High Resolution NMR Spectra

Also, I must add, exam boards tend not to set any molecule with more than 3 carbons attached together but they may ask you for something which has 8 carbons with oxygens in the middle somewhere.

Edit to the last post. Of course! 3 hydrogens on carbon 1 and 2 hydrogens on carbon 2. Therefore 2 + 3 = 5. N + 1 rule = 6. If that isn't right then my head might explode.

Better get the bandages and savlon out...

the hydrogens are split by adjacent hydrogens. If there are two adjacent protons then it splits the signal into 3 peaks.

To understand why you need to look at the possible spin states that the 2 adjacent protons can adopt. If they are both spinning 'up' then this is one environment. If they are both spinning down' then this is another. If they have different spins (one up and one down) then this is the third. Each of these different spin combinations provides a different magnetic anvironment for the measured hydrogens. Three environments = triplet.

The simplest case is splitting by a single proton, in for example CH3CHCl2

The measured signal from the CH3 is split by the adjacent CH into a doublet. This is because the CH proton can adopt two different spin states (up or down) each of which provides a different magnetic environment for the CH3 protons. One of the environments will increase the applied external magnetic field and the other spin state opposes and decreases the external field. Hence, two signals.

The CH proton is split into four signals by the four possible combined spin states of the CH3 protons. (up,up,up: up,up,down: up,down,down: down,down,down). Each of the combined spin states providing a different local magnetic field that adds to (or subtracts from) the external applied field strength.

In the case of CH3-CH2-CH2-CH3 the two CH3 groups are totally equivalent as are the two CH2 groups. This behaves like a double CH3-CH2 group in terms of splitting. The CH2 protons are split by the CH3 into a quartet. The CH3 protons are split by the CH2 protons into a triplet.

The integrals under the peaks gives the ratio of protons being measured - in this case 3:2

The size of the split peaks can be obtained from Pascal's triangle.
For a doublet the peak height ratio is 1:1. If its a triplet the peaks are in a ratio of 1:2:1
For a quartet the ratio is 1:3:3:1

Another useful observation is that the multiplets generally increase slightly in height towards the group that splits them.

Thanks a lot, that's explained why spin-spin splitting happens, but I still don't really understand why the CH2 in your example are only affected by the CH3? Why aren't they effected by the hydrogens on the other CH2 which is the other adjacent carbon?

lol google isn't always your friend - at A-level you are taught the n+1 rule so you would be expected to write;

CH3 = triplet as its next to 2 protons
CH2 = sextet as its next to 5 protons

the reason why the 'real' spectrum doesn't match the rule is because the CH2 and CH3 are not quite equal, giving these 'mulitplets' plus the 'long range' couplings involved

Sure but I've already said that above

Sure but I've already said that above

you said the CH3's would give a quadruplet

Ok thanks for your help guys. So the CH2 is sextet but isn't really effected by the other CH2 group because they are in the same environment, therefore it is only the protons in the CH3 group that effect it? And is this a general rule meaning that it spin-spin splitting will not be any more than into quadruplets?

The CH₂ group will produce a sextet as I said. 3+2=5 plus 1 from n+1 = 6.

What was being said above I think was that there will only be 2 peaks because of this symmetry

OK! Sorry for the questions but as I said, there isn't anything in the book about this. So sextet (normally), would produce peaks of 1:5:10:10:5:1?

Sweet. And so in an organic molecule you can only have a singlet to septet if you had two CH3 groups attached to it.