On the planarity of the tropolone molecule in the excited state: A time dependent DFT geometry optimisation

Density functional theory along with the time-dependent formalism (TD-DFT) are used to directly localise the stationary points in the A ˜ 1 B 2 first singlet excited state of tropolone. The optimisation reveals that the equilibrium geometry of tropolone in the excited state is planar, a result that seems in contradiction with recent analysis using the high-resolution degenerate four-wave mixing technique that finds a large inertial defect. A theoretical evaluation of the vibrational modes reveals the presence of a very small out-of-plane frequency that could also account for the observed large inertial defect. CASSCF and CASPT2 results validate the TD-DFT method to deal with excited intramolecular proton-transfer reactions.