FLUORESCENCE-DIP INFRARED-SPECTROSCOPY OF JET-COOLED 5-HYDROXYTROPOLONE

Ground-state infrared spectra of the syn and anti conformers of jet-co oled 5-hydroxytropolone (5-HOTrOH) monomer have been recorded free fro m interference from one another using a fluorescence depletion method. The 5-hydroxy group ''free'' O-H stretch, 2-hydroxy intramolecularly hydrogen-bonded O-H stretch, and C-H stretch transitions are observed for both conformers. The assignment of these bands is clear by compari son with previous infrared studies of bare tropolone and is confirmed by MP2 and DFT level vibrational frequency and intensity calculations. Each of the conformers is predicted to have four allowed C-H stretch transitions. There is a reasonable one-for-one correspondence between calculation and experiment for the anti conformer's C-H stretch bands, while the syn conformer shows effects of Fermi resonance mixing. The 5-OH stretch bands of the syn and anti conformers are single, sharp tr ansitions located at 3654 and 3664 cm(-1), respectively. The OH stretc h fundamentals of the 2-OH group involved in the intramolecular hydrog en bond are centered on 3170 and 3195 cm(-1). These bands are extensiv ely broadened in both conformers and contain reproducible substructure with irregular spacing of about 10-15 cm(-1). The substructure of the intramolecularly H-bonded 2-hydroxy O-H stretch band reflects a selec tive first-tier vibrational-state mixing, most likely with a set of mo des which are in near two-to-one resonance with it. Finally, the 249 c m(-1) asymmetry in the double-minimum potential well for H atom tunnel ing appears to be sufficient to quench syn-anti tunneling in S-0 even following excitation of the 2-OH stretch fundamental. No evidence is f ound for syn <----> anti ''crossover'' transitions despite careful sea rches capable of detecting such transitions at depletion levels of abo ut 1%.