First-principles simulation of a photoinduced carbocation formation

Highlights: ► First-principles simulation of the photodissociation of diphenylmethyl chloride. ► Excited-state dynamics described with restricted open-shell Kohn–Sham (ROKS). ► Formation of radicals in the gas phase. ► In aqueous solution ultrafast formation of a carbocation. ► Ion pair formation in two steps: radical-pair formation, electron transfer. - Abstract: The photodissociation of diphenylmethyl chloride (DPMCl) can lead to the formation of radicals or ions depending on the environment. We use first-principles molecular dynamics simulations to study the photochemistry of DPMCl in the gas phase and in aqueous solution. In the gas phase simulations the photoexcitation leads to the formation of a radical pair. For the photoreaction of DPMCl in a polar solution a two-step process is observed: the actual photoreaction proceeds diabatically and leads to the formation of a radical pair. This radical pair is subsequently transformed into an ion pair via an electron transfer reaction. The ultrafast process explains the experimentally observed formation of ions on a sub-picosecond timescale.