Fundamental studies of ionic reactions in the gas phase

The intrinsic (solvent-free) reactivity and thermochemical properties of various organic radical cations were studied in the gas phase by Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR). In Chapter 2, several factors that control competition between exothermic electron and proton transfer reactions of various aldehyde and ketone radical cations are described. Electron transfer dominates many of these reactions. Proton transfer successfully competes with electron transfer when the former pathway is more exothermic (\u3e30 kJ/mol) than the latter. These results are readily rationalized by the curve-crossing model which correlates the deprotonation barrier of an aldehyde or ketone radical cation to the recombination energy of the radical cation, as well as the strength of the C-H bond being cleaved. Ab initio MO calculations further predict that the geometric distortion of the acidic site in the radical cation is another factor that strongly contributes to the deprotonation barrier. In Chapter 4, the reactivity of the cytosine radical cation toward various hydrogen atom donors was examined. The cytosine radical cation was found to efficiently abstract a hydrogen atom from most reagents studied. This reaction may occur via direct hydrogen abstraction or indirectly by consecutive electron transfer and proton transfer within the ion-molecule complex. In Chapter 5, collision-activated dissociation threshold energy measurements on the iodobenzene radical cation were carried out in the dual-cell FT-ICR instrument to examine the effects of cooling gas, target gas pressure and collision/dissociation times on the quality of the threshold data. The measured threshold energies for loss of the iodobenzene atom from the radical cation were ca. 0.5 eV greater than the reported values. This is likely due to a kinetic shift in the threshold data due to the limited dynamic range of the 3 Tesla instrument