NMR multiplet

[ggxoxo]

Hey guys; what is a multiplet???

And there is a qn that gives 3 esters: CH3CH2COOCH3; CH3COOCH2CH3 and HCOOCH2CH2CH3

and then it asks you that if a multiplet is seen in the high resolution 1H NMR spectrum of the ester, select the ester from above that it must be.

[jasoN-]

Wow, I thought all of those would show multiplets.. what the hell have I learnt? :/

Edit: If what they mean for multiplet is many peaks, and not just doublet or triplet (which are still multiplets by definition..), I would take a stab and choose the last structure, HCOOCH2CH2CH3, which would have a sextet within the spectra

[jaydee]

multiplet shows up in the splitting of peaks due to nuclear spin spin coupling (i think). So for the last ester you'd have a 12 multiplet show up for the middle CH2 in propyl methanoate

[sam0001]

howd you get 12? are you using the multiplicity rule?

[jaydee]

yeah you'd have to multiply it since they aren't in the same chemical environments. multiplication has only ever appeared in trial exams (vcaa hasnt done them yet)

[Hellrocks]

Yes, its the last structure because the CH2 is between 2 hydrogen environments that are non equivalent.
The others only have a maximum of 1 non equivalent hydrogen environment.

[ggxoxo]

howd you get 12? are you using the multiplicity rule?

Yea jaydee is right- can someone please explain the multiplicity rule very briefly?

And from your answers would it be safe to assume that a multiplet is just more than 3 peaklets

[jaydee]

well you can say 4-multiplet or a quartet. i don't think theres a specific number you definitely have to say multiplet but for convenience i usually use multiplet for 5 and above. Multiplicity rule is basically (n+1) x (n+1). for example if u have HCOOCH2CH2CH3 u mentioned from before the middle CH2 is surrounded by CH2 and CH3 (both are different hydrogen environments to the middle CH2 group). (2+1) x (3+1) = 3 x 4=12 multiplet. As i said, highly unlikely that itll come up on the exam but good to know since some trail exams do have it

[Hellrocks]

Its good that you know that rule, but you can also think of it logically.
If we're analysing the CH2 between the other CH2 and CH3, either the CH2 or CH3 would split the single peak into a triplet or quartet.

Case 1:

Single peak into triplet by CH2 (1 peak replicated twice 1+[1+1])
Then triplet into 12 peaks by CH3 (3 peaks replicated 3 times 3+[3+3+3])

Case 2:

Single peak into quartet by CH3 (1 peak replicated 3 times 1+[1+1+1])
Then quartet into 12 peaks by CH2 (4 peaks replicated 2 times 4+[4+4])

Either way we get 12 peaks

[ggxoxo]

well you can say 4-multiplet or a quartet. i don't think theres a specific number you definitely have to say multiplet but for convenience i usually use multiplet for 5 and above. Multiplicity rule is basically (n+1) x (n+1). for example if u have HCOOCH2CH2CH3 u mentioned from before the middle CH2 is surrounded by CH2 and CH3 (both are different hydrogen environments to the middle CH2 group). (2+1) x (3+1) = 3 x 4=12 multiplet. As i said, highly unlikely that itll come up on the exam but good to know since some trail exams do have it

Sorry- I still don't understand (though I am really curious!); if CH2 is between CH2 and CH3 wouldn't the peak just split to 6 because it is the (n+1) rule? Why do you multiply (n+1) x (n+1)?

[Mao]

It's that time again.

There is a lot of misconceptions re: multiplicity. The word 'multiplet' itself is misused, the concept of multiplicity is also taken too far in this context.

In a nutshell:
- a 'sextet' and so forth are not multiplets. Multiplets are overlaps of ridiculously many peaks such that the end result is not classifiable. i.e. 'multiplets' are used when you cannot assign the mess of peaks.
- For alkanes and other relatively simple compounds, neighbouring environments on the same alkane chain (i.e. very similar, but slightly different environments) do not really deviate from the n+1 rule that much. This is because the difference in coupling frequency is very small, thus the second splitting will essentially overlap, giving you n+1 peaks. For the purposes of VCE, the (n+1)*(n+1) rule is almost never useful.
- Also, when you refer to these splitting, these are not a '12 multiplet', they are quartet of triplets or triplet of quartets (depending on which splitting frequency is larger). You can see the problem here is VCAA never gives you the splitting frequencies, so you cannot actually this. Anyhow, none of these are taught until 2nd or 3rd year uni.

So, don't use the word 'multiplet', only use the n+1 rule.

For more reading, you can check out my posts in these threads:

http://vce.atarnotes.com/forum/index.php/topic,25637.msg260490.html#msg260490
http://vce.atarnotes.com/forum/index.php/topic,26044.msg263934.html#msg263934
http://vce.atarnotes.com/forum/index.php/topic,26218.msg265278.html#msg265278
http://vce.atarnotes.com/forum/index.php/topic,26005.msg263938.html#msg263938