Rk. Frost et al., FLUORESCENCE-DIP INFRARED-SPECTROSCOPY OF JET-COOLED 5-HYDROXYTROPOLONE, Journal of physical chemistry, 100(42), 1996, pp. 16835-16842
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%.