Infrared spectroscopy autoionization-detected

Autoionization-detected infrared spectroscopy (ADIR, which is a new technique)66 makes it possible to record infrared spectra of isolated bare molecular cations in molecular beams. Conventional time-of-flight mass spectroscopy (at zero kinetic energy) and photoelectron spectroscopy also enable the study of such cations. 

N—H vibrational frequencies of substituted anilines are linearly correlated with the sigma values with a positive slope, while a plot of vibrational frequencies of the anilinium ions (obtained by autoionization-detected infrared spectroscopy) results in a similar plot showing a negative slope66. 

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