An infrared spectroscopic study of protonated and cationic indazole

The mid-infrared vibrational spectra of the cationic and protonated forms of the molecule indazole (C7N2H6) are recorded in the gas phase, applying the method of free electron laser induced multiple photon dissociation spectroscopy in a quadrupole ion trap. The spectra are compared to density functional theory calculations, which for the protonated species suggests that the proton attaches to the pyridine-like nitrogen atom. The spectrum of the indazole cation raises the question whether indazole undergoes an N1-N2 H atom shift upon ionization. The spectra for the charged species are further discussed in comparison with the spectrum of neutral indazole. Although the spectral range probed in this study, 600-1800 cm−1, does not cover the hydrogen stretching modes, the spectra are found to be very distinct, indicating how a subtle change in electron distribution can have major effects on the vibrational spectrum of a conjugated system.