Off-campus UNL users: To download campus access dissertations, please use the following link to log into our proxy server with your NU ID and password. When you are done browsing please remember to return to this page and log out.
Non-UNL users: Please talk to your librarian about requesting this dissertation through interlibrary loan.
Unimolecular reaction dynamics of gaseous ions with selected internal energies
Abstract
There are two aspects of unimolecular dissociation that are major interest and importance. These are the rate of the reaction and the energy distribution in the products. A powerful and generally applicable experimental approach to the investigation of such problems is that of threshold photoelectron photoion coincidence (TPEPICO) mass spectrometry. In order to achieve a better understanding of the reaction dynamics, the unimolecular dissociation of several gas phase molecular ions was studied by TPEPICO technique. The ion chemistry of these species was also elucidated based on the dissociation behavior of molecular ions with narrowly selected internal energies. Dimethyl sulfoxide represents an organic molecule containing sulphur as the heteroatom. The breakdown graph of dimethyl sulfoxide calculated based on RRKM/QET was found to agree with experiment. The statistical dissociation of this molecule is not consistent with the generally accepted explanation that nonstatistical dissociation seems to involve the molecules containing heteroatoms. Two major fragment ions produced from ethyl formate at low internal energies are C$\sb2$H$\sb4\sp+$ and C$\sb2$H$\sb4$CO$\sp+$. For a simple direct fragmentation, decay rates should be identical for both processes because only total decay rate reflects itself in TOF distribution of fragment ions. The fact that the decay rate constants observed for formation of these two ions were found different may ascribed to that the ionic dissociation of ethyl formate is a competition of two different isomerization processes. The reaction mechanism of formate esters was determined by both TPEPICO and tandem (MS/MS) mass spectrometry. The formations of major fragment ions at low internal energies are ascribed to a stepwise McLafferty rearrangement. The first step is the formation of a stable isomeric structure, the distonic radical cation. This is the predominant mode of formate ester decomposition at low internal energies. This mechanism is supported by the agreement between measured kinetic energy release and the calculation based on statistical theory. The structure of ionized n-propanol preceding the loss of water was determined to be the distonic radical, not the previously reported ion-dipole complex by the examination of ion lifetime. A factor R related to the state density of reactant was introduced to the RRKM/QET calculation of rate constant. This constant factor, however, reflects the nature of reactant in the unimolecular dissociation involving an isomerization process.
Subject Area
Chemistry
Recommended Citation
Zha, Qingmei, "Unimolecular reaction dynamics of gaseous ions with selected internal energies" (1988). ETD collection for University of Nebraska-Lincoln. AAI8907594.
https://digitalcommons.unl.edu/dissertations/AAI8907594