Beyond the van der Lugt/Oosterhoff model: when the conical intersection seam and the S1 minimum energy path do not cross.

The photoinduced ring-opening reaction of cyclohexadiene (CHD) is a textbook example for electrocyclic reactions. In this paper we report the complete "minimum energy path" of the low-lying region of the conical intersection space reaching from the closed to the open ring side. The general role of conical intersections (CoIns) is to provide the locus for ultrafast transfer between electronic states, in the present case, to the closed or open form in the ground state after photoexcitation. The seam was calculated with use of an analytic approach in which the intersection space in the vicinity of a CoIn is described to second order. The topography of the seam was investigated, revealing minimum energy and transition state structures. In addition the energy profile of the seam was rationalized with valence bond (VB) theory. The geometrical changes along the seam have been related to the motions along the excited state minimum energy path (S(1)-IRC-MEP) in a conceptual model highlighting the quasiparallel orientation of seam and IRC-MEP. Our model shows that even though the van der Lugt and Oosterhoff concept predicts the formation of an avoided crossing along the S(1)-IRC-MEP, it provides an incomplete description of the decay process to the ground state. The latter requires, in addition, vibrational motions orthogonal to the MEP, directed toward the CoIn seam.