Masters Theses
Date of Award
5-1997
Degree Type
Thesis
Degree Name
Master of Science
Major
Chemistry
Major Professor
John E. Bartmess
Abstract
The Ion Cyclotron Resonance (ICR) Mass Spectrometer has been useful in the study of dissociative electron attachment and other chemical gaseous reactions.
In the production of negative ions, dissociative attachment is one of several processes that can be used to successfully generate a large selection of low energy anions. It is understood that some molecules that undergo dissociative attachment have low cross sections (low probability of formation). Hence, unless a molecule has a sizable cross section very little signal in the ICR is generated from attempts to produce a primary anion. The primary focus of this study is to determine if cert^ molecules follow the necessary criteria used to predict whether the reaction of a molecule undergoing dissociative attachment will be favored.
Alkyl halides, a thio sulfonate, nitrite ester, acetyl halide and an anhydride are used for the determination of favorable anion formation. In dissociative electron attachment there is initially the attachment of an electron to a neutral molecule that produces a stable negative intermediate that can go through either non-dissociative or autodetachment as competing pathways. It is the repulsive state of the radical anion that results in the formation of the desired fragment anion.
We proposed that a precursor molecule that gave the results favoring the dissociative pathway could be attributed to the reactions being exothermic, having a bound LUMO and having only one ion produced that doesn't react with its own neutral precursor. In all the cases the precursor molecules give the desired anions. However, the sulfonate molecule gave the desired anion along with traces of other anions. This was attributed to the primary anion reacting with the neutral precursor as well as the relative instability of the S-S bond. If the precursor molecular reaction is not exothermic then it must be close to thermoneutral for the excited molecular anion to undergo dissociative attachment.
In addition, cross sections were assigned based on carbon tetrachloride. chloroform and methylene chloride dissociative attachment formation. Previous literature sources, as well as this study, determined and verified the cross sections of the halomethanes. This was to see if these molecules could serve as models used to aide in the assignment of cross sections for methyl methanethiosulfonate, isoamyl nitrite, acetyl chloride and acetic anhydride, which successfully generated negative ions in this ICR study. This was done by attempting to establish a correlation between the halomethane's experimental corrected slopes and the literature cross sections. Discrepancies were found concerning the required electron energy spread needed for the verification of the halomethane's integrated cross sections and experimental corrected slopes. These discrepancies resulted in a lack of correlation of dissociative attachment probabilities with literature cross sections. However, the literature cross sections and the experimental corrected slopes for the halomethanes both agreed in their magnitude order. As a result, roughly estimated integrated cross sections for methyl methanethiosulfonate, isoamyl nitrite, acetyl chloride and acetic anhydride were assigned.
Recommended Citation
Andrews, Kim, "Gas-phase ion molecule chemistry : 1. ion generation and detection 2. the practical use of dissociative attachment 3. collision of electrons with molecules determined cross sections for dissociative attachment precursors. " Master's Thesis, University of Tennessee, 1997.
https://trace.tennessee.edu/utk_gradthes/10445