Henri Edouard Audier

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

Abstract Fourier transform ion cyclotron resonance experiments show that a variety of molecules catalyze the hydrogen transfer which converts ionized acetaldehyde CH 3 CHO ·+ 1 to its vinyl alcohol counterpart CH 2 CHOH ·+ 2 . Each of these ions has been characterized by its specific bimolecular reactions with selected reactants. Calculations show that two pathways, for which the rate determining barriers have almost the same energy, are feasible. The first transition state involves a direct catalyzed 1,3-H transfer, while the second involves two successive 1,2-H transfers. A detailed experimental study, using methanol as a catalyst as well as labeled reactants, indicates that only the first pathway operates in the isomerization process. The different steps of these two independent pathways were elucidated. The first begins with the formation of a highly stabilized complex 3 , involving a two-center-three-electron interaction between the two oxygen atoms and an interaction between a hydrogen of the methyl group of 1 and the oxygen of methanol. This complex isomerizes into a complex 4 , which in turn gives the complex 5 , via a transition state located 6.3 kcal mol −1 below the energy of the reactants. This complex 5 corresponds to ionized vinyl alcohol hydrogen bonded to the oxygen of methanol, which dissociates to yield ion 2 . The second pathway begins with the interaction between the hydrogen of the CHO group and the oxygen of methanol and gives the complexes 6 and then 7 , which correspond to protonated methanol hydrogen bonded to a CH 3 CO · radical. Dissociation of 7 to give protonated methanol is favoured with respect to further isomerization leading to ionized vinyl alcohol. Compared to the unimolecular conversion between energetic ions 1 and 2 , which can occur either by a direct 1,3-H transfer or by a double 1,2-H transfer, the reaction of 1 with methanol catalyzes the first pathway while inhibiting the second one. In the case studied, catalysis is perhaps better described as a hydrogen atom transport.

Experimental verification of the intermediacy and interconversion of ion–neutral complexes as radical cations dissociate

Specific hydrogen exchange provides experimental verification of the intermediacy of ion–neutral complexes in the reactions of simple alcohol and ether radical cations; interconversion of ion–neutral pairs can occur when loss of alkyl radicals and loss of alkane molecules have similar energy requirements.

Two different pathways for unimolecular and for catalyzed keto-enol isomerization of ionized acetophenone

Abstract Studies of the unimolecular reactions in the gas phase of the C 6 H 5 COCH 3 +· ( 1 ) and C 6 H 5 C(OH)CH 2 +· ( 2 ) ions have shown (1) that ion 1 does not convert to ion 2 prior to methyl radical loss, (2) that ion 2 isomerizes into ion 1 prior methyl radical loss, and (3) that this keto-enol isomerization does not occur by a direct 1,3-H transfer but by two successive 1,4-H transfers. Fourier transform ion cyclotron resonance experiments show that acetone catalyses the isomerization 1 → 2 . Further, by using labeled reactants, it is demonstrated that this isomerization occurs by a direct catalyzed 1,3-H transfer whereas the less energy demanding pathway connecting bare ions 1 and 2 is a double 1,4-H transfer. This represents the first description of a system for which the pathways connecting two isomeric ions are different for the unimolecular and for the catalyzed isomerizations.

Gas phase catalyzed keto-enol isomerization of cations by proton transport☆

Abstract In the gas phase, the unimolecular isomerization of the H 3 COC(O)CH 2 CO + cation 1 ( m / z 101) into the H 3 CO(HO)CCHCO + enol ion 2 by a 1,3-H shift possesses a high energy barrier and is therefore not observed. In contrast, in the cell of a FT-ICR mass spectrometer, interaction with gaseous methanol catalyzes the isomerization of 1 into its more stable isomer 2 , which can be characterized by low energy collision with argon. This exothermic reaction is irreversible. Reaction with labeled methanol and ligand exchange experiments indicate the existence of two distinct reactions. By formation of a covalent bond, one reaction yields protonated dimethyl malonate while the second one leads to ion 2 by a 1,3-H transfer catalyzed by methanol. Conversely, loss of methanol from collisionally activated long-lived m / z 133 cations formed by protonation of dimethyl malonate yields some m / z 101 ions with structure 2 , which shows that methanol catalyzes the isomerization of ion 1 within a [ 1 , CH 3 OH] complex. The efficiency of different catalysts is studied in order to probe the mechanism of the isomerization processes.

Intramolecular hydrogen atom abstraction with an eight-membered cyclic transition state in open-chain aliphatic aminium radicals

Abstract Hydrogen abstraction by the nitrogen in long-chain aliphatic gas-phase aminium radicals (amine molecular ions) takes place via six-, seven-, and eight-membered cyclic transition states. The subsequent fragmentation is particularly facile after eight-membered ring hydrogen transfer.

