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UNIMOLECULAR DISSOCIATION DYNAMICS OF ENERGY-SELECTED MOLECULAR IONS (PROPANE, ETHYLENE, METHANE, NITROMETHANE, METHYLNITRITE)
Abstract
The unimolecular dissociations of several gas-phase molecular ions has been investigated using primarily threshold photoelectron-photoion coincidence (TPEPICO) mass spectrometry. The formation and dissociation of molecular ions with well defined internal energies has been analyzed from breakdown curves, photoionization onsets, and rate constant determinations. The threshold photoelectron detector used in this TPEPICO mass spectrometer (i.e., a collimated hole structure) has a high collection efficiency and good energy resolution in terms of FWHM; however, distortion of breakdown graphs may occur from the transmission of a long "tail" of non-zero kinetic energy electrons transmitted through this detector. A method has been developed in which the contribution of these electrons to the breakdown curves can be estimated. The inherent assumptions and limitations of this method are discussed in detail. The energetics and mechanism of decomposition of propane ion to ethylene ion and methane have been examined using isotopically labeled propane, propane-2-('13)C. A significant fraction of all methane ions lost from the parent ion involves the elimination of the central carbon; this fragmentation is believed to occur by a stepwise cleavage involving a tight transition state rather than by an isomerization of the molecular ion. Elucidation of the reaction mechanism leading to the formation of a fragment ion may be complicated by rearrangement of the molecular ion prior to fragmentation. Such a case is observed in the fragmentation of the nitromethane ion (CH(,3)NO(,2)('+)). At onset, fragmentation of the nitromethane ion to NO('+) is preceded by an isomerization to the methylnitrite ion; the barrier to isomerization is 0.64 eV. Symmetry considerations indicate that the nitromethane ion in its ('2)A(,2)((pi)) state correlates with the ('2)A"((pi)) state of the methylnitrite ion through which dissociation to the NO('+) fragment occurs. The unimolecular dissociation of energy-selected methylnitrite and deuterated methylnitrite ions (CH(,3)ONO('+) and CD(,3)ONO('+), respectively) was examined also and found to show exceptional behavior near threshold. Unlike the other molecular ions studied, these ions involve dissociation from non-competing, non-interconverting electronic states characterized by the formation of the CH(,3)O('+), CD(,3)O('+) and NO('+) ions.
Subject Area
Chemistry
Recommended Citation
GILMAN, JEROME PAUL, "UNIMOLECULAR DISSOCIATION DYNAMICS OF ENERGY-SELECTED MOLECULAR IONS (PROPANE, ETHYLENE, METHANE, NITROMETHANE, METHYLNITRITE)" (1982). ETD collection for University of Nebraska-Lincoln. AAI8306482.
https://digitalcommons.unl.edu/dissertations/AAI8306482