Energy conical intersection

Figure 7.31. Energy contour maps for the face-to-face [2 -I- 2] cycloaddition of two acrylonitrile molecules in the syn head-to-tail arrangement, showing (a) the lowest excited state S, and (b) the ground state S . Possible reaction paths via the low-energy conical intersection are indicated by arrows.

Fig. 12.6. Structures of lowest-energy conical intersections for the singlet excited state of ethene (a) ethylidene-like structure (b) twisted and pyramidalized strucmre. Reproduced from Chem. Phys., 259, 237 (2000), by permission of Elsevier.

From inspection of the figure, it is apparent that a continuum of conical intersection points connects a higher-energy conical intersection with C2 symmetry C 2-CI) to a lower-energy as3mimetric conical intersection Ci-CI). It has been independently shown via conical intersection... 

Fig. 7. The excited-state reaction path of the penta-2,4-dienimminium cation intercepts the conical intersection point CI700 located ca. 5 kcal mol above the minimum-energy conical intersection CIq2o. FC— Cl7oo MEP ( i open diamonds and So full diamonds), Cl7oo Cl92o MEP (5i open circles and So full circles). The values of the relevant structural parameters are given in A and degrees.

Fig. 13. The computational methods used for constructing a photochemical reaction path. The full path is computed by joining different MEPs, each one providing information on a specific part of the excited- or ground-state potential-energy surface. The IRD method is used to compute the steepest relaxation directions departing from the FC point (excited-state relaxation) or Cl (ground-state relaxation). The IRC method is used to compute the steepest-descent line defined by the computed IRDs. The CIO method is used to compute the lowest-energy conical intersection point directly. With TSO we indicate the standard transition structure optimization procedure.

An extensive theoretical/computational study of the time-dependent behavior of diabatic and adiabatic populations for three well-known examples of low-energy conical intersections, the C2H4, pyrazine and NO2 molecules, has been performed by Koppel et who used a three-... 

The position and energy of the minimum energy conical intersection (MECI) are given by... 

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