NMR spectrum - unusual peak

[Kaeroll]

Hi all,

 

I have a problem interpreting a spectrum I ran recently. It's a simple proton NMR of 2,3-dimethylbutan-1-ol. I don't have access to a scanner so unfortunately I can't post the spectrum here, but a near-identical version is available at sdbs. (Which, by the way, is an excellent resource) If anyone's interested in helping with this problem I recommend accessing it there. Its CAS no is 19550-30-2.

 

I've assigned (hopefully correctly) all peaks other than the peak at 3.5ppm. On my spectrum it appears to be a doublet of quartets, with non-binomial intensities, but I'm not sure. From one end to the other, I see two peaks of roughly equal intensity, two of double intensity, all with equal J values; this is followed by a larger gap, and then the first multiplet's mirror image.

 

I believe this peak arises due to the protons in the [math]CH_{2}OH[/math] group being coupled to the proton at C2, and then further coupled due to the methyl group at C2 (i.e. coupling beyond three bonds). If the intensities were binomial I'd be convinced this is so.

 

Can anyone offer any advice as to why it's non-binomial, or where I'm wrong?

 

Cheers

Kaeroll

[Tartaglia]

The CH2O protons are diastereomeric and form an AB quartet by a roofing second order effect, these are further split by the adjacent CHMe proton

[Kaeroll]

Hi,

 

Thanks for your reply. That confirms my interpretation. I'm not sure how to explain it though - I'm not familiar with second order effects (I believe it came up once in a lecture on 2D spectra... I switched off). I take it this effect is the cause of the abnormal intensities?

 

Cheers

Kaeroll

[Tartaglia]

There is a brief description at the bottom of this page - http://chem.ch.huji.ac.il/nmr/whatisnmr/whatisnmr.html

[Kaeroll]

Fantastic. Thank you very much.

 

Kaeroll