Ultraviolet/infrared-double resonance spectroscopy and ab initio calculations on the indole+ and indole(H2O)1+ cations

In this paper we report on the application of infrared/photoinduced Rydberg ionization (IR/PIRI) and IR-photodissociation spectroscopy to investigate the CH, NH or OH stretching vibrations of ${\mathrm{indole}}^{\mathrm{+}}$ and the ${\mathrm{indole(H}}_{\mathrm{2}}{\mathrm{O)}}_{\mathrm{1}}^{\mathrm{+}}$ cluster cation. All vibrational frequencies of ${\mathrm{indole}}^{\mathrm{+}}$ and ${\mathrm{indole(H}}_{\mathrm{2}}{\mathrm{O)}}_{\mathrm{1}}^{\mathrm{+}}$ are compared with the values obtained from ab initio calculations. In the case of the ${\mathrm{indole}}^{\mathrm{+}}$ cation the NH vibration is observed. This is the first observation of a NH vibration in a bare cation. For ${\mathrm{indole(H}}_{\mathrm{2}}{\mathrm{O)}}_{\mathrm{1}}^{\mathrm{+}}$ a hydrogen-bonded structure with a nearly linear hydrogen bond can be derived both from ab initio calculations and the IR-spectra. By applying the state selective IR/PIRI spectroscopy to ${\mathrm{indole(H}}_{\mathrm{2}}{\mathrm{O)}}_{\mathrm{1}}^{\mathrm{+}},$ no vibrational couplings between the intermolecular O–H⋯N stretching vibration and the intramolecular OH stretching modes of the water moiety are observed. In the IR-photodissociation spectra of ${\mathrm{indole(H}}_{\mathrm{2}}{\mathrm{O)}}_{\mathrm{1}}^{\mathrm{+}}$ the NH, OH, and CH stretching vibrations as well as overtones of bending modes are observed. In agreement with the prediction of ab initio calculations the frequency of the NH stretching vibration of the cation is shifted by $\text{300\hspace{0.5em}}{\mathrm{cm}}^{\text{−1}}$ to lower frequencies compared to the neutral cluster. By analyzing the fragmentation of ${\mathrm{indole(H}}_{\mathrm{2}}{\mathrm{O)}}_{\mathrm{1}}^{\mathrm{+}}$ as a function of excess energies within the ion it can be concluded that only one IR photon is absorbed if OH stretching vibrations of the ion are excited, whereas at least two photons can be absorbed if the NH stretching mode is excited. Furthermore, comparison of IR/PIRI and IR-photodissociation spectroscopy indicates that the frequency of the OH stretching vibration decreases very slightly with increasing excess energy within the ion.