means that every carbon gets a peak signal even the ones without hydrogen, so this is going to be my first type, this is going to be signal a, this, oops, different color, this is going to be signal B as well as this one over here, this is going to be signal C along with this one over here, Now this is where things change a little bit from proton NMR, this one gets its own signal, that's signal d, okay? This one is signal E and then finally have signal f, okay? So, this one would have six signals, okay? So, many times, we'll see with carbon 13 NMR is that you get a different number of signals than you would just proton NMR because you're different, you know, you're looking for a different type of atom, okay? Awesome. So, now let's move on to the next question, just look at all four compounds and try to see which compounds would you know meet these criteria of only having one peak for both, the proton NMR and the carbon 13 NMR. So, go ahead and then we'll just answer all four at once.

And the answer is only compounds B and C will yield one signal each for proton NMR and carbon 13 NMR, if we look at compound A, compound A would only give us one signal for proton NMR, okay? That would be the signal, that's experience right at the end, however we would actually get two separate signals for carbon 13, 2 signals, so that one was out. Now, we look at D, D is kind of in the same boat where we would get one signal for proton NMR but we would get two signals for carbon 13. So, these questions obviously do not work, okay? Because those answers do not work, okay? So, that was just a quick overview of carbon 13 NMR, let's go ahead and once the next topic.