Abstract
Gas-phase studies of ion-molecule reactions shed light on the intrinsic factors that govern reactivity; and even solvent effects can be examined in the gasphase environment by employing microsolvated ions. An area that has received considerable attention with regard to the interplay between intrinsic factors and solvent effects is the enhanced reactivity of α-nucleophiles – nucleophiles with a lone-pair adjacent to the attacking site – referred to as the α-effect.
This thesis concerns the reactivity of microsolvated anions and in particular how the presence of a single solvent molecule affects the gas-phase α-effect. The experimental studies are performed by means of the flowing after glow selected ion flow tube technique, and these are supplemented by electronic structure calculations. The α-nucleophile employed is the microsolvated hydrogen peroxide anion whose reactivity is compared to that of a series of microsolvated oxygen centered anions.
The association of the nucleophiles with a single water or methanol molecule allows the α-effect to be observed in the SN2 reaction with methyl chloride; this effect was not apparent in the reactions of the unsolvated anions. The results suggest that solvent effects as well as intrinsic components contribute to the α-effect. Microsolvation lowers the reactivity of the anions – methanol more than water – and methanol microsolvation allows us to demonstrate the presence of an α-effect in reactions of the more reactive methyl bromide as well. In reactions with methyl formate, in which several different channels are possible, microsolvation was found to lower the affinity for proton abstraction in favor of reaction at the carbon sites. Particularly the reaction channel involving acyl substitution at the carbonyl carbon was favorable for the microsolvated α-nucleophile, and a significant α-effect was observed in this channel.
Quantum chemical calculations reveal that the structure of the microsolvated hydrogen peroxide adduct is distinctly different from the structure of the microsolvated alkoxy nucleophiles, in that it involves transfer a proton from the solvent to the anion, resulting in a HO−(HOOH) rather than a HOO−(H2O) structure. However, the results demonstrate that the reactive nucleophile is nonetheless the HOO− anion.
Finally, microsolvation applied to radical-molecule reactions allows us to demonstrate that a single water molecule cannot be expected to catalyze hydrogen abstraction reactions by the hydroxyl radical under atmospherically relevant conditions.
This thesis concerns the reactivity of microsolvated anions and in particular how the presence of a single solvent molecule affects the gas-phase α-effect. The experimental studies are performed by means of the flowing after glow selected ion flow tube technique, and these are supplemented by electronic structure calculations. The α-nucleophile employed is the microsolvated hydrogen peroxide anion whose reactivity is compared to that of a series of microsolvated oxygen centered anions.
The association of the nucleophiles with a single water or methanol molecule allows the α-effect to be observed in the SN2 reaction with methyl chloride; this effect was not apparent in the reactions of the unsolvated anions. The results suggest that solvent effects as well as intrinsic components contribute to the α-effect. Microsolvation lowers the reactivity of the anions – methanol more than water – and methanol microsolvation allows us to demonstrate the presence of an α-effect in reactions of the more reactive methyl bromide as well. In reactions with methyl formate, in which several different channels are possible, microsolvation was found to lower the affinity for proton abstraction in favor of reaction at the carbon sites. Particularly the reaction channel involving acyl substitution at the carbonyl carbon was favorable for the microsolvated α-nucleophile, and a significant α-effect was observed in this channel.
Quantum chemical calculations reveal that the structure of the microsolvated hydrogen peroxide adduct is distinctly different from the structure of the microsolvated alkoxy nucleophiles, in that it involves transfer a proton from the solvent to the anion, resulting in a HO−(HOOH) rather than a HOO−(H2O) structure. However, the results demonstrate that the reactive nucleophile is nonetheless the HOO− anion.
Finally, microsolvation applied to radical-molecule reactions allows us to demonstrate that a single water molecule cannot be expected to catalyze hydrogen abstraction reactions by the hydroxyl radical under atmospherically relevant conditions.
| Original language | English |
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| Publisher | Department of Chemistry, Faculty of Science, University of Copenhagen |
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| Number of pages | 142 |
| ISBN (Print) | 978-87-91963-34-6 |
| Publication status | Published - 2013 |