Towards experimental determination of conical intersection properties: a twin state based comparison with bound excited states

Abstract

The energy and approximate structure of certain S₀/S₁ conical intersections (CI) are shown computationally to be deducible from those of two bound states: the first triplet (T₁), which is iso-energetic with the CI, and the second excited singlet state (S₂). This is demonstrated for acepentalene (I) and its perfluoro derivative (II) using the twin state concept for three states systems and based on the fact that the triplet T₁ is almost degenerate with the CI. The stable S₂ (C_3v configuration) state exhibits unusual exaltation of Jahn–Teller active degenerate mode—ν_JT = 2058 cm⁻¹ (∼500 cm⁻¹ higher than analogous e-mode of the symmetric (C_3v) T₁ and the dianion I⁻² or any C–C vibration of the Jahn–Teller distorted (C_s) ground state minimum). The acepentalene molecule, whose rigid structure and possibility to attain the relatively high symmetry C_3v configuration, is a particularly suitable candidate for this purpose.