The extended S₁/S₀ conical intersection seam for the photochemical 2 + 2 cycloaddition of two ethylene molecules

Abstract

The extended S₁/S₀ conical intersection seam for the photochemical 2_s + 2_s, 2_s + 2_a and 2_a + 2_a cycloadditions of two ethylene molecules has been documented using the methodology described by Sicilia et al. (J. Chem. Theory Comput. 4, 257 (2008)). Two additional critical points on this prototype crossing seam were found as a result, although both lie more than 100 kcal mol⁻¹ above the rhomboidal crossing located previously, and are not minima in the intersection space. Using VB theory, the branching space conditions for the conical intersection can be derived analytically, in fair agreement with calculated CASSCF results. Woodward–Hoffmann symmetry forbidden and allowed reactions are associated with the same extended conical intersection seam in this case.

Keywords:

• photocycloaddition
• Woodward-Hoffmann rules
• excited state
• conical intersection

Acknowledgements

J.J. Serrano-Perez is grateful for support from the European Research Council under the European Community's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement n° 251955.