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 nitrobenzen e 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 an d contribution from competing channels is very small. At 3.1 eV collision e nergy, 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 coll isional activation step does not change significantly with increase in coll ision energy and corresponds to about 0.6 eV above the thermochemical thres hold even though the molecular ions were formed by 70 eV energy electrons. There are no apparent changes in the dynamics features of the activation/di ssociation process in going from 3 to 50 eV collision energy except the dec rease in the maximum of the CM scattering angle. These results suggest that there is no change in the activation/dissociation mechanism for this CID p rocess, We further suggest, based upon the energy loss measurements, that t his dissociation process is nor direct, instead it proceeds via ion-neutral complex C6H5+. . . NO2, as proposed by Osterheld, Baer, and Brauman (Oster held, T. H.; Baer, T.; Brauman, J. A. J. Aln. Chem. Sec. 1993, 115, 6284) a nd competes with the loss of NO via nitro-nitrite rearrangement.