Study on the catalytic role of hydrogen-bond in the phtoinduoed cis【tautomer】trans isomerization of napththol

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There are two isomeric forms in naphthol, depending on the orientation of hydroxyl group with respect to the naphthalene ring, which are called cis and trans. The existence of the two rotational isomers was first proposed by Ito and coworkers by uses of laser spectroscopic technique combined with a supersonic jet. Later, Pratt and coworkers assigned them by analyzing rotationally resolved electronic spectrum of jet cooled naphthol. Since then, the possibility of the laser manipulation of cis【tautomer】trans isomerization has been discussed by several researchers. Up to now, however, it has not been realized yet. In this research, we aimed to manipulate the cis【tautomer】trans isomerization for electronically excited by use of UV-IR stepwise excitation in a supersonic jet. First, a specific isomer was selectively excited to the S_1 state with a tunable UV laser pulse. The molecule or cluster in S_1 is further excited to its X-H stretching vibration by the IR laser pulse, where X refers to … More the C,O or N atom. The energy put in to the X-H stretching vibration is immediately redistributed into other vibrartional modes by IVR, and part of them will flow into the iromerization coordinate. If the energy is large enough to overcome the isomerization barrier, the reaction will occur and the product emits fluorescence.We investigated not only bare molecule but also the hydrogen-bonded clusters and found that the reaction does not occur for bare molecule within the examined energy region up to 3510 cm^<-1>. On the other hand, the reaction occurred at 2900 cm^<-1> for hydrogen-bonded clusters. Such a larger reduction indicates that H-bond acts as a catalyst for the reaction. Furthermore, we fond in the clusters the cis-trans isomerization occurs very efficiently at low IR frequency but the yield decrease rapidly with the energy due to the competition with the dissociation of H-bond within the same isomer. The isomerization occurred both 1- and 2-napthol H-bonded clusters and the barrier height seems to be similar between them. Parallel to the experiments, we performed DFT and time-dependent DFT calculation to obtain the isomerization barrier height and to investigate the effect of H-bond in S_0, S_1 and S_2 states. Thought the calculation predicted large increase of the barrier height in S_1, it failed the decrease of the height upon the H-bond formation. Further calcuation is in progress. Less