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conical intersection

Point of crossing between two electronic states of the same spin multiplicity (most commonly singlets or triplets).
  1. In a polyatomic molecule two potential energy surfaces are allowed to cross along a (3N − 8)-dimensional subspace of the (3N − 6)-dimensional nuclear coordinate space (the intersection space) even if they have the same spatial/spin symmetry (N is the number of nuclei). Each point of the intersection space corresponds to a conical intersection. If the energy is plotted against two special internal geometrical coordinates, x1 and x2, which define the so-called branching plane, the potential energy surface would have the form of a double cone in the region surrounding the degeneracy. In the remaining (3N − 8) directions, the energies of the ground and excited state remain degenerate; movement in the branching plane lifts the degeneracy.
  2. From a mechanistic point of view, conical intersections often provide the channel mediating radiationless deactivation and photochemical reaction.
PAC, 2007, 79, 293 (Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)) on page 317
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IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8.
Last update: 2014-02-24; version: 2.3.3.
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