Nonstationary electronic states and site-selective reactivity

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 isomenzation 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.