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

You don't need to do much more if you already got the chemical shifts in #"ppm"#.

All you have to do to convert to #"Hz"# is to multiply by the #"MHz"# of your NMR.

As an example, you should notice that the spacing between the shorter and taller peaks near #"3.5 ppm"# is about equal to the spacing between the shorter and taller peaks near #"1.2 ppm"#.

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

These differences are approximately #3.665 - 3.654 = "0.011 ppm"# and #1.184 - 1.171 = "0.013 ppm"#.

If you have a #"300 MHz"# spectrometer, then:

#Deltadelta ~~ "0.012 ppm"#

#=> color(blue)(J_"HH")#

#= "0.012 ppm" xx "300 MHz"#

#= 0.012# #cancel("M"^(-1))# #xx# #300 cancel("M")"Hz"#

#=# #color(blue)("3.6 Hz")#

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