Yesterday, an observant student asked me why she frequently sees a small peak in the triplet for CDCl3 in the 13C spectrum.
The small peak is due to a small amount of CHCl3 in the CDCl3 solvent. The CHCl3 signal is a singlet because proton decoupling was used to collect the data. The CDCl3 signal is a 1:1:1 triplet due to the J coupling to the deuteron which is a spin I=1 nucleus having three energy levels. The chemical shift difference between the CHCl3 and CDCl3 isotopomers is called an isotope shift.
Question 1: Since chlorine has two NMR active isotopes (35Cl and 37Cl) why don't we observe J coupling between the 13C and the 35/37Cl like we do for the 13C and 2H?
Answer 1: The relaxation between the energy levels of the chlorine isotopes is very fast and the 13C "sees" each Cl at an average energy. 2H on the other hand relaxes slowly between the energy levels and the 13C "sees" all three energy states of the deuteron.
Question 2: Why don't we see an isotope shift between CD 35Cl3 and CD 37Cl3?
Answer 2: The effect is there ...... just too small to observe.