Experimental and theoretical studies of ion -molecule complexes and of some ionized diketones in the gas-phase

This thesis describes some research in gas-phase ion chemistry, using mass spectrometry techniques combined with theoretical calculations. Metastable ion and collision-induced dissociation mass spectra, kinetic energy release and isotopic labelling were used to characterize and identify precursor and fragment ions and to predict reasonable reaction mechanisms which were then further elucidated by calculations. MP2, B3-LYP and G3 levels of theory were employed to obtain the structures corresponding to energy minima and transition states and thus to obtain a complete potential energy surface for each system.;The first two systems studied were acetaldehyde associated with the distonic methanol cation (•CH2OH2 +), [CH3CHO/H+O(H)CH2 •], and with its conventional isomer (CH3OH +•), [CH3CHO/+•HOCH3]. The isomerization barrier for ionized acetaldehyde to rearrange to its enol isomer, vinyl alcohol CH2CHOH+•, was lowered significantly (by 133 kJ/mol) by its association with •CH 2OH2+. This reaction resulted from a H +/H• transfer mechanism, involving a transition state of low energy requirement. In contrast, the CH3OH +• ion did not facilitate the rearrangement of acetaldehyde to its enol ion. Similar observations were obtained for ion-molecule complexes of acetone and methanol, [(CH3)2C=O/H+O(H)CH 2•] and [(CH3)2C=O/ +•HOCH3]. One metastable ion dissociation of the latter involved the loss of a methyl radical, producing a new [C3H 7O2]+ isomer, [CH3C+···(H)OCH 3], whose structure was assigned by calculations.;A water molecule is one of the smallest species that have been used as a catalyst within an ion-molecule complex, thus the gas-phase reactions of the ion [CH3CHO/H2O]+• were also investigated in this thesis. The metastable ion (MI) mass spectrum revealed that this ion-molecule complex decomposed spontaneously by the losses of H 2O, CO and •CH3. Hydrogen-bridged water complexes were found to be the major products of the losses of CO and •CH3. The CO loss produced the [•CH 3···H3O+] ion and involved a backside displacement mechanism. The product ions arising from • CH3 loss (via two different reaction channels) were assigned by theory to be the isomers, [OC···H3O +] and [CO···H3O+], and their 298 K enthalpy values, calculated at the G3 level of theory are Delta fH[OC···H3O+] = 420 kJ/mol and DeltafH[CO···H3O+ ] = 448 kJ/mol. (Abstract shortened by UMI.).;A major focus of this work was to investigate the unimolecular reactions of some ion-molecule complexes, especially the isomerizations that take place within them and then to discover the mechanisms by which such reactions take place. Particular attention was devoted to ion-molecule complexes that comprise ketones (including aldehydes), enols and distonic ions of alcohols.