“Proton-transport” catalysis in the gas phase. Keto-enol isomerization of ionized acetaldehyde

Rest, Guillaume van der; Nedev, Hristo; Chamot‐Rooke, Julia; Mourgues, P.; McMahon, T. B.; Audier, H. E.

doi:10.1016/s1387-3806(00)00242-6

Cited by 36 publications

(39 citation statements)

References 24 publications

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“…Actually, numerous experimental and theoretical studies have recently reported rearrangement of ionized carbonyl compound to the more stable enol ion by proton transfer through bimolecular interaction with an appropriate base. Such a process, termed "proton-transfer catalysis" [24], was shown to efficiently catalyze the enolization of acetaldehyde [25], acetone [26], acetophenone [27], and acetamide [28] molecular ions in reducing the corresponding critical barrier. It was demonstrated that the required condition for the efficient proton-transfer catalysis of an [HOXOY] •ϩ ion into its isomeric more stable form [XOYOH] •ϩ , the so-called Radom transport criterion [29], is that the proton affinity (PA) of the catalyst lies between the PA values of the conjugate base [XOY] • at X and at Y.…”

Section: Associative Ion/molecule Reactions Between Ionized Cyclohexamentioning

confidence: 99%

Isomeric recognition by ion/molecule reactions: The ionized phenol-cyclohexadienone case

Trupia

Dechamps

Flammang

et al. 2008

J. Am. Soc. Mass Spectrom.

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The isomerization process between ionized phenol and ionized cyclohexadienone is studied by performing ion/molecule reactions with several alkyl nitrites in a hexapole collision cell inserted in a six-sector mass spectrometer. The distinction between both isomeric species is readily achieved on the basis of the completely different reactivity patterns observed for them in subsequent reactions. When reacting with alkyl nitrite, ionized phenol undergoes two competitive reactions corresponding to the formal radical substitution of the hydroxylic hydrogen atom by respectively (i) the nitrosyl radical (m/z 123) and (ii) an alkoxyl radical (m/z 138 if alkyl ϭ ethyl). Both reactions were theoretically demonstrated by density functional theory calculations p) ϩ ZPE] to involve hydrogen-bridged radical cations as key intermediates. The ion/molecule reaction products detected starting from ionized cyclohexadienone as the mass-selected ions arise from • OAlkyl, • OH, and NO 2 • radical additions. The occurrence of a spontaneous ring-opening of cyclohexadienone radical ion into a distonic species is suggested to account for the observed ion/molecule reaction products. We also demonstrated that ionized cyclohexadienone is partly isomerized during a proton-transfer catalysis process into ionized phenol inside the Hcell with ethyl nitrite as the base. The molecular ions of phenol generated in such conditions consecutively undergo reactions producing m/z 123 and 138 radical cations. The proposed mechanism is supported by results of quantum chemical calculations. . It is now well established that, in both condensed and gas phases, neutral carbonyl compounds are usually more stable than their enol tautomers. Exceptions to this rough rule are nevertheless observed when the carbon-carbon double bond of the enol becomes conjugated with another functional group such as a second carbonyl group or included in an aromatic moiety (see following text). The remarkable feature when dealing with the ionized species is that they show a reversed stability order. Indeed, as a rule, enol ions are always by far the most stable structures within a set of isomeric radical cations. As a consequence, enol ions often appear as reaction intermediates or as the ultimate ionic product of a fragmentation process. The higher stability of ionized enols with regard to the corresponding ionized carbonyl compounds is explained on the basis of the large difference between the ionization energies (IEs) of the neutral molecules, the IE of the neutral enol being always by far lower than the IE of the corresponding carbonyl compound [2]. The usual considerable difference between the IEs being much larger than the energy difference between both neutral molecules explains the inversion in the stability order after ionization [2].The thermochemical stability of an ionized enol is also associated with a large kinetic stability. In other words, enol ions are generally protected against dissociation or isomerization by significant energy barriers [3]. Consequently, ioni...

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Section: Associative Ion/molecule Reactions Between Ionized Cyclohexamentioning

confidence: 99%

Isomeric recognition by ion/molecule reactions: The ionized phenol-cyclohexadienone case

Trupia

Dechamps

Flammang

et al. 2008

J. Am. Soc. Mass Spectrom.

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“…In this process, the ion accepts a H atom from the neutral substrate, forming a protonated species which then donates a different H atom back to the neutral radical. This ATM model has been described in the CH 3 CHO +• /CH 3 OH system by van der Rest et al [14] with calculations at the UMP2/6-31G** level of theory, and is shown below:…”

Section: H • -Atom Transfer Mechanism (Atm)mentioning

confidence: 99%

“…In a previous study [13,14], the enol or keto cations of acetaldehyde were formed in an FT-ICR cell and allowed to react with a neutral methanol molecule. Under these conditions, the methanol molecule was found to catalyze the isomerization of the acetaldehyde ion to its isomeric form.…”

Section: Spectator Mechanismmentioning

confidence: 99%

Exploring the potential energy surface of ion–molecule pairs by experiment and by theory: acetaldehyde and methanol

Wang

Holmes

2005

International Journal of Mass Spectrometry

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“…11 Acetaldehyde can exist in two isomeric forms: keto and enol in the gas phase, which has been investigated both via experiment and theory. [12][13][14] For the neutral acetaldehyde molecule, keto-enol isomerization is endothermic by 9.9 kcal/mol. In contrast, the process in the ionized system is exothermic by about 17.1 kcal/mol.…”

Section: Introductionmentioning

confidence: 99%

“…12 Investigations of ionized acetaldehyde have shown that the assistance of one or more solvent molecules enables proton transfer from a methyl group to oxygen and thus significantly decreases the energy barrier to isomerization. 12,13 It was also shown that methanol is a better catalyst than water for proton transfer. 12 In the case of the reaction of the acetaldehyde cation with methanol, the isomerization to a vinyl alcohol is shown to be a barrierless process.…”

Section: Introductionmentioning

confidence: 99%

Proton transfer in acetaldehyde–water clusters mediated by a single water molecule

Kostko

Troy

Bandyopadhyay

et al. 2016

Phys. Chem. Chem. Phys.

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“Proton-transport” catalysis in the gas phase. Keto-enol isomerization of ionized acetaldehyde

Cited by 36 publications

References 24 publications

Isomeric recognition by ion/molecule reactions: The ionized phenol-cyclohexadienone case

Isomeric recognition by ion/molecule reactions: The ionized phenol-cyclohexadienone case

Exploring the potential energy surface of ion–molecule pairs by experiment and by theory: acetaldehyde and methanol

Proton transfer in acetaldehyde–water clusters mediated by a single water molecule

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