Mechanistic studies of gas-phase negative ion unimolecular decompositions. Alkoxide anions

The unimolecular decompositions of 15 gas-phase alkoxide negative ions have been studied by infrared multiple photon photochemical activation in an ion cyclotron resonance spectrometer. Upon pulsed CO₂ laser irradiation, alkoxide anions undergo elimination of neutral molecules (e.g., alkanes RH) to yield enolate anions. The observed reactivity patterns and kinetic isotope effects further establish a stepwise decomposition mechanism involving initial heterolytic cleavage to an intermediate anion-ketone complex followed by proton transfer to give the ultimate products. A relative order of leaving group propensities CF₃ > Ph > H > t-Bu > Me > i-Pr > Et was observed. The apparent anomalous reactivity order for the alkyl groups can be rationalized by invoking a change in mechanism to one involving an intermediate in which an electron is not bound specifically by the eliminated alkyl group for R = t-Bu, i-Pr, and Et: either a radical-ketone radical anion complex produced by homolytic cleavage or an anionic cluster. This order also leads to the conclusion that methane elimination from alkoxide anions proceeds via the pathway involving heterolytic cleavage. The results of this study have implications for bimolecular ion-molecule reaction dynamics, since the photochemically generated intermediates are also intermediates in bimolecular proton transfer reactions.