Abstract
The dynamics of microhydrated nucleophilic substitution reactions have been studied using crossed beam velocity map imaging experiments and quasiclassical trajectory simulations at different collision energies between 0.3 and 2.6 eV. For F–(H2O) reacting with CH3I, a small fraction of hydrated product ions I–(H2O) is observed at low collision energies. This product, as well as the dominant I–, is formed predominantly through indirect reaction mechanisms. In contrast, a much smaller indirect fraction is determined for the unsolvated reaction. At the largest studied collision energies, the solvated reaction is found to also occur via a direct rebound mechanism. The measured product angular distributions exhibit an overall good agreement with the simulated angular distributions. Besides nucleophilic substitution, also ligand exchange reactions forming F–(CH3I) and, at high collision energies, proton transfer reactions are detected. The differential scattering images reveal that the Cl–(H2O) + CH3I reaction also proceeds predominantly via indirect reaction mechanisms.
Highlights
Reaction dynamics of the important class of bimolecular nucleophilic substitution (SN2) reactions have been studied extensively for combinations of X− + halideCH3Y anions model systems with several and methyl halides.[1−5] Detailed information on gas phase reaction dynamics is obtained by measuring differential cross sections of bimolecular reactions in crossed molecular beams under single collision conditions
The in-depth gas phase picture obtained from experimental evidence and simulations often provides a good starting point to interpret reaction dynamics in liquids, which has become experimentally accessible by time-resolved infrared spectroscopy.[6]
The dissociating to I−, result is shown in the apparent mass inFcigrueareseSs 3b.y2.4FuorμsF−C1 Hfro3Im−
Summary
CH3Y anions model systems with several and methyl halides.[1−5] Detailed information on gas phase reaction dynamics is obtained by measuring differential cross sections of bimolecular reactions in crossed molecular beams under single collision conditions. Already the addition of a few solvent molecules will strongly quench the reaction.[9,10] This effect is important in microsolvated SN2 reactions that typically avoid the energetically favored solvated products as opposed to reactions in solution where different molecules concertedly desolvate the nucleophile and solvate the leaving group.[11] When SN2 reactions become inefficient by stepwise solvation, ligand exchange can become important as has been reported for Cl−(D2O)[1−3] and F−(H2O)[4−5] reactions with CH3Br.[12,13]
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