Free Evolution

During free evolution transverse magnetisation will evolve due to J-coupling and chemical shift. No pulses are applied during the free evolution period.

So for a heteronuclear coupled two spin system (IS) if we begin the free evolution period with transverse magnetisation (I_X) then after the free evolution period we would have in phase transverse magnetisation for I modulated by terms in chemical shift for I and J-coupling between I and S as along with antiphase magnetisation for I with respect to S again modulated by terms in chemical shift for I and j-coupling between I and S.