The involvement of skeletal deformations in the ultrafast excited-state proton transfer of 2-(2‘-hydroxyphenyl)benzothiazole (HBT) and the identification of the vibrational modes active in the process are reported. A multidimensional ab initio calculation of ground and excited states at the HF/DFT and CIS/TDDFT level renders the relevant portions of the potential energy surfaces around the minimum-energy path connecting the enol and keto configuration. The frequencies and potential energy distributions of the normal modes and the corresponding deformations of the molecule are calculated for all minimum-energy geometries. Along the minimum-energy path, the nuclear deformation is projected onto the relevant normal modes. This normal-mode analysis shows that mainly five low-frequency in-plane vibrations are associated with the electronic rearrangement and the transfer of the proton. The theoretical findings are in quantitative agreement with the experimental study presented in the accompanying paper.

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Aspects of the photoinduced ring opening reaction of cyclohexadiene with subsequent relaxation to cZc-hexatriene or back to cyclohexadiene are investigated theoretically. In this ultrafast reaction the S1 state plays a central role, providing the possibility for the molecule to rapidly decay from the excited to the ground state via multiple conical intersections. To better understand the dynamics of the molecule on its way toward the product states, we perform a normal mode analysis of its vibrations in the vicinity of two prominent conical intersections and find several eigenmodes to characterize its motion. This raises hope that our findings could lead to an experimental scheme of detection for the wave packet motion in the hot product states by a time resolved transient absorption measurement. In the end this might be useful in designing an input signal for an optimal control experiment to selectively steer the molecule through a specific conical intersection and influence the product yield.

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Many photochemical processes involving the breaking and formation of chemical bonds take place on a femtosecond time scale. By means of ultrafast laser pulses it became possible to monitor these events in real time with a temporal resolution in the sub-20 fs regime. Moving beyond the observation to the control of chemical reactions requires additional theoretical and experimental effort. We illustrate with the aid of selected model systems the concepts and possibilities of "passive" and "active" control and their differences.