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Friday, March 30, 2012

The Extremely Complicated 1H NMR Spectrum of Ethane

It is often incorrectly assumed that simple compounds yield simple NMR spectra. The 1H NMR spectrum of ethane is such an example. The complexity arises when one takes into account the inequivalence between methyl groups in the mono 13C isotopomer which accounts for 2% of the naturally occurring ethane. In this isotopomer, one methyl group experiences a one-bond 1H - 13C coupling (1JH-C) while the other methyl group experiences a two-bond 1H - 13C coupling (2JH-C). Also, the effects of the three-bond 1H - 1H coupling (3JH-H) are exhibited in the spectrum due to the inequivalence. These couplings have a dramatic effect on the spectrum. Furthermore, there is a very small isotope effect on the 1H chemical shifts of each methyl group due to the presence of 13C vs 12C. This effect however, is very small (~0.002 ppm) and has very little effect on the spectrum. The left panel of the figure below shows a simulation of the 1H NMR spectrum of the 12CH3-12CH3 which accounts for 98% of naturally occurring ethane. As expected, the spectrum is a singlet as both methyl groups are equivalent to one another. The middle panel of the figure shows a simulation of the 1H NMR spectrum of the 13CH3-12CH3 isotopomer which accounts for 2% of naturally occurring ethane. In this case the spectrum is extremely complex due to the 1JH-C , 2JH-C and 3JH-H coupling. The panel on the right shows a simulation of a scaled up representation of what one would expect for naturally occurring ethane.
The parameters for the simulations are as follows: ΔδH between -12CH3 and -13CH3= 0.002 ppm, 1JH-C = 125 Hz, 2JH-C = -4.67 Hz, 3JH-H = 8 Hz and LB = 0.5 Hz.

6 comments:

Anonymous said...

Hi Glenn,

Since there are 2 carbon atoms in ethane the abundance of the 12C-13C isotopomer should be about 2.1%.

Cheers
Michael

Glenn Facey said...

Michael,

Thank you for your comment. You are correct and I have corrected the error in my post.

Glenn

Anonymous said...

Glenn, those simulated spectra look very nice! could you tell me what software you used for them? is there a free version? thank you.

Glenn Facey said...

Anonymous,
Thank you for your comment. These spectra were simulated using the NMR-SIM routine within Bruker's TOPSPIN program. There may be a simulation package included with ACD's free NMR processor software. Ian Gay of Simon Fraser University offers a free very useful (and small)simulation program available for download here: http://www.sfu.ca/~gay/

Glenn

Razieh Annabestani said...

Hello

Have you also experimentally obtained the ethane spectra? My data shows a second order coupling J_CH (carbon with the further mehtyl group) in the order of 66 Hz not 5 Hz. And I was wondering to find an actual data (other than this simulation) to analyze my observation.

Glenn Facey said...

Razieh,
I have not measured an experimental spectrum.
Glenn