10.1021/jp506011e.s001
Ron Simkovitch
Ron
Simkovitch
Shay Shomer
Shay
Shomer
Rinat Gepshtein
Rinat
Gepshtein
Dan Huppert
Dan
Huppert
How Fast
Can a Proton-Transfer Reaction Be beyond
the Solvent-Control Limit?
American Chemical Society
2015
100 fs
proton
time limit
regime
ESPT rate
protic solvents
time scales
photoacid
2015-02-12 00:00:00
Journal contribution
https://acs.figshare.com/articles/journal_contribution/How_Fast_Can_a_Proton_Transfer_Reaction_Be_beyond_the_Solvent_Control_Limit_/2196613
In
this article, we review the field of photoacids. The rate of
excited-state proton transfer (ESPT) to solvent spans a wide range
of time scales, from tens of nanoseconds for the weakest photoacids
to short time scales of about 100 fs for the strongest photoacids
synthesized so far. We divide the photoacid strength into four regimes.
Regime I includes the weak photoacids 0 < p<i>K</i><sub>a</sub>* < 3. These photoacids can transfer a proton only to water
or directly to a mild-base molecule in solution. The ESPT rate to
other protic solvents, like methanol or ethanol, is too small in comparison
with the radiative rate. The second regime includes stronger photoacids
whose p<i>K</i><sub>a</sub>*’s range from −4
to 0. They are capable of transferring a proton to other protic solvents
and not only to water. The third regime includes even stronger photoacids.
Their p<i>K</i><sub>a</sub>* is ∼ –6,
and the ESPT rate constant, <i>k</i><sub>PT</sub>, is limited
by the orientational time of the solvent which is characterized by
the average solvation correlation function ⟨<i>S</i>(<i>t</i>)⟩. The fourth regime sets a new time limit
for the ESPT rate of the strongest photoacids synthesized so far.
The <i>k</i><sub>PT</sub> value of such photoacids is 10<sup>13</sup> s<sup>–1</sup>, and τ<sub>PT</sub> = 100 fs.
We attribute this new time limit (beyond the solvent control) to intermolecular
vibration between the two heavy atoms of the proton donor and the
proton acceptor, which assist the ESPT by lowering the height and
width of the potential barrier, thus enhancing the ESPT rate.