We have measured the OH-stretching fundamental and overtone spectra of resorcinol and hydroquinone in a supersonic jet using nonresonant ionization detected infrared/near-infrared spectroscopy. Anharmonic oscillator local mode calculations of the OH-stretching frequencies and intensities and Boltzmann populations of the stable rotamers have been calculated at the B3LYP/6-311++G(3df,2pd) level to help interpret the observed spectra. Resorcinol has three stable rotamers and in the recorded second and third OH-stretching overtone spectra there is evidence of two distinguishable rotamers. Hydroquinone has two stable rotamers; however, the OH-stretching oscillators of each rotamer are so similar in nature that even up to the fourth OH-stretching overtone the transitions coincide. These results place a limit on the ability of the jet-cooled overtone spectroscopy technique to distinguish between rotamers.
Infrared (IR) spectra of resorcinol (Rs)−Ar n clusters (n = 1 and 2) have been measured in the neutral and cationic ground states (S 0 and D 0 ) by IR dip and resonanceenhanced multiphoton ionization (REMPI)-IR spectroscopy. The OH stretching vibrations in S 0 keep their frequency regardless of the number of Ar atoms and the conformation of the OH groups in Rs (rotamers RsI and RsII), demonstrating that the Ar atoms are attached to the aromatic π-ring (π-bound structure) in S 0 . In the D 0 state, the IR spectra of Rs + −Ar n reflect the difference in the Rs conformations (RsI + and RsII + ). For n = 1, the IR spectra of both rotamers are almost the same as those of the corresponding monomer cations, indicating that Ar ligands essentially remain π-bonded after ionization. In contrast, the IR spectra of Rs + −Ar 2 show hydrogen-bonded and free OH stretching vibrations, demonstrating that for a significant fraction of the clusters, the Ar atoms migrate from the π-bound site to the OH groups. The ionization-induced π → H migration yields are not unity for both rotamers RsI + −Ar 2 and RsII + −Ar 2 . This result is in sharp contrast to phenol + −Ar 2 , in which one of the Ar atoms migrates to the OH site with 100% yield. The mechanism leading to the nonunity yield in Rs + −Ar 2 is discussed in terms of the number of OH binding sites and Franck−Condon factors. The ionization excess energy dependence of the IR spectra of Rs + −Ar 2 and its Rs + −Ar fragments is discussed in terms of the Ar binding energies estimated from the photoionization and photodissociation efficiency spectra.
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