The fluorescence excitation and dispersed fluorescence spectra of jet-cooled o-, m-, and p-fluorotoluene were observed. It was found that the spectra exhibit a feature characteristic of internal rotation of the methyl group in the ground and excited states. Detailed analyses of the spectra provided us with the potentials for the internal rotation in both the ground and excited states. In o-fluorotoluene, a large potential barrier for internal rotation in the ground state was found to be practically removed in the excited state (S,). It was also found that the most stable conformation of the methyl group with respect to the benzene plane in the ground state is the most unstable conformation in the excited state. The situation is entirely different in m-fluorotoluene, where the almost free rotation of the methyl group in the ground state is greatly hindered in the excited state. These interesting results are discussed in terms of intramolecular hydrogen bonding and hyperconjugation in the excited state.
The fluorescence excitation and dispersed fluorescence spectra of jet-cooled uracil, thymine, and their derivatives have been observed. Two band systems having well-resolved vibrational structures were found for uracil and thymine in the frequency region from 31 000 to 38 000 cm"1, which corresponds to the region of the long tail in the vapor absorption spectrum. The shorter wavelength band system (system I) was identified as the 8,( , *) *-S0 transition of the diketo tautomer, while the longer wavelength system (system II) was assigned to the Si(n,ir*) *-S0 transition of one of the keto-enol tautomers. Successful detection of a very small amount of the keto-enol tautomer is due to a high fluorescence yield of the keto-enol tautomer.The nature of the ^ , *) states of the tautomers and the hydrogen-bonded complexes with water are also discussed.
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