# When I made my NMR spectrum, I forgot to print it in "Hz" as well; how do I get coupling constants in "Hz"?

Jun 23, 2016

You don't need to do much more if you already got the chemical shifts in $\text{ppm}$.

All you have to do to convert to $\text{Hz}$ is to multiply by the $\text{MHz}$ of your NMR. As an example, you should notice that the spacing between the shorter and taller peaks near $\text{3.5 ppm}$ is about equal to the spacing between the shorter and taller peaks near $\text{1.2 ppm}$.

That likely means those protons are "communicating" and they happen to be neighboring.

These differences are approximately $3.665 - 3.654 = \text{0.011 ppm}$ and $1.184 - 1.171 = \text{0.013 ppm}$.

If you have a $\text{300 MHz}$ spectrometer, then:

$\Delta \delta \approx \text{0.012 ppm}$

$\implies \textcolor{b l u e}{{J}_{\text{HH}}}$

$= \text{0.012 ppm" xx "300 MHz}$

$= 0.012$ $\cancel{{\text{M}}^{- 1}}$ $\times$ 300 cancel("M")"Hz"

$=$ $\textcolor{b l u e}{\text{3.6 Hz}}$

So once you find your chemical shifts, just choose the ones you want to find the coupling constants for, convert to $\text{Hz}$, subtract them, and that's your coupling constant ($J$ value).