In article ,
Kyle Legate wrote:

Mobility has nothing to do with it. Heavier isotopes have a higher
activation energy, so they react slower. A phenomenon called hydrogen
tunneling can overcome this but it is extremely rare.

Fine. Activation energy is increased by isotopic mass,
reducing the reaction rate. And you state categorically,
without anticipating any evidence to the contrary, that
mobility has nothing to do with it.

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Michael Press