A new infrared spectroscopic technique for jet-cooled molecular cations is applied to observe intramolecular hydrogen bonds in substituted phenol ions. Vibrational transitions of an ion core of high Rydberg states are measured by detecting molecular ions prepared through vibrational autoionization. The observed infrared spectra practically provide vibrational frequencies of the corresponding bare molecular ion. The OH stretching vibrations of ortho-, meta-, and para-isomers of fluorophenol and methoxyphenol cations are observed. The OH stretching vibrational frequency of the ortho-isomer shows a characteristic redshift due to the intramolecular hydrogen bond. The redshift increases with ionization, indicating a significant enhancement of the intramolecular hydrogen bond strength.
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We applied autoionization-detected infrared (ADIR) spectroscopy in order to observe OH stretching vibrations of jet-cooled 1-and 2-naphthol cations. In this technique, high Rydberg states, the vibrational levels of which are essentially the same as those of the corresponding bare molecular ion, were prepared by two-color doubleresonance excitation. Vibrational transitions in the ion core of the high Rydberg states were measured by detecting the vibrational autoionization signal. For rotational and structural isomers of naphthol, similar lowfrequency shifts of the OH frequencies upon ionization were found. The OH frequency shifts of naphthols were much smaller than that found for phenol, although both molecules have quite similar OH frequencies in their neutral ground state. This remarkable difference in frequency shifts was qualitatively explained in terms of the charge delocalization in the aromatic ring. In addition, the OH stretching vibrations of the 1-and 2-naphthol-Ar cluster cations were observed by infrared photodissociation spectroscopy. It was found that perturbations from the Ar atom to the hydroxyl group of naphthol are negligible in both the neutral and cationic ground states.
NH stretching vibrations of jet-cooled aniline and its various derivatives were observed in the neutral and cationic ground states. Infrared-ultraviolet double-resonance spectroscopy was utilized for the observation in the neutral ground state, and autoionization-detected infrared spectroscopy was used in the measurement of the cationic ground state. Low-frequency shifts of the NH frequencies were seen upon ionization, though the magnitude of the shifts was much smaller than those of the OH stretching vibrations observed for phenol derivatives. NH vibrational frequencies of meta and para isomers of aniline derivatives showed a positive correlation with the Hammett σ parameters of the substituents in the neutral ground state, while a negative correlation was found for the cationic ground state. Intramolecular hydrogen bond formation was found for o-cyanoaniline in both the neutral and cationic ground states based on the infrared spectra of deuterated isotopomers as well as theoretical calculations, showing that the hydrogen bond strength is remarkably enhanced upon ionization.
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