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 (¹J_H-C) while the other methyl group experiences a two-bond ¹H - ¹³C coupling (²J_H-C). Also, the effects of the three-bond ¹H - ¹H coupling (³J_H-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 ¹J_H-C , ²J_H-C and ³J_H-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: Δδ_H between -¹²CH₃ and -¹³CH₃= 0.002 ppm, ¹J_H-C = 125 Hz, ²J_H-C = -4.67 Hz, ³J_H-H = 8 Hz and LB = 0.5 Hz.