The intramolecular proton transfer in tropolone has been theoretically analyzed. Ab initio calculations using a variety of basis sets have been performed for both the singlet ground state (X 1 A 1 ) and the first excited singlet state (à 1 B 2 ). A configuration interaction all single excitation method ͑CIS͒ has been used to deal with the excited singlet state. Tunneling splittings in both electronic states have been obtained by fitting a bidimensional surface into the ab initio results. This way, a new strategy designed to avoid calculations of the intrinsic reaction coordinate ͑IRC͒, which require a very long computer time, is proposed and shown to give accurate results. Our calculations provide a theoretical interpretation of previous extensive spectroscopical data from which the tunneling splitting for the excited à 1 B 2 state was shown to be clearly higher than for the ground X 1 A 1 state. Finally, the experimentally observed diminution of the splitting upon deuteration of the transferring hydrogen is also accounted for by our theoretical results.

In this paper we perform ab initio calculations for the stable conformations and the transition states for the isomerization processes in 5-hydroxytropolone in both the ground (S0) and first excited (S1) singlet electronic states. The Hartree–Fock self-consistent field (SCF) level and a complete active space SCF (CASSCF) level for S0 are considered, whereas the configuration interaction all single excitation method (CIS) and the CASSCF levels are used to deal with the S1 state. Energies are reevaluated at all levels through perturbation theory up to second order: Mo/ller–Plesset for the Hartree–Fock and CIS methods, and the CASPT2 method for CAS results. The ab initio results are then used to perform different monodimensional fits to the potential energy surfaces in order to analyze the wave functions for the nuclear motions in both electronic states. Our best results predict that for the S0 state two stable conformers, syn and anti, can exist in thermal equilibrium. In accordance with experimental expectations the syn isomer is the most stable. As for the S1 state, and again in accord with experimental spectroscopical data, the order of stability reverses, the anti being the most stable. A more interesting result is that analysis of the nuclear wave functions shows an important syn–anti mixing in the S1 state that does not appear in S0. This result explains the appearance of syn–anti and anti–syn crossover transitions observed in the electronic spectra of 5-hydroxytropolone so that syn–anti reaction may take place through photoisomerization.

A combined ab initio+nuclear dynamics study is performed to theoretically analyze the intramolecular H-atom transfer process in 5-aminotropolone in both the ground (S0) and first excited (S1) singlet electronic states. A complete active space self-consistent field (CASSCF) method is used to optimize the geometries. Energies are then corrected through the second order Mo/ller–Plesset perturbation theory. These results are used to build up reduced bidimensional energy surfaces so that the nuclear wave functions for the nuclear motions in both electronic states are obtained. In particular we have analyzed the six isotopomers that result from deuteration of the amino and hydroxy groups of 5-aminotropolone. It is found that for symmetric structures (−OH/−NH2, −OH/−ND2, −OD/−NH2, and −OD/−ND2), the two lowest vibrational levels in both S0 and S1 appear as a quasidegenerated tunneling doublet. The tunneling splitting in S0 is much lower so that the doublet at the origin, seen in the fluorescence excitation spectra of 5-aminotropolone, can be entirely assigned to the S1 state. In agreement with the experimental findings, this splitting greatly diminishes when the transferring hydrogen is substituted by a deuterium, whereas deuteration of the amino group produces only a modest decrease of such a splitting. A quite different result is found for the nonsymmetric isotopically substituted structures (−OH/−NHD and −OD/−NHD), as the isotope induced asymmetry, combined with the high energy barrier in the S0 potential energy surface, leads to a complete localization of the two lowest vibrational wave functions in S0. On the other hand, for S1 the asymmetry and energy barriers are low enough so that an important degree of delocalization of the two lowest vibrational wave functions is found. These results are again in agreement with the presence of an isotope induced quartet in the fluorescence excitation spectra of these species.

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