Rearrangements of ionized carbonyl compounds

Lactone enol cation-radicals: gas-phase generation, structure, energetics, and reactivity of the ionized enol of butane-4-lactone

The cation-radical of 2-hydroxyoxol-2-ene (1(+*)) represents the first lactone enol ion whose structure and gas-phase ion chemistry have been studied by experiment and theory. Ion 1(+*) was generated by the McLafferty rearrangement in ionized 2-acetylbutane-4-lactone and characterized by accurate mass measurements, isotope labeling, metastable ion and collisionally activated dissociation (CAD) spectra. Metastable 1(+*) undergoes competitive losses of H-4 and CO that show interesting deuterium and (13)C isotope effects. The elimination of CO from metastable 1(+*) shows a bimodal distribution of kinetic energy release and produces (*)CH(2)CH(2)CHdbond;OH(+) (14(+*)) and CH(3)CHdbond;CHOH(+*) (15(+*)) in ratios which are subject to deuterium isotope effects. Ab initio calculations at the G2(MP2) level of theory show that 1(+*) is 105 kJ mol(-1) more stable than its oxo form, [butane-4-lactone](+*)(2(+*)). The elimination of CO from 1(+*) involves multiple isomerizations by hydrogen migrations and proceeds through ion-molecule complexes of CO with 14(+*) and 15(+*). In addition, CO is calculated to catalyze an exothermic isomerization 14(+*) --> 15(+*) in the ion-molecule complexes. Multiple consecutive hydrogen migrations in metastable 1(+*), as modeled by RRKM calculations on the G2(MP2) potential energy surface, explain the unusual deuterium kinetic isotope effects on the CO elimination.

Contributions of C3H6O+. ions with the oxygen on the middle carbon to gas phase ion chemistry

The numerous ways in which studies of C3H6O+. ions with the oxygen on the second carbon have added to our knowledge of gas phase ion chemistry are reviewed. The enol form of this ion (1) first attracted interest during early investigations of the mechanism of the McLafferty Rearrangement and later in characterizing the double McLafferty Rearrangement. Next, it was found that 1 isomerizes to the higher energy acetone ion (2). This discovery sparked studies of the relative stabilities of ionized and neutral enol and ketone species. It also led to the discovery that 1-->2 surmounts a substantial barrier, and that 2 dissociates faster than the excess energy deposited in it by the isomerization can become randomly distributed; i.e., dissociation following 1-->2 is nonergodic. This fact is manifested by a more abundant loss of the methyl formed by isomerization and a greater associated translational energy release. Propene oxide and methyl vinyl ether ions also appear to ionize to 2 and to dissociate nonergodically. Methane is eliminated from 2 through a methyl-acetyl ion complex. Characterization of this reaction by photoionization mass spectrometry, ab initio theory, and RRKM calculations helped to establish that complex-mediated alkane eliminations are generally confined to a region just above threshold. At higher energies, because attractions between ions and nonpolar neutrals are weak, simple dissociation is too fast for complex-mediated H-transfer to compete with it. Studies of the collision-induced dissociations of 2 demonstrate that the first electronically excited A state of 2 is very long-lived and efficiently releases its energy into translational energy when the ion collides with a neutral at low impact energies. Finally, ion-molecule reactions of 2 and acetone-containing ion clusters in the gas phase are described.

Elimination of methane from protonated acetaldehyde: The Ab initio transition state

The transition state (TS) for loss of CH4 from protonated acetaldehyde has been located at the second-order Moller-Plesset (MP2)/6-31G(d, p) level of theory. The activation energy is predicted to be 263.9 kJ/mol starting from the more stable form (methyl and hydrogen E) and 261.6 kJ/mol starting from the less stable form (methyl and hydrogen Z) that is required for reaction. The products (methane and the formyl ion) are predicted to lie 136.6 kJ/mol below the TS for their formation. MP2 methods underestimate the heats of formation of both the TS and the reaction products by about 40 kJ/mol when compared with experiment. Restricted Hartree-Fock (RHF) calculations give much more accurate relative energies. The MP2 TS leads directly to fragmentation and is described as a protonation of the methyl group by the acidic proton on oxygen. Under RHF theory the reaction is stepwise. An RHF TS similar to the MP2 TS leads to a nonclassical intermediate (which is stable at this level of theory) that has one of the C-H bonds protonated. This mechanism (protonation of an alkyl group) appears to be a general one for high energy 1,2 eliminations from organic cations.