Collision-induced dissociation of nitrobenzene molecular cations at low energies by crossed-beam tandem mass spectrometry

The dynamics of the collision-induced dissociation (CID) of the nitrobenzene molecular ipn have been investigated in the energy range of 3 eV-50 eV in the center-of-mass (CM) reference frame using a crossed-beam tandem hybrid mass spectrometer-supersonic molecular beam instrument. At these collision energies the dominant CID channel is loss of NO2 from the molecular ion and contribution from competing channels is very small. At 3.1 eV collision energy, fragment ions, C6H5+, are fully backward scattered and as collision energy is increased the most probable scattering angle moves in the forward scattering region but remains above zero up to 50 eV energy. These results suggest that small impact parameter impulsive collisions dominate this CID process in the low collision energy range, The energy transfer in the collisional activation step does not change significantly with increase in collision energy and corresponds to about 0.6 eV above the thermochemical threshold even though the molecular ions were formed by 70 eV energy electrons. There are no apparent changes in the dynamics features of the activation/dissociation process in going from 3 to 50 eV collision energy except the decrease in the maximum of the CM scattering angle. These results suggest that there is no change in the activation/dissociation mechanism for this CID process, We further suggest, based upon the energy loss measurements, that this dissociation process is nor direct, instead it proceeds via ion-neutral complex C6H5+. . . NO2, as proposed by Osterheld, Baer, and Brauman (Osterheld, T. H.; Baer, T.; Brauman, J. A. J. Aln. Chem. Sec. 1993, 115, 6284) and competes with the loss of NO via nitro-nitrite rearrangement.