TAILIEUCHUNG - Ideas of Quantum Chemistry P31

Ideas of Quantum Chemistry P31 shows how quantum mechanics is applied to chemistry to give it a theoretical foundation. The structure of the book (a TREE-form) emphasizes the logical relationships between various topics, facts and methods. It shows the reader which parts of the text are needed for understanding specific aspects of the subject matter. Interspersed throughout the text are short biographies of key scientists and their contributions to the development of the field. | 266 6. Separation of Electronic and Nuclear Motions Fig. . The photochemical funnel effect. We can see two adiabatic surfaces upper and lower which resulted from intersection of two diabatic surfaces white and gray . The lower surface corresponds to an electronic ground state the upper to an excited electronic state. The molecule is excited from its ground state at the nuclear configuration P to the excited state point FC at the same nuclear configuration according to the Franck-Condon rule . The point FC representing the system is usually located on a slope of the potential energy corresponding to the excited state and this is why it slides downhill towards the energy minimum M . Its kinetic energy may be sufficient to go through M and pass a barrier saddle point corresponding to the point TS. Then the system inevitably slides down into the conical intersection point C funnel effect and lands in the ground state surface at the configuration of the conical intersection with nearly 100 efficiency. The future of the system may correspond to different products it may roll down to product P or slide back to product P . Modified and adapted from F. Bernardi M. Olivucci . Robb Chem. Soc. Rev. 1996 321. The electronic excitation takes place so fast that the nuclei do not have enough time to move. Thus the positions of the nuclei in the excited state are identical to those in the ground state Franck-Condon rule . The point FC in Fig. shows the very essence of the Franck-Condon rule -vertical a vertical transition. The corresponding nuclear configuration may differ quite sig- Uansifion nificantly from the nearest potential energy minimum M in the excited state PES E_ . In a few or a few tens of femtoseconds the system slides down from P to the Polyatomic molecules and conical intersection 267 neighbourhood of M transforming its potential energy into kinetic energy. Usually point M is separated from the conical intersection configuration C by a barrier with

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