摘要：

Summary In large molecules, the electronic ground-state and the first excited state are degenerate in many nuclear configurations. At these configurations, termed conical intersections, radiationless decay from the electronically excited state to the ground state is very efficient. Conical intersections are therefore believed to be important in determining the outcome of photochemical reactions. A general method for locating them is presented; it is based on the phase-change rule of Longuet-Higgins and Herzberg. It is shown that the arduous search for conical intersections in polyatomic molecules can be systemized using the method. The overall phase change of the total electronic wave function when transported adiabatically around a closed loop, is a necessary and sufficient condition for the existence of a conical intersection inside the loop. It is shown that a convenient method to determine the loop is by considering the phase change incurred by elementary chemical reactions on the ground-state surface. Each conical intersection is defined by two nuclear coordinates leading to the degeneracy point. These coordinates can be assigned to two different elementary reactions, by which three distinct chemical species are connected. In contrast with thermal reactions, photochemical ones require in principle motion along two independent coordinates. The removal of the degeneracy by motion along these coordinates is analogous to the distortion leading to lowering the energy in the Jahn–Teller effect. The theoretical basis for this approach is the permutational symmetry of the polyelectronic wave function. The phase change accompanying an elementary reaction is defined on this basis, using the concepts of aromaticity and antiaromaticity. The method is helpful in directing and focusing computer-based searches on the multidimensional potential surface of large molecules.

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