Abstract
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.
| Original language | English |
|---|---|
| Pages (from-to) | 2714-2722 |
| Number of pages | 9 |
| Journal | Journal of the American Chemical Society |
| Volume | 110 |
| Issue number | 9 |
| Publication status | Published - 1988 |
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Mechanistic studies of gas-phase negative ion unimolecular decompositions. Alkoxide anions. / Tumas, William; Foster, Robert F.; Brauman, John I.
In: Journal of the American Chemical Society, Vol. 110, No. 9, 1988, p. 2714-2722.Research output: Contribution to journal › Article
}
TY - JOUR
T1 - Mechanistic studies of gas-phase negative ion unimolecular decompositions. Alkoxide anions
AU - Tumas, William
AU - Foster, Robert F.
AU - Brauman, John I.
PY - 1988
Y1 - 1988
N2 - 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.
AB - 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.
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M3 - Article
VL - 110
SP - 2714
EP - 2722
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
SN - 0002-7863
IS - 9
ER -