Transition states photobiology

Quantum chemistry plays vital central roles in clarifying and understanding the mechanisms of these photobiological events. Electronic structures and transitions of active centers in proteins obey the principles of quantum mechanics, and molecular properties dramatically change after the transitions. In addition, photochemical events in excited states are often transient and sometimes difficult to study in experimental approaches. If an accurate and reliable theory exists and can be applied to photobiological subjects, one can obtain not only rational explanations but also predictions on the photo-functions of the active centers and proteins. 

In the chapters of Part II of this book it has been demonstrated for a variety of examples that conical intersections can provide the mechanism for extremely fast chemical processes, e.g. photodissociation, photoisomerization and internal conversion to the electronic ground state. Time-dependent quantum wave-packet calculations have established that radiationless transitions between electronic states can take place on a time scale of the order of 10 fs, if a conical intersection is directly accessible after preparation of the wave packet in the excited state, see, e.g. Chapters 8-11 and 14 15. In view of these findings and the omnipresence of conical intersections in polyatomic molecules (cf. Chapter 6), it is now widely accepted that conical intersections are of fundamental importance for the understanding of the reaction mechanisms in photochemistry and photobiology. 

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