Nonadiabatic unimolecular reactions. III. Dissociation mechanisms of methylnitrite and deuterated methylnitrite ions J. Chem. Phys. 88, 5606 (1988); 10.1063/1.454571The unimolecular decomposition rates of energy selected methylnitrite and deuterated methylnitrite ions Major fragmentation pathways of the nitromethane and methylnitrite ions have been examined. Ionization and fragmentation onsets for these processes have been determined and breakdown graphs have been constructed for both compounds using a threshold photoelectron-photoion coincident (TPEPICO) mass spectrometer. At onset, fragmentation of the nitromethane ion to NO+ is preceded by isomerization to the energetically favored methylnitrite ion; the barrier to isomerization is 0.64 eV. Symmetry considerations indicate that the nitromethane ion in its 2A2 (1T) state correlates with the 2A "(1T) state of the methylnitrite ion through which dissociation to the NO+ fragment occurs.1174
The threshold photoelectron (TPE) and TPE-coincident photoion (CPI) mass spectra of SO2 have been determined over the energy range 15.844 to 16.674 eV with a resolution of about 0.018 eV FWHM. This is sufficient to prepare most individual vibronic states in populations exceeding 85%. In the 2B2 state, excitation of the stretching mode up to ν1=5 is observed, while in the same state the ν2 bending mode may be excited concurrently with not more than one quantum. In this energy range SO+ formation sets on at 15.930±0.005 eV, suggesting that the onset at 0° K should be 15.965 eV±0.005 eV. S+ is produced over a very narrow energy range only; the onset is at 16.334 eV, coincident with the (310 2B2 state of SO2+; S+ disappears at 16.674 eV. This is interpreted in terms of competition between the assymetric stretch mode and the bending mode for intermodal energy transfer from the symmetric stretch mode, with excitation of the bending mode required to give S+.
The photoionization and fragmentation of CH3ONO+ were studied using threshold photoelectron-coincident photoion mass spectrometry, photoelectron spectroscopy, and triple sector mass spectrometry.(AIP)
The elimination of methane from the propane molecular ion has been investigated using propane-2-13C. Breakdown graphs obtained by threshold photoelectron-coincident photoion mass spectrometry indicate that approximately 11% of all methane loss includes the central carbon and that this amount is independent of internal energy over nearly 1.2 eV. These results are interpreted in terms of a potential energy surface and rate constant curves. We conclude that isomerization of the propane molecular ion does not precede fragmentation to ethylene ion and methane, but that fragmentation occurs by a concerted stepwise cleavage involving a tight transition state.
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