The [+CH2OH, H2O] and [+CH2OH, 2H2O] solvated ions

Abstract The reaction of CH3CH2OH·+ with H182O in the gas phase has been studied by FT-ICR spectroscopy. The main reaction yields the product ion [CH2O⋯H+⋯18OH2]. This ion slowly exchanges its second oxygen with H182O. Two possible channels for this reaction will be successively discussed. (i) In the first, the permutation of both oxygens take place within [CH182O⋯H+⋯18OH2], leading to [CH182O⋯H+⋯OH2]. This last ion can in turn undergo a ligand exchange with H182O, yielding [CH182O⋯H+⋯18OH2]. In this first hypothesis, a second molecule of water intervenes after an unimolecular rearrangement process within the [CH2O⋯H+⋯18OH2] ion. (ii) Conversely, a second possible mechanism involves as a first step a nucleophilic attack of H182O at the carbon atom. In that case the rearrangement takes place within a [+CH2OH, 2H2O] solvated ion. It will be shown that the first process is strongly endothermic and therefore less likely than the second, in which the solvation effect strongly decreases the energy of the intermediate, and where H2O catalyses migrations of H.

On the release of translational energy when stable intermediate ion-neutral complexes dissociate

Abstract The loss of alkane molecules from metastable aliphatic alcohol and ether radical cations in the gas phase can be used to probe the properties of intermediate weakly bonded ion-neutral complexes. Very little translational energy is released when the final products form a stable complex prior to dissociation, whereas the energy released can directly reflect the heat of reaction when dissociation takes place subsequent to an exothermic intra-complex reaction. The heats of formation of the intermediate oxonium ions and the product radical cations of enols and enol ethers were determined using the composite G2(MP2) and CBS-Q high-level ab initio methods.

Ter-Body Intermediates in the Gas Phase: Reaction of Ionized Enols with tert-Butanol

In the gas phase, the CH2CHOH.+ enol radical cation 1 as well as its higher homologues CH3CHCHOH.+2 and (CH3)2CCHOH.+3, undergo exactly the same sequence of reactions with tert-butanol, leading to the losses of isobutene, water and water plus alkene. Fourier transform ion cyclotron resonance (FT-ICR) experiments using labeled reactants as well as ab initio calculations show that independent pathways can be proposed to explain the observed reactivity. For ion 1, taken as the simplest model, the first step of the reaction is formation of a proton bound complex which gives, by a simple exothermic proton transfer, the ter-body intermediate [CH2CHO., H2O, C(CH3)3+]. This complex, which was shown to possess a significant lifetime, is the key intermediate which undergoes three reactions. First, it can collapse to yield tert-butylvinyl ether with elimination of water. Second, by a regiospecific proton transfer, this complex can isomerize into three different ter-body complexes formed of water, isobutene and ionized enol. Within one of these complexes, which does not interconvert with the others, elimination of isobutene leads to the formation of a solvated enol ion. Within the others, a cycloaddition—cycloreversion process can proceed to yield the ionized enol 3 (loss of water and ethylene channel).

RING VERSUS OXYGEN PROTONATION IN METASTABLE ION DECOMPOSITIONS OF PROTONATED ISOPROPYL PHENYL ETHER

Abstract Protonated alkyl phenyl ethers possess more than one stable tautomer. A debate has arisen over whether only one of them gives rise to the principal dissociation pathway observed in their mass-analyzed ion kinetic energy (MIKE) spectra. Alkene loss constitutes the major (often the exclusive) metastable ion decomposition, yielding protonated phenol ions. A hydron deposited by chemical ionization exchanges with some of the alkyl hydrogens (but none of the ring hydrogens) prior to fragmentation. Previously published MIKE spectra have shown that [(CD3)2CHOPh]D+ gives only m/z 97 (C6H5D2O+), but that [(CD3)2CHOPh]H+ gives a mixture of m/z 96 (C6H6DO+) and m/z 97. Exchange must arise via ion-neutral complexes that result from O-protonated ions, (CD3)2CHO(H)Ph+. Current controversy centers around the contribution of ring-protonated ions to the production of unexchanged fragment ions. Here we determine the mole fractions of ring-protonated (X) and O-protonated (1 − X) parent ions using m/z 95:m/z 96:m/z 97 MIKE ion abundance ratios from H2O and D2O CI of (CH3)2CHOPh, CH3(CD3)CHOPh, and (CD3)2CHOPh. Data from the first two compounds give unbiased assessments of X and four other relative rate constants that are obtained using a steady-state kinetic model that gives a set of five equations in five unknowns. The values calculated from the data predict an m/z 96:m/z 97 ratio of 4.7 for [(CD3)2CHOPh]H+ that turns out to be the same ratio as is measured experimentally. This validation of the data analysis corroborates the value of X ≤ 0.01 extracted from the experimental results. The contribution of ring-protonated parent ions to the MIKE spectra of chemically ionized isopropyl phenyl ether is therefore negligible.

The gas-phase reactions of the methoxymethyl cation with aldehydes and ketones

The results of both mass-analysed ion kinetic energy spectroscopy and Fourier transform ion cyclotron resonance experiments involving reactions of the methoxymethyl cation CH3OCH2+ with a variety of aldehydes and ketones are reported. With ketones, the reaction yields a covalent complex whose dissociation either gives back the CH3OCH2+ cation or leads to a methyl cation transfer to the ketone. Except for CH2O, a third pathway is open with aldehydes. A hydride transfer to CH3OCH2+ yields a RCO+ acylium product ion. The branching ratio of these three pathways strongly depends on the structure of the aldehyde. Ab initio calculations confirm that the results can be explained by the interconversion of covalent structures and electrostatic complexes on the potential energy surface.