π-Complex Structure of Gaseous Benzene-NO Cations Assayed by IR Multiple Photon Dissociation Spectroscopy

[C6H6NO]+ ions, in two isomeric forms involved as key intermediates in the aromatic nitrosation reaction, have been produced in the gas phase and analyzed by IR multiple photon dissociation (IRMPD) spectroscopy in the 800-2200 cm-1 fingerprint wavenumber range, exploiting the high fluence and wide tunability of a free electron laser (FEL) source. The IRMPD spectra were compared with the IR absorption spectra calculated for the optimized structures of potential isomers thus allowing structural information on the absorbing species. [C6H6NO]+ ions were obtained by two routes, taking advantage of the FEL coupling to two different ion-traps. In the first one, an FT-ICR mass spectrometer, a sequence of ion-molecule reactions was allowed to occur ultimately leading to an NO+ transfer process to benzene. The so-formed ions displayed IRMPD features characteristic of a [benzene,NO]+ pi complex structure, including a prominent band at 1963 cm-1, within the range for the N-O bond stretching vibration of NO (1876 cm-1) and NO+ (2344 cm-1). A quite distinct species is formed by electrospray ionization (ESI) of a methanol solution of nitrosobenzene. The ions transferred and stored in a Paul ion trap showed the IRMPD features of substituent protonated nitrosobenzene, the most stable among conceivable [C6H6NO]+ isomers according to computations. It is noteworthy that IRMPD is successful in allowing a discrimination between isomeric [C6H6NO]+ species whereas high energy collision induced dissociation fails in this task. The [benzene,NO]+  complex is characterized by IRMPD spectroscopy as an exemplary non-covalent ionic adduct between two important biomolecular moieties.