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

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.