Autoionization-detected infrared spectroscopy of intramolecular hydrogen bonds in aromatic cations. II. Unconventional intramolecular hydrogen bonds

A newly developed infrared spectroscopic technique, called autoionization-d etected infrared (ADIR) spectroscopy, was applied for a study on hydroxyl-a lkyl interactions in cresol and ethylphenol cations. In this technique, vib rational transitions in the ion core of high Rydberg states, which has almo st the same vibrational structure as the corresponding bare molecular ion, are measured by detecting the vibrational autoionization signal. The OH str etching vibrations in the rotational isomers of the ortho-, meta-, and para -cresol cations and those of the ethylphenol cations were observed. Remarka ble low-frequency shifts of the OH vibration were found only for the cis ro tational isomers of the ortho-cresol and ortho-ethylphenol cations, whereas no such shift was found for all the other rotational and structural isomer cations. On the other hand, no remarkable shift of the OH stretch frequenc y was found for all the isomers in the neutral ground state. These results indicate that an intramolecular hydrogen bond is formed between the hydroxy l and alkyl groups in the cationic ground state of ortho-cresol and ortho-e thylphenol. The remarkable low-frequency shift of the OH vibration also ind icates that the alkyl group acts as a proton acceptor in the hydrogen bond. This is a new type of intramolecular hydrogen bond, and the origin of such unconventional hydrogen bond in the cations is discussed. (C) 2000 America n Institute of Physics. [S0021- 9606(00)00301-9].