During nitration of benzene with nitrating mixture, HNO3 acts as

Electrophilic substitution reactions involve two steps, cleavage of the C - H or C - D step (i.e. proton elimination) take place in the second (fast) step, which being fast does not affect the rate of reaction. The first (slow) step is the formation of carbocation in which C - H or C - D bond is not affected. Thus absence of isotopic effect establishes the two-steps mechanism for electrophilic aromatic substitution in which formation of carbocation (first step) is rate determining step.