# basics of NMR (spin-spin splitting)

here's what I wrote from lecture,

1. what i'm confused about is, if two magnetic moment(B0 and the one of red H) have the same direction. wouldn't the magnetic field by red H cancel out the external magnetic field? I tried to visualized it in picture below.

2.1 if the directions of the two magnetic moment are different, is it because aromatic compound can flip upside down?

2.2 If the spin state is always same whether or not compound flip upside down, is it because of E from NMR machine?

1. Why do protons interact only with themselves? there are also O,C and Cl

• 1. The proton magnetic moment is much smaller than the external magnetic field 2. The actual direction of the spin does not matter, it is only the direction of the spin relative to that of the other proton that matters, and no the spin state is not always the same, otherwise there is no way to record NMR spectra (the lines are transitions from one spin to another) 3. 16O and 12C nuclei have no nuclear spin. Coupling to Cl occurs but we don't see it in the spectrum chem.wisc.edu/areas/reich/nmr/07-multi-02-effects.htm – orthocresol Apr 18 '16 at 8:58
• Honestly I would recommend going back again. Try using a textbook perhaps, I can recommend Spectroscopy by Pavia et al. I feel like you have a number of misconceptions. The splitting of spins is not a result of the external magnetic field - it is independent of the external magnetic field. It's wrong to say that the splitting arises because the proton spin is aligned with $B_0$ so if that's what the lecture said, stop watching it. NMR is not an easy topic so don't make it harder for yourself by not understanding the fundamentals. – orthocresol Apr 18 '16 at 9:40
• what orthocresol says, and your example shows second order coupling, which is hard to understand before you've got the basics. – Karl Apr 18 '16 at 16:04
• I can't make any sense of question 2. I think the answer to both is the physical orientation of the molecules in solution are averaged out due to molecular tumbling and other random motion. – Lighthart Apr 18 '16 at 17:41
• and about Q2, I just tried to tell if the physical orientation of molecules has to do with spin state of proton. – NK Yu Apr 19 '16 at 0:31

Regarding Question 1:

NMR measures the energy difference between the field-aligned energy of a nuclei and the field opposed energy of the nuclei.

The biggest field in the situation is the external magnet.

The next biggest field is the magnetic moments of the electrons. Electron density leads to what we refer to as chemical shift.

Lastly, there are the magnetic moments of other nuclei. But, the other nuclei are also undergoing their own spin flipping and such like. So if a hydrogen has its molecular neighbor with a spin aligned with the external magnetic field while relaxing, the total field it experiences is stronger. Otherwise it is weaker. The difference between stronger and weaker is essentially the distance between the two peaks on the NMR spectrum. It is important to remember the external magnetic field is the strongest here, so when the nuclear spins oppose and weaken (or align and strengthen) the magnetic field, it is only a tiny amount.

Regarding question 3:

Carbon and Oxygen have complete Nuclear quantum shells, and as a result have no net magnetic moment. The concept of 'spin up' or 'spin down' means nothing to these nuclei.

Since splitting is the result of a magnetic interaction between spins, there will be no splitting.

Regarding chlorine, The coupling constants are small, and generally not observed due to limitations in the machines making the measurements.

• about the sentence you mentioned, "So if a hydrogen has its molecular neighbor with a spin aligned with the external magnetic field while relaxing, the total field it experiences is stronger. ". ------ – NK Yu Apr 19 '16 at 0:37
• I don't get it. and I tried to explain why I have difficulty understanding it in a second picture. is it too messed up? T_T; – NK Yu Apr 19 '16 at 0:38
• Your picture is fine. The two vectors B0 (up) and Red (up) add together making a larger "up". It's just vector math. – Lighthart Apr 19 '16 at 2:02
• but the direction of magnetic field that came out of Red(up) becomes down at the position of blue H ;; – NK Yu Apr 19 '16 at 8:55
• NO!! Coupling is a through bond phenomenon NOT through space. It arises via the Fermi contact interaction, NOT by the adjacent proton reinforcing/opposing the external magnetic field. Get that idea out of your head now! – orthocresol Apr 19 '16 at 10:13