COupling constant in nmr? - Answers

Here is how you calculate a coupling constant J: For the simple case of a doublet, the coupling constant is the difference between two peaks. The trick is that J is measured in Hz, not ppm. The first thing to do is convert the peaks from ppm into Hz. Suppose we have one peak at 4.260 ppm and another at 4.247 ppm. To get Hz, just multiply these values by the field strength in mHz. If we used a 500 mHz NMR machine, our peaks are at 2130 Hz and 2123.5 respectively. The J value is just the difference. In this case it is 2130 - 2123.5 = 6.5 Hz This can get more difficult if a proton is split by more than one other proton, especially if the protons are not identical.