Hydrogen radical abstraction by small ionized molecules, distonic ions and ionized carbenes in the gas phase
Résumé
In the gas phase and within ion-neutral complexes, H. abstraction by the ion from the neutral moiety was studied by using FT-ICR experiments and molecular orbital calculations. Ionized methanol abstracts rapidly H. from methane and other alkanes while its a-distonic counterpart, .CH2OH2+, is completely unreactive. On the other hand, ß-distonic ions, such as .CH2CH2XH+ (X=OCH3, NH2), are also unreactive towards methane but can abstract H . from ketones and ethers. Finally, ionized carbenes, such as HO-C-NH2.+, react with methane by a slow H. abstraction. Ab initio molecular orbital calculations at the G3(MP2) level were performed in order to understand these behaviors. For ionized methanol and its a-distonic counterpart, the reacting structure that could lead to H . abstraction is the highly stabilized complex between protonated methanol and a methyl radical, which yields the final state (CH 3OH2++.CH3) by simple cleavage. In the case of methanol the encounter complex with methane leads easily by rotation of the methane molecule to this reacting structure. In contrast, in the case of the a-distonic ion, the almost linear structure of the encounter complex [.CH2OH2+?CH4] and the high energy required for its isomerization into the reacting structure prevent the reaction. Two factors are required to observe H. abstraction in ß-distonic ions: the interaction energy of the encounter complex and the distance between the hydrogen to be transferred and the carbon radical. Reaction of the HO-C-NH 2.+ carbene with methane lies between these two extreme cases. The encounter complex is poorly stabilized (-8.7kcalmol-1) and the transition state for H. abstraction is very close to the reactants energy. © 2003 Elsevier B.V. All rights reserved.