Unimolecular rearrangement and fragmentation mechanisms of gas phase ions

Some unimolecular rearrangement and fragmentation mechanisms of enolate and alkoxide ions in the gas phase are studied using Fourier transform ion cyclotron resonance mass spectrometry and infrared multiple photon activation techniques. Three novel reaction mechanisms were found in the unimolecular dissociation of enolate ions. First, an enolate ion can undergo a 1,3-hydrogen rearrangement which is normally forbidden by the Woodward-Hoffmann rules. Second, an enolate ion can also undergo a 1,3-methyl rearrangement. Third, one fragmentation pathway of the enolate ions is consistent with an alkyl radical-ketene radical anion complex as an intermediate.;The effect of various degrees of alkyl substitution on the relative rates of deprotonation from the two proton transfer sites in several unsymmetrical ketones is examined. The infrared multiple photon activation of an appropriately deuterium-labeled alkoxide ion generates the ion-molecule complex for the half reaction of the bimolecular proton transfer process between an alkyl anion and an unsymmetrical ketone with one deprotonation site selectively deuterated. The alkyl anion then removes either a deuteron or a proton to generate enolate ions that are distinguishable by mass. The measurement of the enolate ion product ratios, along with an independent measurement of the kinetic isotope effect, allowed the kinetic effect of the alkyl environment on the relative proton transfer rates to be determined. The kinetic alkyl group effect on the proton transfer rate is due to a combination of factors (proton transfer, charge delocalization, and rehybridization) competing in the transition state.;Since alkanes are very weak acids, the gas phase acidities of alkanes are studied indirectly by examining the infrared multiple photon dissociation of several alkoxide ions. For an alkoxide ion consisting of two distinct alkyl groups, two different elimination products that are distinguishable by mass can be formed. The relative alkyl leaving group propensities are determined from the product ratios of the two different elimination products. The relative alkyl leaving group propensity correlates with the acidity of the associated alkane. This correlation gave an anomalous ordering for the gas phase acidity of alkanes that indicates that the methyl group destabilizes a negative charge on a carbon.