Kyle Legate wrote:

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.


I didn't define it quite correctly at first, I think. It's also confusing
because there are two fractionation processes going on in plants, the first
as CO2 goes across the stoma into the leaf, the second as the CO2 gets used
to make sugar and cellulose and whatever. The first process is a true
mobility effect, the second is the activation effect.

--
Bill Asher