NH stretching vibrations of jet-cooled aniline and its derivatives in 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 sigma 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.