Wierd signal in 3-methyl-2-pentanone NMR?
The NMR and C13 NMR in question are in this link:
http://www.hanhonggroup.com/nmr/nmr_en/B11014.html
They say that the multiplet looking signal has two 1H protons from the CH2 showing an individual signal. My main question is why does this signal show up.
I have a higher resolution NMR on hand and both of those 1H signals have 7 peaks each.
More Questions From This User See All
Answers & Comments
Verified answer
AlisonM has correctly pointed out that the CH2 group is attached to a chiral *CABC moiety and hence the molecule does not have a mirror plane and the Hs of the CH2 are chemically different so two resonances and will couple Ha, Hb; this will result in a doublet of doublets (dd). But it is a mistake that each of the other H couplings are to be considered equal. Coupling to the neighboring C-H proton will split each of the four signals into 2, hence a ddd (8 signals); we then apply the coupling to the nearest CH3 group and split each signal into a further 1:3:3:1quartet hence the pattern is strictly a dddq (I make that 32 lines!). The couplings can be accidentally degenerate and signals can overlap which can simplify the spectrum. Oh yes the CH2CH system will probably be an ABC NMR system where the chemical shift differences between A,B and C are comparable to their coupling constants: e.g., Δδ ≈ JAC. When this happens chaos breaks loose: more signals jump out of the base line and the intensities go haywire. This is called second order effects. Because couplings don`t change with the field but shifts do you have a better chance of getting a first order spectrum at higher fields.
http://www.users.csbsju.edu/~frioux/nmr/Speclab4.h... Never mind the quantum mechanics just look at the spectra at 60 and 300 MHz in the figures.
So that the CH2 signal is weird is perfectly understandable!! (I think:))
Go to http://riodb01.ibase.aist.go.jp/sdbs/cgi-bin/direc...
and plug in your beastie (3-methyl-2-pentanone) and click H NMR spectrum. View the spectra at 90 and 400 MHz. And don`t tell anyone about this site!
H-H Coupling over more than three bonds is incidentally not detectable (except in aromatic systems).
The two CH2 protons are different because of the presence of a chiral centre in the molecule (at C2 if you number according to the 13C spectrum you have provided). I would actually expect the signals of Hc and Hd to be 9-line multiplets rather than 7 - each should couple to He (CH3), Hf (CH3), Ha (CH), and Hc (if we're looking at Hd, or vise versa). This gives a total of eight neigbouring protons - so nine lines. The two lines on the either end of these multiplets could very easily get lost in the baseline noise.
At higher field strength, you start to see things that are not resolved at lower magnet field strength. You would expect to see seven peaks, due to the contribution of long range coupling of the methyl group.
3-pentanone Nmr