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Concert along the Edge: Dynamics and the Nature of the Border between General and Specific Acid–Base Catalysis
journal contribution
posted on 2017-04-05, 00:00 authored by Hannah
R. Aziz, Daniel A. SingletonReactions that involve a combination
of proton transfer and heavy-atom
bonding changes are normally categorized by whether the proton transfer
is occurring during the rate-limiting step, as in the distinction
between general and specific acid–base catalysis. The experimental
and computational study here of a β-ketoacid decarboxylation
shows how the distinction between the two mechanisms breaks down near
its border due to the differing time scales for proton versus heavy-atom
motion. Isotope effects in the decarboxylation of benzoylacetic acid
support a transition state in which the proton transfer is complete.
In quasiclassical trajectories passing through this transition state,
the new O–H bond after proton transfer undergoes several vibrations
before heavy-atom motion completes the reaction. The bonding changes
are thus temporally separated at a “dynamic intermediate”
structure that acts equivalently to an ordinary intermediate in the
trajectories, including the reversal of trajectories at the intermediate
when the second “step” fails, but the structure is not
an energy minimum. The results define a border between mechanisms
where the usual energetic definition of intermediates is not meaningful.