Fig. 3: TD-DFT and DFT investigations.
From: Triplet state reactivity of iminium ions in organocatalytic asymmetric [2 + 2] photocycloadditions
a, Relative electronic energy profile for the unconstrained TD-DFT minimization (relaxation) of 4 (S1) after photoexcitation of 4 (S0), viewed with out-of-plane iminium distortion towards the Re face of the C=C indole bond involved in the [2 + 2] cycloaddition. The reaction coordinate is the CCCN dihedral marked in green in 4 (0° when fully planar): surfaces for S0, T1 and T2 are also shown for comparison; all other low-lying excited states are omitted for clarity. b, Representations of the S1 and the T2 states at MECP1 (S1–T2) as electron–hole pairs (difference between squared natural transition orbitals (NTOs); orange areas denote maximum localization of the excited electron). c, The relative free energy profile (∆G) of the proposed reaction pathway for the organocatalytic enantioselective [2 + 2] photocycloaddition. Only the pathway leading to the observed enantiomer is presented.
