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UNIMOLECULAR REACTIONS OF MOLECULAR IONS WITH SELECTED INTERNAL ENERGIES (PHOTOIONIZATION, MASS SPECTROMETRY, TPEPICO, TIME-OF-FLIGHT)
Abstract
The principal aim of this research is a better understanding of the formation and reaction of gas-phase ions and of the energetic and kinetic factors which control them. The unimolecular fragmentations of some gas-phase molecular ions were investigated using primarily threshold photoelectron-photoion coincidence (TPEPICO) time-of-flight mass spectrometry. The ion chemistry of these species was elucidated from breakdown curves, appearance potentials, photoionization yield curves, and the dependence on internal energy of rate constants and kinetic energy release. Below 14 eV, four major fragments were observed from the ionic dissociation of anisole. Near threshold, the rearrangement product, C(,6)H(,6)('+), exhibited asymmetric time-of-flight peak shapes indicating rates corresponding to metastable decay. Rate constants were found to agree well with those calculated using the RRKM theory. Most of the excess energy at threshold was found to be due to kinetic shift with only a small contribution from reverse activation energy. Successive CO loss is the only dissociation route for hexacarbonylchromium except for producing Cr('+) which has two precursors, Cr(CO)('+) and Cr(CO)(,2)('+). The breakdown curve of Cr('+) has a lower onset at about 14 eV, concurrent with the decrease in Cr(CO)(,2)('+) intensity and another around 17.5 eV coinciding with the decrease in Cr(CO)('+) intensity. The first onset is associated mainly with ionization from CO 1(pi) and 5(sigma) orbitals while the second with ionization occurring largely from ligand 4(sigma) orbital. The breakdown graph of dimethyl ether indicates that autoionization may occur over fairly broad absorption regions. Kinetic energy release for m/z 31 shows about 80% conversion of excess energy into product translation suggesting its formation by a fast, state-specific process, possibly from a repulsive surface. CH(,3)('+) is formed as a secondary product from (M-H)('+). Its average energy release remains nearly constant for an internal energy range of 3.75 eV over three electronic states. Apparently it is formed through a specific dissociation channel, possibly resulting from the surface crossing between the CH(,3)OCH(,2)('+) + H and CH(,3)('+) + H(,2)CO + H potential surfaces.
Subject Area
Chemistry
Recommended Citation
DAS, PRADIP RANJAN, "UNIMOLECULAR REACTIONS OF MOLECULAR IONS WITH SELECTED INTERNAL ENERGIES (PHOTOIONIZATION, MASS SPECTROMETRY, TPEPICO, TIME-OF-FLIGHT)" (1985). ETD collection for University of Nebraska-Lincoln. AAI8526617.
https://digitalcommons.unl.edu/dissertations/AAI8526617