Autoionization-detected infrared spectroscopy of intramolecular hydrogen bonds in aromatic cations. I. Principle and application to fluorophenol and methoxyphenol

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 measure d by detecting molecular ions prepared through vibrational autoionization. The observed infrared spectra practically provide vibrational frequencies o f the corresponding bare molecular ion. The OH stretching vibrations of ort ho-, meta-, and para-isomers of fluorophenol and methoxyphenol cations are observed. The OH stretching vibrational frequency of the ortho- isomer show s a characteristic redshift due to the intramolecular hydrogen bond. The re dshift increases with ionization, indicating a significant enhancement of t he intramolecular hydrogen bond strength. (C) 1999 American Institute of Ph ysics. [S0021-9606(99)00609-1].