Ultrafast Charge Transfer and Structural Dynamics Following Outer-Valence Ionization of a Halogen-Bonded Dimer.

We study the temporal evolution of the CH2O···ClF halogen-bonded dimer following vertical ionization out of outer-valence molecular orbitals on a femtosecond time scale, employing mixed quantum-classical molecular dynamics simulations. The charge density pattern in the ground state that is suitable for the formation of the ground-state halogen bond can be changed upon ionization. Depending on the molecular orbital that gets ionized, the change in the charge density transiently strengthens or weakens the halogen bond through altering the electrostatic interaction. A transient increase in the halogen bond strength is observed if the ionization enhances the positive charge on the halogen atom provided that the electron donor site possesses some negative charge. The evolution of the system following ionization is driven by energetic stabilization through transferring the electronic charge from the halogen bond acceptor/electron donor (CH2O) to the halogen bond donor/electron acceptor (ClF). The charge transfer oscillations are at the same time governed by the covalent bond vibrations.