William Asher wrote:
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.



And they have a higher activation energy because they vibrate differently
for a given thermal energy level, in general the motion is slower and the
vibrational excursions smaller. That is a "mobility" issue in the sense
that the intermolecular atomic motion is less because the mass is greater
and the overall energy at a given temperature is constant.


Aha. When I saw mobility I thought you were referring to the rate of
substrate delivery to enzyme active sites. Thinking like a biochemist,
and not as a chemist.