Effect of deuteration on the fluorescence excitation spectrum of tropolone: a theoretical study Original Research Article

A combined ab initio+nuclear dynamics study is performed to theoretically analyze the intramolecular H-atom transfer process in mono- and bi-deuterated tropolones in both the ground (S0) and first excited singlet (S1) electronic states. From previous ab initio electronic calculations, reduced bidimensional energy surfaces are built up so that the nuclear wave functions for the nuclear motions in both electronic states are obtained. In particular we have analyzed the three isotopomers that result from deuteration of the tropolone ring: tropolone-3-d (3DT), tropolone-4-d (4DT) and tropolone-5-d (5DT). It is found that for the symmetric 5DT case the two lowest vibrational levels in both S0 and S1 appear as a quasi-degenerated tunneling doublet. The effect of the isotopic substitution is very modest so that it is predicted that the electronic spectrum of 5DT will be very similar to the one of the parent tropolone molecule. Conversely, for the non-symmetric structures 3DT and 4DT, the isotope induced asymmetry combined with the energy barrier in S0, lead to a complete localization of the two lowest vibrational wave functions in S0, whereas for S1 the asymmetry and energy barriers are low enough so that some degree of delocalization of the two lowest vibrational wave functions is still seen. In light of our results we try to explain the appearance of the reported fluorescence excitation spectra for these species.