The Genuine Article Number: YB795
Weinkauf R., Schlag EW., Martinez TJ., Levine RD.
NONSTATIONARY ELECTRONIC STATES AND SITE-SELECTIVE REACTIVITY
Journal of Physical Chemistry. 101(42):7702-7710, 1997 Oct 16.
Born-oppenheimer approximation. Transfer matrix-elements.
Mass-spectrometry. Large molecules. Energy-transfer. Multiphoton ionization.
Charge separation. Laser desorption. Nuclear-dynamics. Peptide cations.
An efficient route to the site-selective reactivity of electronically excited states
of multicentered molecules is discussed. In the first stage the migration of the
electronic excitation occurs. This can operate over an extensive range without
extensive draining of energy into the nuclear frame. Only in a second stage,
once the optimal site has been reached, does the excess energy become
available for bond breaking or isomerization at the new, optimal, site. This
two-stage mechanism, where electronic excitation (or the charge) is the
scout, avoids the pitfall of conventional large molecule kinetics. (In that view,
known as the quasi equilibrium theory, the electronic excitation is first
converted to nuclear modes. But then there are so many available vibrational
states that the probability for the excitation energy to become localized at
the necessary site, is too small and the resulting reaction rate is too slow.) By
confining the site search to the electronic manifold, it becomes a highly
efficient process. The recent novel experiments of Weinkauf et al. on
(positive) charge migration and dissociation of peptide ions are suggested as
an example of the considerations above where there is a facile migration of
the positive charge followed by reactivity at the selected site. The peptide is
modeled as beads on a chain. Interbead and intrabead coupling are
discussed in terms of adiabatic and diabatic states. We find a multistep
mechanism (unlike superexchange): a charge-directed reactivity (CDR) model.
Such efficient ranging could also take place in other chain structures and
suggests that there will be examples where electronic processes set the time
scale for the chemical change. [References: 73]
Physical chemistry/chemical physics.
Current Contents/Physical, Chemical & Earth Sciences
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